CHAPTER 23
Answers to “What Did You Learn?”
1.
An anastomosis occurs where two or more arteries (or veins) converge to
supply the same body region. Arterial anastomoses provide alternate blood
supply routes to reach body tissues or organs.
2.
Continuous capillaries are the most common type of capillary. The endothelial
cells form a complete continuous lining and are connected by tight junctions.
These capillaries are found in muscle, skin, the thymus, the lungs, and the CNS.
Fenestrated capillaries have holes (or fenestrations) within each endothelial cell,
but the basement membrane of the endothelial cells is continuous. Fenestrated
capillaries are seen in tissues where a great deal of fluid transport between the
blood and tissues occurs, such as the intestinal villi, ciliary process of the eye,
most endocrine glands, and the kidney. Sinusoids (discontinuous capillaries) have
larger gaps than fenestrated capillaries, and a discontinuous or absent basement
membrane. Sinusoids are found in bone marrow, the anterior pituitary,
parathyroid glands, adrenal glands, the spleen and the liver.
3.
Valves assist in the movement of blood because they only open one way, so blood
is pushed through them but cannot flow back. This prevents both pooling of
blood in the limbs and backflow of blood in the veins. Many deep veins pass
between skeletal muscle groups and as the skeletal muscles contract, they help
pump the blood toward the heart.
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4.
Blood pressure is the force per unit area that blood places on the blood vessel
wall. Blood pressure is measured in millimeters of mercury (mm Hg). Systolic
blood pressure is the pressure in the blood vessels during systole (ventricular
contraction), while diastolic blood pressure occurs during diastole (ventricular
relaxation). Systolic pressure typically is greater than diastolic pressure due to the
greater force from ventricular contraction. Blood pressure readings are expressed
in a ratio format, where the numerator (upper number) is the systolic pressure and
the denominator (lower number) is the diastolic pressure.
5.
The three main branches that arise from the aortic arch are the brachiocephalic
trunk, the left common carotid artery, and the left subclavian artery. The
brachiocephalic trunk further divides into the right common carotid artery
(supplies arterial blood to right side of head and neck), and the right subclavian
artery (supplies right upper limb and some thoracic structures). The left common
carotid artery supplies the left side of the head and neck. The left subclavian
artery supplies right upper limb and some thoracic structures.
6.
The internal carotid arteries give off the anterior cerebral and middle cerebral
arteries, which help supply the brain. The vertebral arteries merge to form the
basilar artery, which then subdivides into the posterior cerebral arteries that
supply the posterior portion of the cerebrum. The cerebral arterial circle is an
important anastomosis of arteries that surround the base of the brain. The circle
is formed from posterior cerebral arteries and posterior communicating arteries
(branches of the posterior cerebral arteries), internal carotid arteries, anterior
cerebral arteries (branches of the internal carotid arteries) , and anterior
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communicating arteries (connect the two anterior cerebral arteries). This arterial
circle equalizes blood pressure in the brain and can provide collateral channels
should one vessel become blocked.
7.
The hemiazygos and accessory hemiazygos veins are on the left side of the
vertebrae, and drain the left-side veins. The azygos vein drains the right side
veins and also receives blood from the hemiazygos veins. The azygos vein also
receives blood from the esophageal veins, bronchial veins, and pericardial veins.
The azygos vein merges with the superior vena cava.
8.
The celiac trunk gives off three branches: the left gastric artery, the splenic artery,
and the common hepatic artery.
9.
The hepatic portal system consists of a network of veins that drain the
gastrointestinal tract and shunt the blood to the liver for processing. The hepatic
portal vein is the large vein that receives blood from the gastrointestinal organs.
The 3 main branches that merge to form this vein include the inferior mesenteric
vein, the splenic vein, and the superior mesenteric vein.
10.
The uterine and vaginal arteries, which supply the uterus and vagina, are only
found in females.
11.
On the dorsum of the hand, a dorsal venous network of veins drain into the basilic
vein and the cephalic vein. The median cubital vein connects the cephalic and
basilic veins in the cubital fossa. The basilic vein eventually helps form the
axillary vein. The cephalic vein drains into the axillary vein.
12.
The main arterial supply for the lower limb comes from the external iliac artery (a
branch of the common iliac artery). The external iliac artery travels inferior to the
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inguinal ligament, where it is renamed the femoral artery. The femoral artery
gives off several branches, including the deep femoral artery. The femoral artery
is renamed the popliteal artery when it enters the popliteal fossa. The popliteal
artery divides into an anterior tibial artery and a posterior tibial artery. The
posterior tibial artery gives off a fibular artery and the posterior tibial artery then
branches into medial and lateral plantar arteries. The anterior tibial artery crosses
over the anterior surface of the ankle, where it is renamed the dorsalis pedis
artery. The dorsalis pedis artery and a branch of the lateral plantar artery unite to
form the plantar arch of the foot. Digital arteries come from the plantar arch.
13.
The bronchial arteries carry blood high in oxygen to the bronchi, bronchioles and
connective tissues of the lung. Bronchial veins receive blood low in oxygen from
these same areas. These vessels are very small and thin. In contrast, the
pulmonary vessels are much larger and are responsible for transporting blood to
and from the lungs for oxygenation purposes. Pulmonary arteries take blood low
in oxygen from the heart to the lungs, while pulmonary veins carry blood high in
oxygen from the lungs back to the heart.
14.
Oxygenated blood leaves the left ventricle and enters the ascending aorta before
entering the aortic arch and then the descending thoracic aorta.
15.
As adults get older, the heart and blood vessels become less resilient. Many of
the elastic arteries are less able to withstand the forces from the pulsating blood.
Systolic blood pressure may increase with age, exacerbating this problem.
16.
The right vitelline vein is primarily responsible for forming the hepatic portal
system.
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17.
The umbilical artery becomes the medial umbilical ligament after the baby is
born.
Answers to “Content Review”
1.
The tunica intima is the innermost layer. It has an endothelium, a layer of
areolar connective tissue and a basement membrane. The tunica media is the
middle layer of the vessel wall. It has circularly arranged layers of smooth
muscle cells. The tunica externa is the outermost layer. It is composed of
areolar connective tissue as well as some elastic and collagen fibers.
2.
Arteries conduct blood away from the heart toward capillary beds, while veins
drain capillaries and conduct blood toward the heart. The lumen of an artery is
narrower than that of a corresponding vein, due to the thicker tunica media wall in
an artery. Arteries must withstand high blood pressures while veins transport
blood under low pressures. The tunica media is thicker in the artery than in the
companion vein, while the tunica externa is thicker in the vein than in the
companion artery.
3.
Elastic arteries are the largest arteries. They have a large proportion of elastin
throughout all three tunics, especially in the tunica media. The abundant elastin
allows the artery to stretch when a ventricle ejects blood into it. Examples of
elastic arteries include the aorta, pulmonary trunk, and brachiocephalic trunk.
Muscular arteries are medium-sized arteries. The elastin in muscular arteries is
confined to two circumscribed rings; the internal elastic lamina and the external
elastic lamina. Muscular arteries have a proportionately thicker tunica media with
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multiple layers of smooth muscle fibers. The greater amount of muscle and less
amount of elastic tissue results in less elasticity but better ability to vasoconstrict
and vasodilate. Examples of muscular arteries include the brachial, anterior
tibial, and coronary arteries. Arterioles are the smallest arteries and generally
have six or less layers of smooth muscle in their tunica media. Larger arterioles
have all three tunics, whereas the smallest arterioles may have a thin layer of
endothelium surrounded by a single layer of smooth muscle fibers.
4.
The thin wall and the narrow vessel diameter of capillaries are optimal for
diffusion of gases, nutrients, and wastes between blood and body tissues. There
are three basic kinds of capillaries: continuous capillaries, fenestrated capillaries,
and sinusoids. Continuous capillaries have endothelial cells that form a complete
continuous lining and are connected by tight junctions. Fenestrated capillaries
have fenestrations within each endothelial cell, yet the basement membrane
remains continuous. Fenestrated capillaries are seen where a great deal of fluid
transport across the blood occurs. Sinusoids have larger gaps than fenestrated
capillaries, and they have a discontinuous or absent basement membrane.
Sinusoids tend to be wider, larger vessels and the large openings in these vessels
allow for transport of larger materials, such as proteins or leukocytes.
5.
Blood pressure is greater in arteries than in veins, because the arteries receive
blood from the heart and must withstand the forceful pumping action of the blood
coming from the heart. Hypertension causes functional and structural changes in
the blood vessel walls making them more prone to further injury. As damage
increases, the blood vessels are more prone to atherosclerosis. Hypertension
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causes undue stress on arterioles, resulting in thickening of the arteriole walls and
reduction in luminal diameter (a condition called arteriolosclerosis). Furthermore,
hypertension is a major cause of heart failure owing to the extra workload placed
on the heart.
6.
Oxygenated blood leaves the left ventricle of the heart and enters the ascending
aorta. The left and right coronary arteries emerge from the wall of the ascending
aorta and supply the heart. The ascending aorta curves toward the left side of the
body and becomes the aortic arch (arch of the aorta). Three main arterial
branches emerge from the aortic arch. (1) the brachiocephalic trunk bifurcates
into the right common carotid artery supplying arterial blood to right side of head
and neck, and the right subclavian artery supplying right upper limb and some
thoracic structures; (2) the left common carotid artery supplying the left side of
the head and neck; and (3) the left subclavian artery supplying the left upper limb
and some thoracic structures.
7.
Both the upper and lower limbs are supplied by a main arterial vessel (subclavian
artery for the upper limb, femoral artery for the lower limb). The artery bifurcates
(branches into two vessels) at the elbow or knee. Arterial and venous arches are
foundin both the hand and foot. Both the lower limb and upper limb have a
superficial and deep network of veins. Most of the deep veins are found in pairs,
and travel alongside the artery of the same name.
8.
The arteries of systemic circulation carry oxygenated blood in its arteries from the
left side of the heart to body tissue capillary beds and the systemic veins carry
deoxygenated blood from these capillary beds back to the right side of the heart.
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The pulmonary circulation is responsible for carrying deoxygenated blood in its
arteries from the right side of the heart to the lungs, and then returning the newly
oxygenated blood to the left side of the heart in its veins.
9.
As adults age, the heart and blood vessels become less resilient. Many of the
elastic arteries are less able to withstand the forces from the pulsating blood.
Systolic blood pressure may increase with age, exacerbating this problem. As a
result, older individuals are more prone to developing an aneurysm, whereby part
of the arterial wall thins and balloons out. In addition, aging increases the
incidence and severity of atherosclerosis.
10.
When the umbilical cord is clamped after birth, the umbilical vein and umbilical
arteries constrict and become nonfunctional. They turn into the round ligament of
the liver and the medial umbilical ligaments, respectively. Since there is no blood
going through the umbilical vein and ductus venosus after bith, the ductus
venosus ceases to be functional and constricts, becoming the ligamentum
venosum. Since pressure now is greater on the left side of the heart, the two flaps
of the interatrial septum close off the foramen ovale. The only remnant of the
foramen ovale will be a thin, oval depression in the wall of the septum called the
fossa ovalis. Within 10-15 hours after birth, the ductus arteriosus closes and
becomes a fibrous structure called the ligamentum arteriosum.
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