Saladin 5e Extended Outline
Chapter 20
The Circulatory System: Blood Vessels and Circulation
I. General Anatomy of the Blood Vessels (pp. 756–764)
A. The three principal categories of blood vessels are arteries, veins, and capillaries. (pp. 756–757)
(Fig. 20.1)
1. Arteries are the efferent vessels of the cardiovascular system—they carry blood away
from the heart.
2. Veins are the afferent vessels, carrying blood back to the heart.
3. Capillaries are microscopic, thin-walled vessels the connect the smallest arteries to the
smallest veins.
B. The vessel walls of arteries and veins are composed of three layers called tunics. (pp. 757–759)
(Fig. 20.2)
1. The tunica interna (tunica intima) lines the inside of the vessel and is exposed to the
blood.
a. It consists of a simple squamous epithelium called the endothelium overlying
a basement membrane and a sparse layer of loose connective tissue.
b. The endothelium acts as a selectively permeable barrier.
c. It secretes chemicals that stimulate dilation or constriction of the vessel.
d. It normally repels blood cells and platelets.
e. If damaged, platelets may adhere to it and form a blood clot.
f. When tissue around a vessel is inflamed, the endothelial cells produce celladhesion molecules that induce leukocytes to adhere to the surface, so that they
congregate in the area.
2. The tunica media, the middle layer, is usually the thickest.
a. It consists of smooth muscle, collagen, and in some cases elastic tissue..
b. The relative amounts of muscle and elastic tissue vary greatly and form a
basis for classifying vessels.
c. The tunica media strengthens vessels and prevents blood pressure from
rupturing them.
d. It provides vasomotion, or changes in the diameter of the blood vessel.
3. The tunica externa (tunica adventitia) is the outermost layer.
a. It consists of loose connective tissue that often merges with that of
neighboring blood vessels, nerves, or other organs. (Fig. 20.1a)
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b. It anchors the vessel and provides passage for nerves, lymphatic vessels, and
smaller blood vessels.
c. Small vessels called the vasa vasorum supply blood to at least the outer half of
the wall of a larger vessel, while tissues on the inner half are thought to be
nourished by diffusion from blood in the lumen.
C. Arteries are sometimes called the resistance vessels of the cardiovascular system because they
have relatively strong structure that resists blood pressure. (pp. 758–760)
1. Arteries are built to withstand the surges of pressure created by each beat of the heart.
2. Being more muscular than veins, they retain a round shape even when empty.
3. They are divided into three categories by size, but with a smooth transition between
categories.
a. Conducting (elastic or large) arteries are the biggest; examples are the aorta,
common carotid and subclavian arteries, pulmonary trunk, and common iliac
arteries.
i. They have a layer of elastic tissue, the internal elastic lamina, at the
border between the interna and media, but it is incomplete and difficult
to distinguish.
ii. The tunica media consists of 40 to 70 layers of elastic sheets,
perforated with holes and alternating with thin layers of smooth muscle,
collagen, and elastic fibers.
iii. An external elastic lamina is found at the border between the media
and externa, but is also difficult to distinguish.
iv. The tunica externa is less than half as thick and the media; it is well
supplied with vasa vasorum.
v. Conducting arteries expand during ventricular systole to receive
blood, and recoil during diastole.
vi. Recoil between heartbeats prevents blood pressure from dropping
too low.
vii. Atherosclerosis stiffens the arteries, lessening their recoil and
causing stress to arteries downstream.
Insight 20.1 Aneurysm
b. Distributing (muscular or medium) arteries are smaller branches that
distribute blood to specific organs.
i. Examples are the brachial, femoral, renal, and splenic arteries.
ii. Distributing arteries have up to 40 layers of smooth muscle
constituting about ¾ of the wall thickness.
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iii. The smooth muscle is more conspicuous than the elastic tissue but
internal and external elastic laminae are thick and often visible.
c. Resistance (small) arteries usually do not have individual names and are
variable in number and location.
i. They exhibit up to 25 layers of smooth muscle and relatively little
elastic tissue.
ii. Compared to large arteries, they have a thicker tunica media in
proportion to the lumen.
iii. The smallest, about 40 to 200 μm in diameter and with only one to
three layers of smooth muscle, are called arterioles.
4. Metarterioles are short vessels that link arterioles and capillaries.
a. Instead of a continuous tunica media, they have individual muscle cells
spaced a short distance apart, each forming a precapillary sphincter that
encircles the entrance to one capillary. (Fig. 20.3)
b. Constriction of these sphincters reduces or shuts off blood flow, diverting
blood elsewhere.
5. Arterial sense organs that detect blood pressure and chemistry changes are located in
certain major arteries above the heart; they transmit signals to the brainstem to regulate
heartbeat, vasomotion, and respiration and are of three kinds. (Fig. 20.4)
a. Carotid sinuses are baroreceptors that respond to blood pressure.
i. A common carotid artery ascends the neck on each side and branches
near the angle of the mandible, forming the internal carotid artery to the
brain and external carotid artery to the face.
ii. The carotid sinuses are located in the wall of the internal carotid
artery just above the branch point.
iii. the carotid sinus has a relatively thin tunica media and an abundance
of glossopharyngeal nerve fibers in the tunica externa.
iv. A rise in blood pressure easily stretches the thin media and
stimulates the nerve fibers.
v. Signals to the vasomotor and cardiac centers of the brainstem bring
about lowering of heart rate and dilating of blood vessels.
b. Carotid bodies are also located near the branch of the common carotid
arteries.
i. These are oval receptors about 3 × 5 mm in size, innervated by fibers
of the vagus and glossopharyngeal nerves.
ii. They are chemoreceptors that monitor changes in blood composition.
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iii. They transmit signals primarily to the brainstem respiratory centers,
which adjust breathing to stabilize blood pH and levels of O2 and CO2.
c. Aortic bodies are one to three chemoreceptors located in the aortic arch near
the arteries to the head and arms.
i. They are structurally similar to the carotid bodies and have the same
function.
D. Capillaries are the vessels where exchange takes place with other body tissues. (pp. 760–762)
(Fig. 19.3)
1. Some venules also allow exchange, but capillaries greatly outnumber them.
2. Blood capillaries are composed only of an endothelium and a basal lamina.
3. Their walls are as thin as 0.2 to 0.4 μm and they average about 5 μm in diameter at the
proximal end and widen to about 9 μm at the distal end.
4. Erythrocytes are about 7.5 μm in diameter, and they often have to stretch to squeeze
through the smallest capillaries.
5. The number of capillaries is estimated at about a billion and their total surface area at
6,300 m2.
a. Hardly any cell in the body is more than 60 to 80 μm away from the nearest
capillary.
b. Capillaries are scarce in tendons and ligaments, only occasionally found in
cartilage, and absent from epithelia and from the cornea and lens.
5. Three types of capillaries are distinguished by their permeability and structure.
a. Continuous capillaries occur in most tissues. (Fig. 20.5)
i. Their endothelial cells are held together by tight junctions and form a
continuous tube.
ii. The basal lamina is a thin protein–carbohydrate layer surrounding
the endothelium.
iii. The endothelial cells are separated by narrow intercellular clefts
about 4 nm wide, through which small solutes such as glucose can pass,
but not larger molecules or formed elements.
iv. In the brain, the continuous capillaries lack intercellular clefts and
form the blood–brain barrier.
v. Some continuous capillaries exhibit pericytes, cells that lie external
to the endothelium and that have elongated tendrils that wrap around
the capillary.
α. It is thought that pericytes contract to regulate blood flow
through the capillaries.
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β. They can differentiate into endothelial and smooth muscle
cells to contribute to vessel growth and repair.
b. Fenestrated capillaries have endothelial cells riddles with holes called
filtration pores (fenestrations). (Fig. 20.6)
i. The pores are about 20 to 100 nm in diameter and are often spanned
by a glycoprotein membrane.
ii. They allow for rapid passage of small molecules but still retain most
proteins and larger particles.
iii. They are important in organs that engage in rapid absorption or
filtration—kidneys, endocrine glands, small intestine, etc.
c. Sinusoids (discontinuous capillaries) are irregular blood-filled spaces in the
liver, bone marrow, spleen, and some other organs. (Fig. 20.7)
i. They are twisted, tortuous passageways 30 to 40 μm wide that
conform to the shape of the surrounding tissue.
ii. The endothelial cells are separated by wide gaps with no basal
lamina and have large fenestrations.
iii. Even proteins and blood cells can pass through; this is how proteins
and factors synthesized by the liver enter the blood, as well as how new
blood cells enter the circulation from bone marrow.
6. Capillaries are organized into networks called capillary beds, consisting of 10 to 100
capillaries supplied by a single metarteriole. (Fig. 20.3)
a. Beyond the origins of the capillaries, the metarteriole continues as a
thoroughfare channel leading directly to a venule; capillaries empty into the
distal end of the thoroughfare or directly into the venule.
b. When the precapillary sphincters are open, the capillaries are well perfused
with blood; when they are closed, the blood bypasses the capillaries, flowing
through the thoroughfare channel.
c. About three-quarters of the body’s capillaries are shut down at any given
time, since there is not enough blood to fill the entire vascular system at once.
d. Capillaries in different tissues are opened or closed depending on activity.
E. Veins are considered the capacitance vessels of the cardiovascular system because they are
relatively thin-walled and flaccid and expand easily. (pp. 762–763)
1. At rest, 54% of the blood is found in systemic veins as compared with only 11% in
systemic arteries. (Fig. 20.8)
2. Being distant from the ventricles, veins are subjected to relatively low blood pressure
and thus have thinner walls.
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a. In large arteries, blood pressure averages 90 to 100 mm Hg and surges to 120
mm Hg, whereas in veins it averages about 10 mm Hg.
b. The blood pressure is steady rather than pulsing as in arteries.
c. Veins collapse when empty and have flattened, irregular shapes in tissue
sections. (Fig. 20.1a)
3. Blood flow in the arterial system splits off repeatedly into smaller and smaller
branches; in the venous system, conversely, small veins merge to form larger and larger
tributaries.
4. Description of the venous system begins with the smallest vessels and works upward.
a. Postcapillary venules are the smallest of the veins, beginning with diameters
of about 15 to 20 μm.
i. They receive blood from capillaries or by way of the thoroughfare
channels.
ii. They have a tunica interna with only a few fibroblasts surrounding it
and no muscle.
iii. They are often surrounded by pericytes.
iv. They are more porous than capillaries and also exchange fluid with
surrounding tissues.
v. Most leukocytes emigrate from the blood stream through venule
walls.
b. Muscular venules receive blood from the postcapillary venules.
i. They are up to 1 mm in diameter.
ii. They have a tunica media of one or two layers of smooth muscle and
a thin tunica externa.
c. Medium veins range up to 10 mm in diameter.
i. Most veins with individual names are in this category.
ii. They have a tunica externa with an endothelium, basement
membrane, loose connective tissue, and sometimes a thin internal
elastic lamina.
iii. The tunica media is much thinner than in medium arteries, with a
bundles of smooth muscle but not a continuous muscular layer.
iv. The tunica externa is relatively thick.
v. Many medium veins, especially in limbs, exhibit infoldings of the
tunica interna that meet in the middle of the lumen forming venous
valves directed toward the heart. (Fig. 10.19)
vi. The valves keep blood moving upward toward the heart rather than
dropping down again when muscles relax.
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vii. Contraction of muscles surrounding a vein forces blood through the
valves and back toward the heart; this mechanism is called the skeletal
muscle pump.
viii. Varicose veins result in part from the failure of the valves.
ix. The valves are absent from very small and large veins, veins of the
abdominal and thoracic cavities, and veins of the brain.
d. Venous sinuses are veins with especially thin walls, large lumens, and no
smooth muscle.
i. The coronary sinus of the heart and the dural sinuses of the brain are
examples.
e. Large veins have diameters greater than 10 mm.
i. They have some smooth muscle in all three tunics.
ii. They have a relatively thin tunica media with only a moderate
amount of smooth muscle.
iii. The tunica externa is the thickest layer and contains longitudinal
bundles of smooth muscle.
iv. Example are the venae cavae, pulmonary veins, internal jugular
veins, and renal veins.
Insight 20.2 Varicose Veins
F. The simplest and most common circulatory route of blood flow is heart  arteries  capillaries
 veins  heart, but there are exceptions, notably portal systems and anastomoses. (p. 763) (Fig.
20.9)
1. In a portal system, blood flows through two consecutive capillary networks before
returning to the heart.
a. Portal systems occur in the kidneys; between the hypothalamus and anterior
pituitary, and connecting the intestines to the liver.
2. An anastomosis is a point where two blood vessels merge.
a. In an arteriovenous anastomosis (shunt), blood flows from an artery directly
into a vein, bypassing capillaries. (Fig. 20.9c)
i. Shunts occur in the fingers, palms, toes, and ears, where they reduce
heat loss in cold weather by allowing warm blood to bypass these
surfaces.
ii. These areas are thus more susceptible to frostbite.
b. The most common anastomoses are venous anastomoses, in which one vein
empties directly into another. (Fig. 10.9)
i. These provide alternative routes for drainage from an organ.
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c. Arterial anastomoses, in which two arteries merge, provide collateral
(alternative) routes of blood supply to a tissue. (Fig. 20.9e)
i. These may be found in the coronary circulation.
ii. They are common around joints where movement may obstruct a
pathway.
II. Blood Pressure, Resistance, and Flow (pp. 764–771)
A. The blood supply to a tissue can be expressed in terms of flow and perfusion. (p. 764)
1. Flow is the amount of blood flowing through an organ, tissue, or blood vessel in a
given time, such as mL/min.
2. Perfusion is the flow per given volume or mass of tissue, such as mL/min/g.
3. A large organ could have a greater flow but less perfusion than a small organ.
B. Total flow in a resting individual is equal to cardiac output, typically 5.25 L/min; flow through
organs, however, varies from minute to minute depending on demand. (p. 764)
C. Hemodynamics is the study of physical principles of blood flow, which are based mainly on
pressure and resistance.
1. The relationship is F  ΔP/R, which means that the greater the pressure difference
(ΔP) between two points, the greater the flow (F); and the greater the resistance (R), the
lower the flow.
D. Blood pressure (BP) is the force that the blood exerts against a vessel wall. (pp. 764–766)
1. For routine clinical purposes, the systemic arterial BP is measured with a
sphygmomanometer connected to a cuff that is wrapped around the arm, to record the
brachial artery pressure.
2. Systolic pressure is the peak arterial BP attained during ventricular contraction;
diastolic pressure is the minimum arterial BP occurring during ventricular relaxation
between heartbeats.
a. For a healthy person age 20 to 30, these pressures are typically 120 and 75
mm Hg, written as 120/75.
3. The difference between systolic and diastolic pressure is called the pulse pressure.
a. For a BP of 120/75, the pulse pressure would be 120 – 75 = 45 mm Hg.
b. This is a measure of stress exerted on small arteries by the pulse.
4. Mean arterial pressure (MAP) is the pressure obtained if measurements were taken at
several intervals throughout the cardiac cycle, say every 0.1 sec.
a. This average BP most influences risk level for disorders such as edema,
syncope, atherosclerosis, kidney failure, and aneurysm.
b. MAP is not a simple arithmetic mean because low pressure diastole lasts
longer than systole.
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c. An estimate of MAP is diastolic pressure plus one third of pulse pressure; for
120/75, MAP would be 75 + 45/3 = 90 mm Hg.
d. MAP varies with gravity, however, and is lower in the head than in the legs.
5. Hypertension (high BP) is commonly considered to be a chronic resting BP of greater
than 140/90.
a. Hypertension can weaken small arteries and cause aneurysms, and it promotes
development of atherosclerosis.
6. Hypotension (low BP) is a chronic low BP that may be a consequence of blood loss,
dehydration, anemia, or other factors; it is normal in people approaching death.
7. One of the body’s chief mechanisms for preventing excessive blood pressure is the
ability of the arteries to stretch and recoil during the cardiac cycle.
8. Blood flow in the arteries is pulsatile—it rushes forward in the aorta at 120 cm/sec
during systole and has an average speed of 40 cm/sec over the cardiac cycle.
9. When measured farther away from the heart, systolic and diastolic pressures are lower
with less difference between them. (Fig. 20.10)
10. In capillaries and veins, blood flows without pulsation.
a. An injured vein exhibits slow, steady bleeding, whereas a severed artery may
spurt blood.
b. In the inferior vena cava, however, venous flow fluctuates with the respiratory
cycle, and there is some fluctuation in the jugular veins.
11. Arteries become less distensible with age, and blood pressure therefore rises. (Table
20.1)
12. Three principal variables determine blood pressure.
a. Cardiac output.
b. Blood volume, which is regulated mainly by the kidneys; they have a greater
influence than any other organ on BP.
c. Resistance to flow, which results from friction of blood against the walls of
the vessels and hinges on blood viscosity, vessel length, and vessel radius.
E. Peripheral Resistance is the opposition to flow that the blood encounters in vessels away from
the heart. (pp. 766–768)
1. A moving fluid has no pressure unless it encounters at least some resistance.
2. Blood viscosity is determined by several factors, the most significant of which are the
erythrocyte count and albumin concentration.
a. A deficiency of erythrocytes (anemia) or albumin (hypoproteinemia) reduces
viscosity and speeds blood flow.
b. Excessive erythrocytes (polycythemia) and dehydration increase viscosity and
flow declines.
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3. The farther a liquid travels through a tube, such as a vessel, the more cumulative
friction it encounters, so pressure and flow decline with distance.
a. In a reclining person, mean arterial pressure would be higher in the arm than
in the ankle.
b. A strong pulse in the dorsal pedal artery is a good sign of adequate cardiac
output.
4. Vessel radius changes are a significant way of controlling peripheral resistance from
moment to moment.
a. Vasoconstriction is the narrowing of a vessel, and vasodilation is the
widening.
b. Vasoconstriction occurs when smooth muscle of the tunica media contracts;
vasodilation is the result of relaxation.
c. The effect of vessel radius results from friction of moving blood against the
walls.
i. Blood normally exhibits laminar flow—it flows in layers that move
faster toward the center of the vessel and slower near the walls.
ii. When a blood vessel is dilated, a greater portion of blood is in the
middle and average flow may be quite swift.
iii. When a vessel constricts, more of the blood is close to the wall and
average flow is slower. (Fig. 20.11)
d. Blood flow is proportional not merely to vessel radius, but to the fourth power
of radius: F  r4.
e. Blood vessels are capable of substantial changes in radius. (Fig. 20.12)
5. The result of many factors is that velocity of blood flow differs from one part of the
systemic circuit to another. (Table 20.2)
a. Flow is fastest in the aorta.
b. From the aorta to the capillaries, velocity diminishes for three reasons.
i. The blood has traveled a greater distance.
ii. The arterioles and capillaries have smaller radii.
iii. Even though the radii become smaller, the number of vessels and
total cross-sectional area become greater and greater, from 3 to 5 cm2 in
the aorta to 4,500 to 6,000 cm2 in all the capillaries collectively; the
effect is to slow blood down.
c. From capillaries to vena cava, velocity rises again.
i. Veins have larger diameters, so they create less resistance.
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ii. Because many capillaries converge on one venule, and many venules
on a larger vein, blood is being forced into a progressively smaller
channel.
iii. Nevertheless, blood in veins never regains the velocity it had in
large arteries.
6. The arterioles are the most significant point of control over resistance and blood flow.
a. They are on the proximal sides of the capillary beds.
b. They greatly outnumber any other class of arteries and thus provide the most
numerous control points.
c. They are more muscular in proportion to their diameters than any other class
of blood vessels and are highly capable of vasomotion.
d. Artieroles alone account for about half of the total peripheral resistance; but
larger arteries and veins are also capable of vasomotion and control of
resistance.
F. Vasomotion is a powerful way of regulating blood pressure and flow; vasomotion can be
controlled by local, neural, and hormonal mechanisms. (pp. 768–769)
1. With regard to local control, autoregulation is the ability of tissues to regulate their
own blood supply.
a. According to the metabolic theory of autoregulation, an inadequately perfused
tissue becomes hypoxic and waste products accumulate.
i. These conditions stimulate vasodilation that in turn increases
perfusion.
ii. As oxygen is delivered and metabolites are removed, the vessels
constrict.
b. Platelets, endothelial cells, and perivascular tissues secrete vasoactive
chemicals—substances that stimulate vasomotion.
i. Histamine, bradykinin, and prostaglandins stimulate vasodilation.
ii. Endothelial cells secrete prostacyclin and nitric oxide, which are
vasodilators, and polypeptides called endothelins, which are
vasoconstrictors.
c. If a tissue’s blood supply is cut off for a time and then restored, it often
exhibits reactive hyperemia—an increase above the normal level of flow.
d. Over a longer time, a hypoxic tissue can increase its own perfusion by
angiogenesis—the growth of new blood vessels.
i. This occurs in regrowth of the uterine lining after each menstrual
period, in the development of denser capillaries in muscles of athletes,
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and in growth of arterial bypasses around obstructions in coronary
circulation.
ii. Several growth factors and inhibitors control angiogenesis, but its
regulation is unclear.
iii. Malignant tumors secrete growth factors that stimulate vessels to
grow into them.
2. Blood vessels are also under remote neural control by the central and autonomic
nervous systems.
a. The vasomotor center of the medulla oblongata exerts sympathetic control
over blood vessels throughout the body, with the exception of precapillary
sphincters.
i. Sympathetic nerve fibers stimulate most blood vessels to constrict,
but cause dilation of vessels of skeletal and cardiac muscle for the
demands of exercise.
b. The vasomotor center is an integrating center for three autonomic reflexes—
baroreflexes, chemoreflexes, and the medullary ischemic reflex.
c. A baroreflex is an autonomic, negative feedback response to changes in blood
pressure detected by carotid sinuses. (Fig. 20.13)
i. Glossopharyngeal nerve fibers transmit signals continually to the
brainstem, with rising input as BP rises.
ii. This input inhibits sympathetic cardiac and vasomotor neurons to
reduce sympathetic tone and dilate arteries and veins.
iii. It also excites vagal fibers to the heart, slowing heart rate.
iv. When blood pressure drops too low, the opposite reactions occur
and BP rises back to normal.
v. Baroreflexes are important in short-term regulation, but are not
effective in correcting chronic hypertension.
d. A chemoreflex is an autonomic response to changes in blood chemistry,
especially pH and concentrations of O2 and CO2.
i. It is initiated by aortic bodies and carotid bodies.
ii. The primary role of these reflexes is to adjust respiration, but they
have a secondary role in stimulating vasomotion.
iii. Hypoxemia and acidosis stimulate the chemoreceptors, which act
through the vasomotor center to induce vasoconstriction.
iv. Chemoreceptors also stimulate breathing, so increased ventilation of
the lungs matches their increased perfusion.
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e. The medullary ischemic reflex is an autonomic response to a drop in perfusion
of the brain.
i. The medulla oblongata monitors its own blood supply and activates
correction when it senses a state of ischemia.
ii. Signals from the cardiac and vasomotor centers then induce an
increase in heart rate and contraction force and widespread
vasoconstriction.
iii. Input to these centers from other brain centers under conditions of
stress, anger, and arousal can also raise the blood pressure.
3. Several hormones influence blood pressure, some through vasoactive effects and some
through means such as regulating water balance.
a. Angiotensin II is a potent vasoconstrictor that raises blood pressure.
i. One of the enzymes required for synthesis is angiotensin converting
enzyme (ACE).
ii. Hypertension is often treated with ACE inhibitors.
b. Aldosterone is the “salt-retaining hormone” that promotes Na+ retention by
the kidneys.
i. Na+ promotes water retention through osmotic effects.
ii. This retention promotes a higher blood volume and pressure.
c. Atrial natriuretic peptide (ANP), secreted by the heart, antagonizes
aldosterone.
i. It increases NA+ excretion, thus reducing blood volume and pressure.
ii. It has a generalized vasodilator effect; Brain natriuretic peptide
(BNP) has a similar effect.
d. Antidiuretic hormone (ADH) primarily promotes water retention, but at
pathologically high levels it is also a vasoconstrictor and has the alternate name
vasopressin; both effects raise blood pressure.
e. Ephinephrine and norepinephrine are adrenal and sympathetic
catecholamines.
i. They bind to α-adrenergic receptors on smooth muscle of most blood
vessels, stimulating vasoconstriction.
ii. In the coronary blood vessels and vessels of the skeletal muscles,
however, they bind to β-adrenergic receptors, increasing blood flow to
the myocardium and muscular system during exertion.
G. Vasomotion can serve either of two physiological purposes: (1) a generalized raising or
lowering of blood pressure, or (2) selectively modifying the perfusion of a particular organ and
rerouting blood from one region to another. (pp. 769–771)
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1. A generalized increase in BP requires centralized control via the medullary vasomotor
center or via circulating hormones.
a. Widespread vasoconstriction raises the overall blood pressure because the
whole system squeezes on a fixed amount of blood.
b. It can be important in supporting cerebral perfusion in situations such as
hemorrhage or dehydration.
c. Generalized vasodilation lowers BP through the system.
2. Rerouting of blood flow and individual-organ changes in perfusion can be achieved by
central or local control.
a. During exercise, the sympathetic nervous system can selectively reduce blood
flow to the kidneys and digestive tracts, yet increase perfusion of skeletal
muscles.
b. Metabolite accumulation in a tissue can stimulate local vasodilation without
affecting circulation elsewhere.
3. If a specific artery constricts, pressure downstream drops and pressure upstream rises.
(Fig. 20.14)
a. Most blood follows the path of least resistance, and this enables the body to redirect
blood.
a. If you are resting after a big meal, vasoconstriction shuts down blood flow to
90% of capillaries in muscles of lower limbs, directing blood to the superior
mesenteric artery that supplies the small intestine. (Fig. 20.14a)
b. During vigorous exercise, arteries dilate in lungs, coronary circulation, and
muscles; vasoconstriction occurs elsewhere, such as the kidneys and digestive
tract. (Figs. 20.14b, 20.15)
III. Capillary Exchange (pp. 771–774)
A. Only 250 to 300 mL (5%) of the blood is in the capillaries at any given time, but this is where
capillary exchange occurs between the blood and surrounding tissues. (p. 771)
1. Examples of chemicals given off by the capillary blood include O 2, glucose, amino
acids, lipids, other organic nutrients, minerals, antibodies, and hormones.
2. Examples of chemicals taken up by the capillary blood include CO 2, ammonia and
other wastes, and many of the same substances as given off—so many chemicals have a
two-way traffic, entering at one point and leaving at another.
B. Mechanisms of capillary exchange are difficult to study quantitatively because of the vessels’
small size, so theories are still in dispute. (p. 771)
1. BP has been measured at 32 mm Hg at the arterial end and 15 mm Hg at the venous
end, 1 mm away; the drop is due to friction.
2. It takes 1 to 2 seconds for an RBC to pass through a fingernail-bed capillary.
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C. Chemicals pass through the capillary wall by three routes: the endothelial cell cytoplasm;
intercellular clefts between endothelial cells; and filtration pores in the fenestrated capillaries. (p.
771) (Fig. 20.16)
D. The mechanisms of movement are diffusion, transcytosis, filtration, and reabsoption. (p. 771)
E. Diffusion is the most important mechanism of exchange. (pp. 771–772)
1. Glucose and oxygen, being more concentrated in the arterial blood, flow out of
capillaries, while carbon dioxide and wastes, being more concentrated in the tissue fluid,
diffuse into the capillaries.
2. Lipid-soluble substances such as steroid hormones, O2, and CO2, diffuse easily through
the plasma membranes, whereas substances insoluble in lipids, such as glucose and
electrolytes, must pass through channels, pores, or clefts.
3. Large molecules such as proteins are usually held back.
F. Transcytosis is a process in which endothelial cells pick up material on one side of the plasma
membrane and release it on the other side. (p. 772)
1. Endothelial cells pick up material via pinocytosis or receptor-mediated endocytosis.
2. The material, in vesicles, is transported across the cell cytoplasm.
3. Exocytosis releases the material on the other side of the cell. (Fig. 3.23)
4. Fatty acids, albumin, and some hormones, such as insulin, move across the
endothelium by this method.
G. Filtration and reabsorption account for fluid movement. (pp. 772–773)
1. Following principles of osmosis, fluid filters out of the arterial end of a capillary and
reenters it at the venous end. (Fig. 20.17)
2. This shift in transfer is a result of changes in hydrostatic pressure and osmotic balance.
a. A typical capillary has a hydrostatic pressure of about 30 mm Hg at the
arterial end, and the pressure in the interstitial fluid may be about –3 mm Hg.
i. The negative value indicates a slight suction and is represented as
3out.
ii. The positive pressure inside the capillary and the negative interstitial
pressure give a net outward force of 33 mm Hg.
b. Colloid osmotic pressure (COP), the osmotic pressure due to protein, opposes
hydrostatic pressure.
i. The blood has a COP of about 28 mm Hg, due mainly to albumin.
ii. Interstitial fluid has less than one-third of this value, at 8 mm Hg.
iii. The difference between COP of blood and that of tissue fluid is
called oncotic pressure: 28in – 8out = 20in.
3. These opposing forces produce a net filtration pressure (NFP) of 13 mm Hg out at the
arterial end.
Saladin Outline Ch.20
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a. Net hydrostatic pressure is 10out (BP) + 3out (interstitial) = 13out.
b. Oncotic pressure is 28in (blood COP) – 8out (Tissue COP) = 20in.
c. Net filtration pressure is 33out (net hydrostatic) – 20in (oncotic) = 13out.
4. At the venous end, capillary blood pressure is lower, at about 10 mm Hg, producing a
net reabsorption pressure of 7 mm Hg in.
a. Net hydrostatic pressure is 10out (BP) + 3out = 13out.
b. Oncotic pressure is the same, at 20in.
c. Net reabsorption pressure is 20in (oncotic) – 13out (hydrostatic) = 7in.
5. The only pressure that changes significantly from the arterial end to the venous end is
the capillary blood pressure, and this is responsible for the shift from filtration to
reabsorption.
6. Capillaries branch along their length, and they have nearly twice the diameter at the
venous end, so more surface area is available for reabsorption.
7. Capillaries reabsorb about 85% of the fluid they filter; the other 15% is absorbed by
the lymphatic system.
8. Chemicals dissolved in water are dragged with it, a process termed solvent drag.
9. Circumstances vary from place to place and from time to time, which can affect
capillary exchange.
a. Kidneys have capillary networks called glomeruli in which there is little or no
reabsorption.
b. Alveolar capillaries of the lungs are almost entirely dedicated to absorption.
c. In a resting tissue, most precapillary sphincters are constricted, and
reabsorption predominates.
d. In metabolically active tissue, capillary flow increases.
e. In active muscles, capillary pressure rises to the point that filtration overrides
reabsorption along the entire length, and fluid accumulates in the muscle.
f. Traumatized tissue releases chemicals such as substance P, bradykinin, and
histamine, which increase permeability and filtration.
H. Edema is the accumulation of excess fluid in a tissue. (p. 774)
1. Edema shows as swelling in the face, fingers, abdomen, or ankles, but also occurs in
internal organs; it has three fundamental causes.
a. Increased capillary filtration. Numerous conditions can increase the rate of
filtration and accumulation of fluid.
i. Kidney failure leads to water retention and hypertension, raising
capillary blood pressure and filtration.
ii. Histamine dilates arterioles and raises capillary pressure and makes
capillary walls more permeable.
Saladin Outline Ch.20
Page 17
iii. Capillaries become more permeable in old age.
iv. Capillary blood pressure also rises in cases of poor venous return,
which depends on muscular activity; edema is a common problem
among people confined to bed or to a wheelchair.
v. Failure of the right ventricle can cause pressure to back up in the
systemic veins, whereas failure of the left ventricle causes pulmonary
edema.
b. Reduced capillary reabsorption.
i. Capillary reabsorption depends on oncotic pressure, which is
proportional to the concentration of blood albumin; a deficiency of
albumin (hyoproteinemia) produces edema by reducing reabsoprtion.
ii. Liver diseases such as cirrhosis tend to lead to hypoproteinemia.
iii. Edema is commonly seen in regions of famine because of dietary
protein deficiency.
iv. Edema also commonly results from severe burns because of the loss
of protein from body surfaces lacking skin.
v. It may also result from kidney diseases that allow protein to escape
in the urine.
c. Obstructed lymphatic drainage.
i. The lymphatic system is a network of one-way vessels that collect
fluid and return it to the bloodstream.
ii. Obstruction of lymphatic vessels or surgical removal of lymph nodes
can interfere with drainage.
2. Edema has multiple pathological effects.
a. As tissues become congested with fluid, oxygen delivery and waste removal
are impaired and the tissues may begin to die.
b. Pulmonary edema present a threat of suffocation.
c. Cerebral edema can produce headaches, nausea, and sometimes delirium,
seizures, and coma.
d. In severe edema, so much fluid may transfer to the tissue spaces that a patient
can enter circulatory shock.
IV. Venous Return and Circulatory Shock (pp. 774–777)
A. Hieronymus Fabricius argued that the valves in veins would allow only one-way flow, and his
student William Harvey demonstrated that this is the case.(pp. 774–775) (Fig. 20.18)
B. The flow of blood back to the heart, called venous return, is achieved by five mechanisms. (pp.
775–776)
Insight 20.3 Air Embolism
Saladin Outline Ch.20
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1. The pressure gradient. Pressure generated by the heart is the most important force
although it is weaker in the veins than in the arteries.
a. Pressure in venules ranges from 12 to 18 mm Hg.
b. Pressure where the venae cavae enter the heart, called central venous
pressure, averages 4.6 mm Hg.
c. Therefore the pressure gradient (ΔP) is about 7 to 13 mm Hg, toward the
heart.
d. When blood volume increases, both the pressure gradient and rate of venous
return increase.
e. Venous return also increases in the event of widespread vasoconstriction.
2. Gravity. When you are sitting or standing, blood from head and neck returns to the
heart by flowing downward.
a. The large veins of the neck are normally collapsed or nearly so in this
position, and their pressure is close to zero.
b. The dural sinuses of the brain have more rigid walls and cannot collapse.
i. Their pressure is as low as –10 mm Hg and there is a risk of air
embolism if they are punctured.
3. The skeletal muscle pump. In the limbs, the veins are surrounded and massaged by the
muscles.
a. Contracting muscles squeeze blood out of the compressed part of a vein, and
the valves ensure that the blood goes in one direction, toward the heart. (Fig.
20.19)
4. The thoracic (respiratory) pump. This mechanisms aids flow of venous blood from the
abdominal to the thoracic cavity.
a. When you inhale, the thoracic cavity expands and its internal pressure drops,
while downward movement of the diaphragm raises pressure in the abdominal
cavity.
b. The inferior vena cava (IVC), passes through both these cavities.
c. If abdominal pressure rises on the IVC while thoracic pressure on it drops,
blood is squeezed upward toward the heart; veins prevent backward flow.
d. Venous pressure in the IVC fluctuates from 2 mm Hg when you inhale to 6
mm Hg when you exhale.
5. Cardiac suction. During ventricular systole, the tendinous cords pull the AV valve
cusps downward, slight expanding the atrial space and creating a slight suction that draws
blood from the venae cavae and pulmonary veins.
C. Level of activity affects venous return. (p. 776)
1. Exercise and other physical activity increases venous return for many reasons.
Saladin Outline Ch.20
Page 19
a. The heart beats faster and harder.
b. Blood vessels of the skeletal muscles, lungs, and coronary circulation dilate.
c. The increase in respiratory rate and depth enhances action of the thoracic
pump.
d. Muscle contractions increase venous return.
e. Increased venous return increases cardiac output.
2. Conversely, when a person is not moving, blood accumulates in the limbs, which is
called venous pooling.
a. Venous pooling is troublesome to those who stand or sit for prolonged
periods.
b. Periodically tensing the calf and other muscles can help keep the skeletal
muscle pump active.
D. Circulatory shock is any state in which cardiac output is insufficient to meet the body’s
metabolic needs; all forms can be divided into two categories. (pp. 776–777)
1. Cardiogenic shock is caused by inadequate pumping by the heart, usually as a result of
myocardial infarction.
2. Low venous return (LVR) shock is caused when too little blood is returning to the
heart; it has three principal forms.
a. Hypovolemic shock is produced by a loss of blood volume as a result of
hemorrhage, trauma, burns, or dehydration.
i. Dehydration is a major cause of death from heat exposure.
ii. As much as 1.5 L of sweat may be excreted in an hour in hot
weather, and transfer of water from the bloodstream may cause blood
volume to drop too low.
b. Obstructed venous return shock occurs when any object, such as a growing
tumor or aneurysm, compresses a vein and impedes its flow.
c. Venous pooling (vascular) shock occurs when the body has normal blood
volume, but too much accumulates in the limbs.
i. Neurogenic shock is a form of venous pooling shock that results from
sudden loss of vasomotor tone, causing vasodilation.
ii. Neurogenic shock can result from something as severe as brainstem
trauma or as slight as an emotional shock.
d. Elements of both venous pooling and hypovolemic shock are present in cases
such as septic shock and anaphylactic shock, which involve both vasodilation
and a loss of fluid through abnormally permeable capillaries.
3. The body’s homeostatic mechanisms attempt to respond to circulatory shock.
a. In compensated shock, these mechanisms bring about spontaneous recovery.
Saladin Outline Ch.20
Page 20
i. Baroreflexes detect lower BP and bring about production of
angiotensin II.
ii. If a person faints and falls to a horizontal position, gravity restores
blood flow to the brain; feet may be elevated to achieve quicker
recovery.
b. Decompensated shock ensues if these mechanisms are inadequate, and several
life-threatening positive feedback loops occur.
i. Poor cardiac output results in myocardial ischemia and infarction,
further weakening the heart.
ii. Slowed circulation can lead to disseminated intravascular
coagulation (DIC), and venous return grows even worse.
iii. Ischemia and acidosis of the brainstem depress the vasomotor and
cardiac centers, causing further drop in BP and cardiac output.
iv. About half of those who go into decompensated shock die from it.
V. Special Circulatory Routes (pp. 777–778)
A. In the brain, total blood flow fluctuates less than that of any other organ (about 700 mL/min at
rest). (p. 777)
1. Even a few seconds of oxygen deprivation to the brain can cause loss of consciousness,
and 4 or 5 minutes of anoxia may cause irreversible damage.
2. Although perfusion is fairly stable, blood flow can be shifted from one part of the brain
to another as different areas engage in motor, sensory, or cognitive functions.
3. The brain regulates its own blood flow in response to changes in BP and chemistry.
a. Cerebral arteries dilate when systemic BP drops, and they constrict when it
rises.
b. Blood flow thus remains quite stable even when MAP fluctuates from 60 to
140 mm Hg.
i. MAP below 60 mm Hg produces syncope.
ii. MAP above 160 mm Hg causes cerebral edema.
c. The main chemical stimulus is pH; poor perfusion allows CO2 to accumulate,
and the pH lowers.
i. Lower pH in the tissue fluid triggers local vasodilation.
ii. Extreme hypercapnia, however, depresses neural activity.
iii. The opposite condition, hypocapnia, raises the pH and stimulates
vasoconstriction, reducing perfusion and allowing the CO2 level to rise.
iv. Hyperventilation induces hypocapnia, which leads to cerebral
vasoconstriction, ischemia, dizziness, and sometimes syncope.
Saladin Outline Ch.20
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4. Transient ischemic attacks (TIAs) are brief episodes of cerebral ischemia characterized
by temporary dizziness, loss of vision or other senses, weakness, paralysis, headache, or
aphasia.
a. TIAs may result from spasms of diseased cerebral arteries.
b. A TIA may last from just a moment to a few hours and is often an early
warning of impending stroke.
c. People with TIAs should receive prompt medical attention to identify the
cause and to provide treatment to prevent a stroke.
5. A stroke, or cerebrovascular accident (CVA), is the sudden death (infarction) of brain
tissue caused by ischemia.
a. Cerebral ischemia can be produced by atherosclerosis, thrombosis, or a
ruptured aneurysm.
b. The effects of a CVA range from unnoticeable to fatal, depending on the
extent of tissue damage.
c. Blindness, paralysis, loss of sensation, and loss of speech are common.
d. Recovery depends on the ability of neighboring neurons to take over lost
functions and the extend of collateral circulation to regions surrounding the
cerebral infarction.
B. Skeletal muscles receive a highly variable blood flow depending on their state of exertion. (pp.
777–778)
1. At rest, arterioles are constricted, most capillary beds are shut down, and total flow
through the muscular system is about 1 L/min.
2. During exercise, the arterioles dilate in response to epinephrine and norepinephrine
from the adrenal medulla and sympathetic nerves; precapillary sphincters dilate in
response to muscle metabolites; and blood flow can increase more than 30-fold.
a. Blood must be diverted from other organs to meet the needs of the working
muscle.
3. Muscular contraction compresses blood vessels and impedes flow, so that isometric
contraction causes fatigue more quickly than intermittent isotonic contraction.
C. After birth, the pulmonary circuit (lungs) is the only route in which arterial blood contains less
oxygen than venous blood. (pp. 742–743)
1. The pulmonary arteries have thin distensible walls with less elastic tissue, and they
have a BP of only 25/10.
2. Capillary hydrostatic pressure is about 10 mm Hg in the pulmonary circuit, as
compared to an average of 17 mm Hg in the systemic capillaries.
a. Blood flows more slowly through pulmonary capillaries and has more time
for gas exchange.
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b. Oncotic pressure overrides hydrostatic pressure, so these capillaries are
engaged almost entirely in absorption, preventing fluid accumulation.
i. Mitral valve stenosis may cause backup of blood into the pulmonary
circuit, causing pulmonary edema, congestion, and hypoxemia.
3. Pulmonary arteries constrict in response to hypoxia, whereas systemic arteries dilate.
a. Pulmonary hypoxia indicates that part of the lung is not being ventilated well.
b. Vasoconstriction redirects blood flow to better ventilated regions.
VI. Anatomy of the Pulmonary Circuit (p. 778) (Fig. 20.20)
A. The pulmonary circuit begins with the pulmonary trunk, which ascends diagonally from the
right ventricle and branches into the right and left pulmonary arteries. (p. 778)
1. The right pulmonary artery branches in two, and both branches enter the lung at a
medial indentation called the hilum. (Fig. 22.9)
a. The upper branch is the superior lobar artery.
b. The lower branch divides again within the lung to form the middle lobar and
inferior lobar arteries.
2. The left pulmonary artery is more variable, giving off several superior lobar arteries
before entering the hilum, and a variable number of inferior lobar arteries once it enters
the lung.
B. These arteries lead to small, basketlike capillary beds that surround the pulmonary alveoli. (p.
778)
1. Here the blood unloads CO2 and picks up O2.
2. After leaving the alveolar capillaries, the blood flows into venules and veins.
C. Ultimately the smaller veins lead to the main pulmonary veins that exit the lung at the hilum.
(p. 778)
1. The left atrium of the heart receives two pulmonary veins on each side. (Fig. 19.5b)
D. The purpose of the pulmonary circuit is gas exchange; the lungs also receive a separate
systemic blood supply by way of the bronchial arteries. (p. 778)
VII. Systemic Vessels of the Axial Region (pp. 779–798)
A. The systemic circuit supplies oxygen and nutrients to all organs and removes their metabolic
wastes; this section surveys arteries and veins of the axial region—head, neck, and trunk. (p. 779)
(Figs. 20.21, 20.22)
B. Tables 20.3 through 20.9 trace arterial outflow and venous return, region by region, but a great
deal of anatomical variation exists. (p. 779)
C. Names of blood vessels often describe their location by indicating the body region traversed,
and adjacent bone, or the organ supplied or drained; in many cases an artery and adjacent vein
have similar names. (p. 779)
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D. Throughout the tables and illustrations, a. and aa. mean artery and arteries; v. and vv. mean
vein and veins. (p. 779)
E. The aorta and its major branches are the first arteries to lead away from the heart. (p. 782) (Fig.
20.23) (Table 20.3)
1. The ascending aorta rises for about 5 cm above the left ventricle.
a. Its only branches are the coronary arteries, which arise behind two cusps of
the aortic valve.
b. They are the origins of the coronary circulation.
2. The aortic arch curves to the left like an inverted U superior to the heart.
a. It gives off three major arteries: the brachiocephalic trunk, the left common
carotid artery, and the left subclavian artery.
3. The descending aorta passes downward posterior to the heart through the thoracic and
abdominal cavities.
a. At first it is to the left of the vertebral column and then anterior to it.
b. It is called the thoracic aorta above the diaphragm and the abdominal aorta
below it.
c. It ends in the lower abdominal cavity by forking into the right and left
common iliac arteries.
F. Blood supply to the brain is a critical function of the arteries in the head and neck region. (pp.
782–784) (Figs. 20.24, 20.25) (Table 20.4)
1. The head and neck receive blood from four pairs of arteries. (Fig. 20.4)
a. The common carotid arteries pass up the anterolateral region of the neck,
alongside the trachea.
i. Shortly after leaving the aortic arch, the brachiocephalic trunk divides
into the right subclavian artery and right common carotid artery.
ii. A little farther along the arch, the left common carotid arises
independently.
b. The vertebral arteries arise from the right and left subclavian arteries and
travel upward through the transverse foramina of vertebrae C1–C6; they enter
the cranial cavity through the foramen magnum.
c. The tiny thyrocervical trunks arise from the subclavian arteries lateral to the
vertebral arteries and supply the thyroid gland and some scapular muscles.
d. The costocervical trunks arise from the subclavian arteries a little farther
laterally; they supply the deep neck muscles and some of the intercostal
muscles.
Saladin Outline Ch.20
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2. The common carotid arteries have the most extensive distribution of all the head and
neck arteries; near the laryngeal prominence each branches into an external and internal
carotid artery.
a. The external carotid ascends the side of the head external to the cranium and
supplies most external head structures except the orbits; it gives rise to the
following arteries:
i. The superior thyroid artery to the thyroid gland and larynx.
ii. The lingual artery to the tongue.
iii. The facial artery to the skin and muscles of the face.
iv. The occipital artery to the posterior scalp.
v. The maxillary artery to the teeth, maxilla, oral cavity, and external
ear.
vi. The superficial temporal artery to the chewing muscles, nasal cavity
lateral aspect of the face, most of the scalp, and the dura mater.
b. The internal carotid artery supplies the orbits and about 80% of the cerebrum;
compressing the internal carotids near the mandible can cause loss of
consciousness.
i. Each artery passes medial to the angle of the mandible and enters the
cranial cavity through the carotid canal of the temporal bone, where it
gives rise to three branches.
ii. The ophthalmic artery to the orbit, nose, and forehead.
iii. The anterior cerebral artery to the medial aspect of the cerebral
hemisphere.
iv. The middle cerebral artery, which travels in the lateral sulcus of the
cerebrum, supplies the insula, and then issues branches to the lateral
region of the frontal, temporal and parietal lobes.
3. The vertebral arteries give rise to small branches that supply the spinal cord and its
meninges, the cervical vertebrae, and deep muscles of the neck.
a. They then enter the foramen magnum, supply the cranial bones and meninges,
and converge to form the basilar artery along the anterior aspect of the
brainstem.
b. Branches of the basilar artery supply the cerebellum, pons, and inner ear.
c. At the pons-midbrain junction, the basilar artery divides and flows into the
cerebral arterial circle.
4. Blood supply to the brain is so critical that it is furnished by several arterial
anastomoses, particularly an array of arteries called the cerebral arterial circle (circle of
Willis), which surrounds the pituitary gland and optic chiasm. (Fig. 20.25)
Saladin Outline Ch.20
Page 25
a. The circle receives blood from the internal carotid and basilar arteries.
b. Most people lack one or more of its components; only 20% have a complete
arterial circle.
c. Knowledge of the distribution of arteries arising for the circle is crucial for
understanding the effects of blood clots, aneurysms, and strokes.
i. Two posterior cerebral arteries arise from the basilar artery and
sweep posteriorly to the rear of the brain to supply the inferior and
medial regions of temporal and occipital lobes as well as midbrain and
thalamus.
ii. Two anterior cerebral arteries arise from the internal carotids, travel
anteriorly, and then arch posteriorly over the corpus callosum as far as
the posterior of the parietal lobe, giving off extensive branches to the
frontal and parietal lobes.
iii. The single anterior communicating artery is a short anastomosis
between the right and left anterior cerebral arteries.
iv. The two posterior communicating arteries are small anastomoses
between the posterior cerebral and internal carotid arteries.
G. The head and neck are drained mainly by three pairs of veins—the internal jugular, external
jugular, and vertebral veins, which empty into the subclavian veins. (pp. 785–786) (Table 20.5)
1. After blood circulates through the brain, it collects in the dural venous sinuses—blood
filled spaces between the layers of the dura mater. (Fig. 20.26a, b)
a. The dura mater has a periosteal layer against the bone and a meningeal layer
against the brain, with a space between these layers in a few places to
accommodate a blood-collecting sinus.
b. Between the two hemispheres is a vertical, sickle-shaped wall of dura called
the falx cerebri, which contains two of the sinuses; there are about 13 sinuses in
all.
c. The superior sagittal sinus is contained in the superior margin of the falx
cerebri and overlies the longitudinal fissure. (Fig. 20.26a)
i. It begins anteriorly near the crista galli and extends to the rear of the
head, ending at the level of the posterior occipital protuberance.
ii. It bends at this point usually to the right and drains into a transverse
sinus.
d. The inferior sagittal sinus is contained in the inferior margin of the falx
cerebri and arches over the corpus callosum.
i. Posteriorly it joins the great cerebral vein, and their union form the
straight sinus, which continues to the rear of the head.
Saladin Outline Ch.20
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ii. The straight sinus meets the superior sagittal sinus in a space called
the confluence of the sinuses.
e. Right and left transverse sinuses lead away from the confluence and encircle
the inside of the occipital bone, leading toward the ears. (Fig. 20.26b)
i. Their path is marked by grooves in the inner surface of the occipital
bone.
ii. The right transverse sinus receives blood mainly from the superior
sagittal sinus, and the left one drains mainly the straight sinus.
iii. Laterally, each transverse sinus makes an S-shaped bend, the
sigmoid sinus, then exits through the jugular foramen.
iv. From here the blood flows down the internal jugular vein.
f. The cavernous sinuses are honeycombs of blood-filled spaces on each side of
the body of the sphenoid bone. (Fig. 20.26b)
i. They receive blood from the superior ophthalmic vein of the orbit and
the superficial middle cerebral vein of the brain, among other sources.
ii. They drain through several outlets including the transverse sinus,
internal jugular vein, and facial vein.
iii. They are clinically important because infections can pass from the
face and other superficial sites into the cranial cavity via this route.
iv. Inflammation of a cavernous sinus can injure important structures
that pass through it, including the internal carotid artery and cranial
nerves CNIII to CNVI.
2. Blood flows down the neck mainly through three veins on each side, all of which
empty into the subclavian vein. (Fig. 20.26c)
a. The internal jugular vein courses down the neck deep to the
sternocleidomastoid muscle.
i. It receives most of the blood from the brain and picks up blood from
the facial vein, superficial temporal vein, and superior thyroid vein.
ii. It passes behind the clavicle and joins the subclavian vein.
b. The external jugular vein courses down the side of the neck superficial to the
sternocleidomastoid muscle and empties into the subclavian vein.
i. It drains tributaries from the parotid gland, facial muscle, scalp, and
other superficial structures.
ii. Some of this blood also follows venous anastomoses to the internal
jugular vein.
c. The vertebral vein travels with the vertebral artery in the transverse foramina
of the cervical vertebrae.
Saladin Outline Ch.20
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i. The vertebral vein does not drain the brain, but rather the cervical
vertebrae, spinal cord, and some small, deep muscles of the neck.
ii. It empties into the subclavian vein.
H. The thorax is by several arteries arising directly from the aorta and from the subclavian and
axillary arteries. (pp. 787–790) (Figure 20.27) (Table 20.6)
1. The thoracic aorta begins distal to the aortic arch and ends at the aortic hiatus, a
passage through the diaphragm, sending off numerous branches to the thoracic viscera
and body wall.
2. The visceral branches of the thoracic aorta are as follows.
a. The bronchial arteries are variable in number and arrangement, but usually
there are two on the left and one on the right.
i. The right bronchial artery usually arises from one of the left bronchial
arteries or from a posterior intercostal artery.
ii. The bronchial arteries supply the bronchi, bronchioles, and larger
blood vessels of the lungs, visceral pleura, pericardium and esophagus.
b. Four or five unpaired esophageal arteries arise from the anterior surface of the
aorta and supply the esophagus.
c. Many small mediastinal arteries supply structures of the posterior
mediastinum.
3. The parietal branches of the thoracic aorta supply chiefly the muscles, bones, and skin
of the chest wall.
a. Nine pairs of posterior intercostal arteries arise from the posterior surface of
the aorta and course around the posterior side of the rib cage between ribs 3
through 12.
i. They then anastomose with the anterior intercostal arteries.
ii. They supply the intercostal, pectoralis, serratus anterior, and some
abdominal muscles, as well as vertebrae, spinal cord, meninges,
breasts, skin and subcutaneous tissue.
iii. They are enlarged in lactating women.
b. A pair of subcostal arteries arise from the aorta inferior to the twelfth rib.
i. They supply the posterior intercostal tissues, vertebrae, spinal cord,
and deep muscles of the back.
c. The superior phrenic arteries, variable in number, arise at the aortic hiatus and
supply the superior and posterior regions of the diaphragm.
4. Branches of the subclavian and axillary arteries arise in the shoulder region, the first
from the subclavian artery and the other three from its continuation, the axillary artery.
Saladin Outline Ch.20
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a. The internal thoracic (mammary) artery supplies the breast and anterior
thoracic wall and has the following branches.
i. The pericardiophrenic artery supplies the pericardium and diaphragm.
ii. The anterior intercostal arteries arise from the thoracic artery as it
descends alongside the sternum; they supply ribs and intercostal
muscles, anastomose with posterior intercostal arteries, and each sends
one branch along the lower margin of the superior rib and another
along the upper margin of the inferior rib.
b. The thoracoacromial trunk provides branches to the superior shoulder and
pectoral regions.
c. The lateral thoracic artery supplies the pectoral, serratus anterior, and
subscapularis muscles as well as branches to the breast; it is larger in females.
d. The subscapular artery is the largest branch of the axillary artery.
i. It supplies the scapula and latissimus dorsi, serratus anterior, teres
major, deltoid, triceps brachii, and intercostal muscles.
I. Veins that drain the thorax ultimately empty into the superior vena cava. (pp. 789–790) (Fig.
20.28) (Table 20.7)
1. The most prominent veins of the upper thorax carry blood from the shoulder region to
the heart and are as follows. (Fig. 20.28)
a. The subclavian vein drains the upper limb.
i. It begins at the lateral margin of the first rib and travels posterior to
the clavicle.
ii. It receives the external jugular and vertebral veins, then changes
name where it receives the internal jugular vein.
b. The brachiocephalic vein is formed by union of the subclavian and internal
jugular veins.
i. The right brachiocephalic is very short, about 2.5 cm, and the left is
about 6 cm long.
ii. They receive tributaries from the vertebrae, thyroid gland, and upper
thoracic wall and breast, then converge to form the superior vena cava.
c. The superior vena cava is formed by the union of the right and left
brachiocephalic veins.
i. It travels inferiorly for about 7 cm and empties into the right atrium
of the heart.
ii. Its main tributary is the azygos system.
2. The azygos system is the principal venous drainage of the thoracic organs. (Fig. 20.28)
Saladin Outline Ch.20
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a. The most prominent vein is the azygos vein, which ascends the right side of
the posterior thoracic wall and is named for the lack of a mate on the left; it
receives the following tributaries, then empties into the superior vena cava at the
level of T4.
b. The right ascending lumbar vein drains the right abdominal wall, then
penetrates the diaphragm and enters the thoracic cavity.
i. The azygos vein begins where the ascending lumbar vein meets the
right subcostal vein beneath rib 12.
c. The right posterior intercostal veins drain the intercostal spaces.
i. The first (superior) one empties into the right brachiocephalic vein.
ii. Intercostals 2 and 3 join to form a right superior intercostal vein
before emptying into the azygos.
iii. Intercostals 4 through 11 each enter the azygos vein separately.
d. The right esophageal, mediastinal, pericardial, and bronchial veins drain their
respective organs into the azygos.
e. The hemiazygos vein ascends the posterior thoracic wall on the left.
i. It begins where the left ascending lumbar vein, having just penetrated
the diaphragm, joins the subcostal vein.
ii. The hemiazygos then receives the lower three intercostal veins,
esophageal veins, and mediastinal veins.
iii. At the level of T9 it crosses to the right and empties into the azygos.
f. The accessory hemiazygos vein descends the posterior thoracic wall on the
left.
i. it receives drainage from posterior intercostals veins 4 through 8 and
sometimes the left bronchial veins.
ii. It crosses to the right at the level of T8 and empties into the azygos
vein.
g. The left posterior intercostals veins 1 to 3 are the only ones on this side that
do not drain into the azygos vein; the unite to form the left superior intercostal
vein, which empties into the left brachiocephalic.
J. The abdomen and pelvic region is supplied by the abdominal aorta and the right and left
common iliac arteries, which are the two forks arising from the aorta at L4; the aorta in the
abdomen is retroperitoneal. (pp. 791–794) (Table 20.8)
1. The abdominal aorta gives off arteries as follows. (Fig. 20.29)
a. The inferior phrenic arteries supply the inferior surface of the diaphragm.
i. They may arise from the aorta, celiac trunk, or renal artery.
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ii. Each issues two or three small superior suprarenal arteries to the
ipsilateral adrenal (suprarenal) gland.
b. The celiac trunk supplies the upper abdominal viscera.
c. The superior mesenteric artery supplies the intestines.
d. The middle suprarenal arteries arise laterally from the aorta, usually at the
same level as the superior mesenteric artery; they supply the adrenal glands.
e. The renal arteries supply the kidneys and issue a small inferior suprarenal
artery to each adrenal gland.
f. The gonadal arteries are long, slender arteries that arise from the midabdominal aorta.
i. They descend along the posterior wall to the female pelvic cavity or
male scrotum.
ii. The gonads begin embryonic development near the kidneys; as they
descend, the arteries grow and acquire their peculiar length and course.
g. The inferior mesenteric artery supplies the distal end of the large intestine.
h. The lumbar arteries arise from the lower aorta in four pairs; they supply the
posterior abdominal wall and the spinal cord and other tissues in the vertebral
canal.
i. The median sacral artery supplies the sacrum and coccyx.
j. The common iliac arteries arise as the aorta forks at its inferior end.
2. The celiac circulation to the upper abdominal viscera is a complex route with
numerous anastomoses; its three main branches are the common hepatic, left gastric, and
splenic arteries. (Fig. 20.30)
a. The common hepatic artery passes to the right and issues two main
branches—the gastroduodenal artery and the hepatic artery proper.
i. The gastroduodenal artery gives of the right gastro-omental
(gastroepiploic) artery to the stomach and continues as the superior
pancreaticoduodenal artery, which splits into two branches that pass
around the anterior and posterior sides of the head of the pancreas;
these then anastomose with two branches of the inferior
pancreaticoduodenal artery.
ii. The hepatic artery proper ascends toward the liver, giving off the
right gastric artery and then branching into right and left hepatic
arteries; the right issues a cystic artery to the gallbladder, and then the
two hepatic arteries enter the liver.
b. The left gastric artery supplies the stomach and lower esophagus.
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i. It then arcs around the lesser curvature (superomedial margin) of the
stomach and anastomoses with the right gastric artery. (Fig. 20.30b)
ii. The right and left gastric arteries approach from opposite directions.
iii. The left gastric also has branches to the lower esophagus, and the
right gastric also supplies the duodenum.
c. The splenic artery supplies blood to the spleen, but gives off the following
branches.
i. Several small pancreatic arteries supply the pancreas.
ii. the left gastro-omental (gastroepiploic) artery arcs around the great
curvature of the stomach and anastomoses with the right gastro-omental
artery; these travel through the superior margin of the greater omentum
and furnish blood to both this and the stomach.
iii. The short gastric arteries supply the upper portion (fundus) of the
stomach.
3. The mesentery is a translucent sheet that suspends the intestines and other abdominal
viscera and contains numerous arteries, veins, and lymphatic vessels that drain the
intestines; the arterial supply arises from the superior and inferior mesenteric arteries that
have numerous anastomoses.
a. The superior mesenteric artery is the most significant intestinal blood supply;
it arises medially from the upper abdominal aorta and gives off the following
branches. (Fig. 20.31a)
i. The inferior pancreaticoduodenal artery, mentioned above, branches
and passes around the pancreas and anastomoses with the two branches
of the superior pancreaticoduodenal artery.
ii. Twelve to 15 jejunal and ileal arteries from a fanlike array that
supplies the jejunum and ileum.
iii. The ileocolic artery supplies the ileum, appendix, and the cecum
and ascending colon.
iv. The right colic artery also supplies the ascending colon.
v. The middle colic artery supplies most of the transverse colon.
b. The inferior mesenteric artery arises from the lower abdominal aorta and
serves the distal part of the large intestine. (Fig. 20.31b)
i. The left colic artery supplies the transverse and descending colon.
ii. The sigmoid arteries supply the descending and sigmoid colon.
iii. The superior rectal artery supplies the rectum.
4. In the pelvic region, the two common iliac arteries descend for another 5 cm, and then
each divides into and external and internal iliac artery; the external one supplies mainly
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the lower limb, while the internal one supplies mainly the pelvic wall and viscera. (Fig.
20.37)
a. Shortly after its origin, the internal iliac divides into two trunks; the anterior
trunk produces the following branches.
i. The superior vesical artery supplies the urinary bladder and distal end
of the ureter, arising indirectly from the anterior trunk via a short
umbilical artery, a remnant from fetal development.
ii. In men, the inferior vesicle artery supplies the bladder, ureter,
prostate gland, and seminal vesicle; in women, the corresponding
vessel is the vaginal artery, which supplies the vagina and part of the
bladder and rectum.
iii. The middle rectal artery supplies the rectum.
iv. The obturator artery exits the pelvic cavity through the obturator
foramen and supplies adductor muscles of the medial thigh.
v. The internal pudendal artery serves the perineum and erectile tissues
of the penis and clitoris.
vi. In women, the uterine artery is the main blood supply to the uterus
and also serves the vagina; it passes up the uterine margin, turns
laterally, and anastomoses with the ovarian artery.
vii. The inferior gluteal artery supplies the gluteal muscles and hip
joint.
b. The posterior trunk of the internal iliac produces the following branches.
i. The iliolumbar artery supplies the lumbar body wall and pelvic
bones.
ii. The lateral sacral arteries (superior and inferior) supply the sacral
canal, skin, and muscles posterior to the sacrum
iii. The superior gluteal artery supplies the skin and muscles of the
gluteal region and the muscle and bone of the pelvic wall.
K. Veins of the abdomen and pelvic region include the inferior vena cava (IVC) and its tributaries,
veins of the abdominal wall, and the hepatic portal system. (pp. 795–797) (Table 20.9)
1. The inferior vena cava is the largest blood vessel with a diameter of about 3.5 cm; it
forms from the right and left common iliac veins at L5, then ascends the posterior body
wall retroperitoneally, to the right of the aorta. It has a number of tributaries. (Fig. 20.32)
a. The internal iliac veins drain the gluteus; the medial thigh, urinary bladder,
rectum, prostate and ductus deferens of the male; and the uterus and vagina of
the female.
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i. They unite with the external iliac veins to form the common iliac
veins, which then converge to form the IVC.
b. Four pairs of lumbar veins empty into the IVC as well as into the ascending
lumbar veins described earlier.
c. The gonadal veins drain the gonads.
i. Like the gonadal arteries, and for the same reason, these are long
slender vessels that end far from their origins.
ii. the left gonadal vein empties into the left renal vein, while the right
gonadal vein empties directly into the IVC.
d. The renal veins drain the kidneys into the IVC.
i. The left renal vein also receives blood from the left gonadal and left
suprarenal veins.
ii. The left vein is also three times as long as the right renal vein, since
the IVC lies to the right of the median plane.
e. The suprarenal veins drain the adrenal (suprarenal) glands.
i. The right suprarenal empties directly into the IVC and the left into
the left renal vein.
f. The hepatic veins drain the liver, extending a short distance from its superior
surface to the IVC.
g. After these inputs, the IVC penetrates the diaphragm and enters the right
atrium of the heart from below; it does not receive any thoracic drainage.
2. The veins of the abdominal wall are drained by a pair of ascending lumbar veins.
a. These veins receive blood from the common iliac veins below and from the
lumbar veins of the posterior body wall. (Fig. 20.28b)
b. The ascending lumbar veins give off anastomoses with the inferior vena cava
as they ascend to the diaphragm.
c. The left ascending lumbar vein passes through the diaphragm via the aortic
hiatus and continues as the hemiazygos vein.
d. The right ascending lumbar vein passes through the diaphragm to the right of
the vertebral column and continues as the azygos vein.
3. The hepatic portal system receives all the blood draining from the abdominal digestive
tracts as well as the pancreas, gallbladder, and spleen. (Fig. 20.33)
a. It is called a portal system because it connects capillaries of the intestines and
other digestive organs to modified capillaries (hepatic sinusoids) of the liver.
b. The liver has first claim to the nutrients in blood from the intestines, and it
also cleanses this blood of bacteria and toxins.
c. The principle veins are as follows.
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i. The inferior mesenteric vein receives blood from the rectum and
distal part of the colon, converging in a fanlike array in the mesentery
and emptying into the splenic vein.
ii. The superior mesenteric vein receives blood from the small intestine,
ascending colon, transverse colon, and stomach; it has a fanlike
arrangement in the mesentery and joins the splenic vein to form the
hepatic portal vein.
iii. The splenic vein drains the spleen and travels across the abdominal
cavity toward the liver, picking up pancreatic veins and the inferior
mesenteric vein, and ending (changing name) where it meets the
superior mesenteric vein.
iv. The hepatic portal vein is the continuation beyond the splenic and
superior mesenteric veins.
α. It travels upward and to the right, receives the cystic vein
from the gallbladder, then enters the inferior surface of the
liver.
β. In the liver, it ultimately leads to the innumerable,
microscopic hepatic sinusoids; blood from these empties into
the hepatic veins and then into the IVC.
v. The left and right gastric veins form an arch along the lesser
curvature of the stomach and empty into the hepatic portal vein.
VIII. Systemic Vessels of the Appendicular Region (pp. 798–806)
A. This section surveys arteries and veins of the appendicular regions—those of the limbs. (p.
798)
B. Tables 20.10 through 20.13 trace arterial outflow and venous return. (p. 798)
C. Although appendicular arteries are usually deep and well protected, veins occur in both deep
and superficial groups. (p. 798)
1. Deep veins run parallel to the arteries and often have similar names.
2. In several cases, the deep veins occur in pairs flanking the corresponding artery.
D. Venous pathways have more anastamoses than arterial pathways, and the route of flow is often
not as clear. (p. 798)
E. The upper limb is supplied by a prominent artery that changes name along its course from
subclavian to axillary to brachial, then issues branches to the arm, forearm, and hand. (pp. 798–
800) (Fig. 20.24) (Table 20.10)
1. The shoulder and arm (brachium) are supplied by this prominent artery, which has
several branches.
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a. The brachiocephalic trunk arises from the aortic arch and branches into the
common carotid artery and left subclavian artery; the right subclavian artery
arises directly from the aortic arch.
i. Each subclavian arches over the respective lung, rising as high as the
base of the neck slightly superior to the clavicle.
ii. It then passes posterior to the clavicle, downward over the first rib,
and changes name at the rib’s lateral margin.
iii. In the shoulder it gives off several small branches to the thoracic
wall and viscera. (Table 20.6)
b. The axillary artery continues past the first rib through the axillary region.
i. It gives off more small thoracic branches and changes name again at
the neck of the humerus.
ii. Here it gives off a pair of circumflex humeral arteries that encircle
the humerus, anastomose laterally with each other, and supply blood to
the shoulder joint and deltoid muscle.
c. The continuation, now called the brachial artery, passes down the medial and
anterior sides of the humerus and ends just distal to the elbow.
i. It supplies the anterior flexor muscles of the brachium.
ii. It is the most common site of blood pressure measurement with the
sphygmomanometer.
d. The deep brachial artery arises from the proximal end of the brachial artery
and supplies the humerus and triceps brachii muscle; midway down the arm, it
continues as the radial collateral artery.
e. The radial collateral artery descends in the lateral side of the arm and empties
into the radial artery slightly distal to the elbow.
f. The superior ulnar collateral artery arises about midway along the brachial
artery and descends in the medial side of the arm; it empties into the ulnar artery
slightly distal to the elbow.
2. Arterial flow continues into the forearm, wrist, and hand as the brachial artery forks
into the radial and ulnar arteries.
a. The radial artery descends the forearm laterally, alongside the radius,
supplying the lateral forearm muscles; the pulse is often taken at the radial artery
just proximal to the thumb.
b. The ulnar artery descends medially through the forearm, alongside the ulna,
nourishing the medial forearm muscles.
c. The interosseous arteries of the forearm lie between the radius and ulna.
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i. They begin with a short common interosseous artery branching from
the upper ulnar artery.
ii. The common interosseous quickly divides into anterior and posterior
branches.
iii. The anterior interosseous travels down the anterior side of the
interosseous membrane, supplying the radius, ulna, and deep flexor
muscles; it ends distally by joining the posterior interosseous artery.
iv. The posterior interosseous artery descends along the posterior side
of the interosseous membrane and nourishes mainly the superficial
extensors.
d. Two U-shaped palmar arches arise by anastomosis of the radial and ulnar
arteries at the wrist.
i. The deep palmar arch is fed mainly by the radial artery; while the
superficial palmar arch is supplied mainly by the ulnar artery.
ii. The arches issue arteries to the palmer region and fingers.
F. Both superficial and deep veins drain the upper limb, ultimately leading to axillary and
subclavian veins. (pp. 800–801) (Fig. 20.35) (Table 20.11)
1. The superficial veins are often externally visible and are larger in diameter than deep
veins.
a. The dorsal venous network is a plexus of veins often visible through the skin
on the back of the hand; it empties into the major superficial veins of the
forearm, the cephalic and basilic.
b. The cephalic vein arises from the lateral side of the network.
i. It travels up the lateral side of the forearm and arm to the shoulder.
ii. It joins the axillary vein at the shoulder; intravenous fluids are often
administered through the distal end of this vein.
c. The basilic vein arises from the medial side of the network.
i. It travels up the posterior side of the forearm and continues into the
arm.
ii. It turns deeper about midway up the arm and joins the brachial vein
at the axilla.
iii. As an aid to remembering which is cephalic and which is basilic,
visualize your arm abducted with the thumb up. The cephalic runs
along the upper side of the arm, closer to the head, and the basilic runs
along the lower (basal) side of the arm although not named for that
reason.
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d. The median cubital vein is a short anastomosis between the cephalic and
basilic veins that obliquely crosses the cubital fossa (anterior bend of the elbow);
it is the most common site for drawing blood.
e. The median antebrachial vein drains a network of blood vessels in the hand
called the superficial palmar venous network.
i. It travels up the medial forearm and terminates at the elbow,
emptying variously into the basilic, median cubital, or cephalic vein.
2. The deep veins are not generally visible and carry less blood than the superficial veins.
a. The deep and superficial venous palmar arches receive blood from the fingers
and palmar region; they are anastomoses that join the radial and ulnar veins.
b. Two radial veins arise from the lateral side of the palmar arches.
i. They course up the forearm alongside the radius.
ii. They converge slightly distal to the elbow and give rise to one of the
brachial veins.
c. Two ulnar veins arise from the medial side of the palmar arches.
i. They course up the forearm alongside the ulna.
ii. They unit near the elbow to form the other brachial vein.
d. The two brachial veins continue up the brachium, flanking the brachial artery,
and converge into a single vein just before the axillary region.
e. The axillary vein forms by union of the brachial and basilic veins.
i. It begins at the lower margin of the teres major muscle and passes
through the axillary region, picking up the cephalic vein along the way.
ii. At the lateral margin of the first rib, its name changes.
f. The subclavian vein, as it is now called, then continues into the shoulder
posterior to the clavicle.
i. It ends where it meets the internal jugular vein of the neck.
ii. There it becomes the brachiocephalic vein.
iii. The right and left brachiocephalics converge and form the superior
vena cava.
G. The right and left common iliac arteries each divide into the internal and external iliac arteries;
the external iliac artery supplies the lower limb. (pp. 802–804) (Figs. 20.36, 20.37) (Table 20.12)
1. The arteries from the pelvic region to the knee include the external iliac artery, the
femoral artery and its branches, and the popliteal artery.
a. The external iliac artery sends small branches to the skin and muscles of the
abdominal wall and pelvis, then passes behind the inguinal ligament and
becomes the femoral artery.
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b. The femoral artery then passes through the femoral triangle of the upper
medial thigh, where its pulse can be palpated.
i. In the triangle, it gives off several small arteries to the skin and then
gives off branches before descending to the knee.
ii. The deep femoral artery arises from the lateral side; it is the largest
branch and the major arterial supply to the thigh muscles.
iii. Two circumflex femoral arteries arise from the deep femoral,
encircle the head of the femur, and anastomose laterally; they supply
the femur, hip joint, and hamstring muscles.
c. The popliteal artery is a continuation of the femoral artery in the popliteal
fossa at the rear of the knee.
i. It begins where the femoral artery merges from an opening (adductor
hiatus) in the tendon of the adductor magnus.
ii. It ends where it splits into the anterior and posterior tibial arteries.
iii. As it passes through the popliteal fossa, it gives off anastomoses
called genicular arteries that supply the knee joint.
2. The three most significant arteries of the leg and foot include the anterior tibial,
posterior tibial, and fibular arteries.
a. The anterior tibial artery arises from the popliteal artery.
i. It immediately penetrates through the interosseous membrane of the
leg to the anterior compartment.
ii. There, it travels lateral to the tibia and supplies the extensor muscles.
iii. Upon reaching the ankle, it give rise to the following dorsal arteries
of the foot:
iv. The dorsal pedal artery, which traverses the ankle and upper medial
surface of the foot and gives rise to the arcuate artery.
v. The arcuate artery, which sweeps across the foot from medial to
lateral and gives rise to vessels that supply the toes.
b. The posterior tibial artery is a continuation of the popliteal artery.
i. It passes down the leg deep in the posterior compartment, supplying
flexor muscles.
ii. Inferiorly, it passes behind the medial malleolus of the ankle and into
the plantar region of the foot; it give rise to additional arteries:
iii. The medial and lateral plantar arteries, which branch at the ankle;
the medial plantar artery supplies mainly the great toe, while the lateral
plantar artery sweeps across the sole of the foot and becomes the deep
plantar arch.
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iv. The deep plantar arch gives off another set of arteries to the toes.
c. The fibular (peroneal) artery arises from the proximal end of the posterior
tibial artery near the knee.
i. It descends through the lateral side of the posterior compartment,
supplying lateral muscles of the leg.
ii. It ends in a network of arteries in the heel.
H. As in the upper limb, veins that drain the lower limb include both deep and superficial veins,
with anastomoses between them. (pp. 804–806) (Figs. 20.38, 20.39) (Table 20.13)
1. The superficial veins are often visible, as with the upper limb.
a. The dorsal venous arch can often be viewed through the skin on the dorsum of
the foot.
i. It collects blood from the toes and more proximal part of the foot.
ii. It has numerous anastomoses similar to the dorsal enough network of
the hand.
iii. It gives right to the following two veins.
b. The small (short) saphenous vein arises from the lateral side of the arch and
passes up that side of the leg to the knee, where it drains into the popliteal vein.
c. The great (long) saphenous vein, the longest vein in the body, arises from the
medial side of the arch and travels all the way up the leg and thigh to the
inguinal region.
i. It empties into the femoral vein slightly inferior to the inguinal
ligament.
ii. It is commonly used as a site for the long term administration of
intravenous fluids.
iii. It is relatively accessible in infants and in patients in shock whose
veins have collapsed.
iv. Portions of this vein are commonly used as grafts in coronary
bypass surgery.
v. The great and small saphenous veins are among the most common
sites of varicose veins.
2. The deep veins of the lower limb are as follows.
a. The deep plantar venous arch receives blood from the toes. (Fig. 20.38b)
i. It gives rise to lateral and medial plantar veins on the respective sides.
ii. The lateral plantar vein gives off the fibular veins, then courses to
the medial side and approaches the medial plantar vein.
iii. The two plantar veins pass behind the medial malleolus of the ankle
and continue as a pair of posterior tibial veins.
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b. The two posterior tibial veins pass up the leg embedded deep in the calf
muscles; they converge like an inverted Y into a single vein about two thirds of
the way up the tibia.
c. The two fibular (peroneal) veins ascend the back of the leg and similarly
converge like a Y.
d. The popliteal vein begins near the knee by convergence of these two inverted
Y’s; it passes through the popliteal fossa at the back of the knee.
e. The two anterior tibial veins travel up the anterior compartment of the leg
between the tibia and fibula. (Fig. 20.38a)
i. They arise from the medial side of the dorsal venous arch.
ii. They converge just distal to the knee and then flow into the popliteal
vein.
f. The femoral vein is a continuation of the popliteal vein into the thigh; it drains
blood from the deep thigh muscles and femur.
g. The deep femoral vein drains the femur and muscles of the thigh supplied by
the deep femoral artery.
i. It receives four principles tributaries along the shaft of the femur and
then a pair of circumflex femoral veins that encircle the upper femur.
ii. It drains ultimately into the upper femoral vein.
h. The external iliac vein is formed by the union of the femoral and great
saphenous veins near the inguinal ligament.
i. The internal iliac vein follows the course of the internal iliac artery and its
distribution.
i. Its tributaries drain the gluteal muscles; the medial aspect of the
thigh; the urinary bladder, rectum, prostate, and ductus deferens in the
male; and the uterus and vagina in the female.
j. The common iliac vein is formed by the union of the external and internal iliac
veins; the right and left common iliacs then unit to form the inferior vena cava.
3. The most common cardiovascular disease are atherosclerosis and hypertension; a few
additional vascular disorders are described in Table 20.14.
Insight 20.4 Arterial Pressure Points (Fig. 20.40)
Insight 20.5 Hypertension—The “Silent Killer”
Connective Issues: Circulatory System Interactions
Cross Reference
Additional information on topics mentioned in Chapter 20 can be found in the chapters listed below.
Chapter 3: Osmosis
Saladin Outline Ch.20
Chapter 14: The blood–brain barrier
Chapter 14: Brain anatomy
Chapter 15: The role of sympathetic tone and vasomotor tone in vessel diameter.
Chapter 17: Portal system between hypothalamus and anterior pituitary
Chapter 19: Coronary circulation
Chapter 19: Collateral arteries of the coronary circulation
Chapter 19: Cardiac output
Chapter 21: The lymphatic system
Chapter 21: Anaphylactic shock
Chapter 23: Portal system in the kidneys
Chapters 23 and 24: Effect of kidneys on blood pressure
Chapter 23: Synthesis and action of angiotensin II
Chapter 25: Circulation within the liver.
Chapter 29: Fetal and placental circulation
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