The cardiovascular system
• The cardiovascular system provides the transport
system that keeps blood continuously circulating.
The heart is no more than the transport system
pump; the hollow blood vessels are the delivery
routes.
• Using blood as the transport medium, the heart
continually propels oxygen, nutrients, wastes, and
many other substances into the blood vessels that
service body cells.
Heart Anatomy
Size, Location, and Orientation
About the size of a fist, the hollow, cone-shaped
heart has a mass of between 250 and 350
grams—less than a pound. enclosed within the
mediastinum (me″de-ah-sti′num), the medial
cavity of the thorax, the heart extends obliquely
for 12 to 14 cm (about 5 inches) from the
second rib to the fifth intercostal space .
As it rests on the superior surface of the
diaphragm, the heart lies anterior to the
vertebral column and posterior to the sternum.
If you press your fingers between the fifth and sixth ribs
just below the left nipple, you can easily feel your heart
beating where the apex contacts the chest wall. Hence,
this site is referred to as the point of maximal intensity
(PMI).
The lungs flank the heart laterally and partially
obscure it.
Approximately two-thirds of its mass lies to the left
of the midsternal line. Its broad, flat base, or
posterior surface, is about 9 cm (3.5 in) wide and
directed toward the right shoulder. Its apex points
inferiorly toward the left hip.
Coverings of the Heart
The heart is enclosed in a double-walled sac called the pericardium.
The loosely fitting superficial part of this sac is the fibrous
pericardium.
This tough, dense connective tissue layer (1) protects the heart, (2)
anchors it to surrounding structures, and (3) prevents overfilling of
the heart with blood.
Deep to the fibrous pericardium is the serous pericardium, a thin,
slippery, two-layer serous membrane. Its parietal layer lines the
internal surface of the fibrous pericardium. At the superior margin of
the heart, the parietal layer attaches to the large arteries exiting the
heart, and then turns inferiorly and continues over the external heart
surface as the visceral layer, also called the epicardium (“upon the
heart”), which is an integral part of the heart wall.
Between the parietal and visceral layers is the slitlike pericardial
cavity, which contains a film of serous fluid. The serous membranes,
lubricated by the fluid, glide smoothly past one another during heart
activity, allowing the mobile heart to work in a relatively friction-free
environment
HOMEOSTATIC IMBALANCE
Pericarditis, inflammation of the pericardium,
roughens the serous membrane surfaces.
Pericarditis is characterized by pain deep to the
sternum. Over time, it may lead to adhesions in
which the visceral and parietal pericardia stick
together and impede heart activity. In severe cases,
excess fluid compresses the heart, limiting its ability
to pump blood. This condition in which the heart is
compressed by fluid is called cardiac tamponade
(tam″pŏ-nād′).
Physicians treat it by inserting a syringe into the
pericardial cavity and draining off the excess fluid.
Layers of the Heart Wall
The heart wall is composed of three layers :
1-The superficial epicardium is the visceral
layer of the serous pericardium. It is often
infiltrated with fat, especially in older
people.
2-The middle layer, the myocardium
(“muscle heart”), is composed mainly of
cardiac muscle and forms the bulk of the
heart. It is the layer that contracts.
3-The third layer, the endocardium (“inside the
heart”), is a glistening white sheet of
endothelium (squamous epithelium) resting
on a thin connective tissue layer.
Located on the inner myocardial surface, it
lines the heart chambers and covers the
fibrous skeleton of the valves. The
endocardium is continuous with the
endothelial linings of the blood vessels
leaving and entering the heart.
Chambers and Associated Great Vessels
The heart has four chambers ,two superior atria
(a′tre-ah) and two inferior ventricles (ven′trĭ-klz).
The internal partition that divides the heart
longitudinally is called the interatrial septum where it
separates the atria, and the interventricular septum
where it separates the ventricles.
The right ventricle forms most of the anterior surface of
the heart.
The left ventricle dominates the inferoposterior aspect
of the heart and forms the heart apex.
The coronary sulcus or atrioventricular groove, encircles
the junction of the atria and ventricles like a crown
(corona = crown).
Atria: The Receiving Chambers
Except for small, wrinkled, protruding appendages
called auricles (or′ĭ-klz; auricle = little ear), which
increase the atrial volume somewhat, the right and left
atria are remarkably free of distinguishing surface
features. The interatrial septum bears a shallow
depression, the fossa ovalis (o-vă′lis), that marks the
spot where an opening, the foramen ovale, existed in
the fetal heart .
Atria are receiving chambers for blood returning to
the heart from the circulation .Because they need
contract only minimally to push blood “downstairs” into
the ventricles, the atria are relatively small, thinwalled chambers. As a rule, they contribute little to the
propulsive pumping activity of the heart.
-Blood enters the right atrium via three veins :
(1) The superior vena cava returns blood from
body regions superior to the diaphragm;
(2) (2) the inferior vena cava returns blood from
body areas below the diaphragm; and
(3) (3) the coronary sinus collects blood draining
from the myocardium.
- Four pulmonary veins enter the left atrium,
which makes up most of the heart’s base.
Ventricles: The Discharging Chambers
Together the ventricles make up most of the volume of
the heart. The conelike papillary muscles, which play a
role in valve function, project into the ventricular cavity.
The ventricles are the discharging chambers or actual
pumps of the heart .When the ventricles contract,
blood is propelled out of the heart into the circulation.
-The right ventricle pumps blood into the pulmonary
trunk, which routes the blood to the lungs where gas
exchange occurs.
-The left ventricle ejects blood into the aorta (a-or′tah),
the largest artery in the body.
Pathway of Blood Through the Heart
The heart is actually two side-by-side pumps, each
serving a separate blood circuit.
• The blood vessels that carry blood to and from the
lungs form the pulmonary circuit (pulmonos =
lung), which serves gas exchange.
• The blood vessels that carry the functional blood
supply to and from all body tissues constitute the
systemic circuit.
The right side of the heart is the pulmonary circuit pump.
Blood returning from the body is relatively oxygen-poor
and carbon dioxide–rich. It enters the right atrium and
passes into the right ventricle, which pumps it to the
lungs via the pulmonary trunk .
-In the lungs, the blood unloads carbon dioxide and picks
up oxygen. The freshly oxygenated blood is carried by
the pulmonary veins back to the left side of the heart.
-Typically, we think of veins as vessels that carry blood
that is relatively oxygen-poor to the heart and arteries
as transporters of oxygen-rich blood from the heart to
the rest of the body.
-Exactly the opposite condition exists in the pulmonary
circuit.
The left side of the heart is the systemic circuit pump.
Freshly oxygenated blood leaving the lungs is
returned to the left atrium and passes into the left
ventricle, which pumps it into the aorta.
From there the blood is transported via smaller
systemic arteries to the body tissues, where gases
and nutrients are exchanged across the capillary
walls.
Then the blood, once again loaded with carbon
dioxide and depleted of oxygen, returns through
the systemic veins to the right side of the heart,
where it enters the right atrium through the
superior and inferior venae cavae.
Although equal volumes of blood are pumped to the
pulmonary and systemic circuits at any moment, the
two ventricles have very unequal workloads.
The pulmonary circuit, served by the right ventricle, is a
short, low-pressure circulation, whereas the systemic
circuit, associated with the left ventricle, takes a long
pathway through the entire body and encounters about
five times as much friction, or resistance to blood
flow.
This functional difference is revealed in the anatomy of
the two ventricles .The walls of the left ventricle are
three times as thick as those of the right ventricle, and
its cavity is nearly circular.
Consequently, the left ventricle can generate much more
pressure than the right and is a far more powerful
pump.
Coronary Circulation
The coronary circulation, the functional blood supply of the heart,
is the shortest circulation in the body. The arterial supply of the
coronary circulation is provided by the right and left coronary
arteries, both arising from the base of the aorta and encircling the
heart in the coronary sulcus.
HOMEOSTATIC IMBALANCE Blockage of the coronary arterial
circulation can be serious and sometimes fatal.
Angina pectoris is thoracic pain caused by a fleeting deficiency in
blood delivery to the myocardium. The myocardial cells are
weakened by the temporary lack of oxygen but do not die.
Myocardial infarction (MI), is far more serious is prolonged coronary
blockage. Because adult cardiac muscle is essentially amitotic, most
areas of cell death are repaired with noncontractile scar tissue.
Whether or not a person survives a myocardial infarction depends
on the extent and location of the damage. Damage to the left
ventricle, which is the systemic pump, is most serious.
• Heart Valves
Blood flows through the heart in one direction: from atria to
ventricles and out the great arteries leaving the superior aspect of
the heart. This one-way traffic is enforced by four valves that open
and close in response to differences in blood pressure on their two
sides.
Atrioventricular Valves
The two atrioventricular (AV) valves, one located at each atrialventricular junction, prevent backflow into the atria when the
ventricles are contracting.
• The right AV valve, the tricuspid valve has three flexible cusps
• The left AV valve, with two flaps, is called the mitral valve (mi′tral).
It is sometimes called the bicuspid valve.
• Attached to each AV valve flap are tiny white collagen cords called
chordae tendineae “heart strings” which anchor the cusps to the
papillary muscles protruding from the ventricular walls.
• When the heart is completely relaxed, the AV valve
flaps hang limply into the ventricular chambers below
and blood flows into the atria and then through the
open AV valves into the ventricles. When the
ventricles contract, compressing the blood in their
chambers, the intraventricular pressure rises, forcing
the blood superiorly against the valve flaps. As a
result, the flap edges meet, closing the valve .The
chordae tendineae and the papillary muscles serve to
anchor the valve flaps in their closed position. If the
cusps were not anchored in this manner, they would
be blown upward into the atria, in the same way an
umbrella is blown inside out by a gusty wind.
• Semilunar Valves
The aortic and pulmonary (semilunar, SL) valves guard the
bases of the aorta and pulmonary trunk, respectively and
prevent backflow into the associated ventricles.
• Each SL valve is fashioned from three pocketlike cusps, each
shaped roughly like a crescent moon .
• Like the AV valves, the SL valves open and close in response
to differences in pressure. In the SL case, when the ventricles
are contracting and intraventricular pressure rises above the
pressure in the aorta and pulmonary trunk, the SL valves are
forced open .
• When the ventricles relax, and the blood (no longer
propelled forward by the pressure of ventricular contraction)
flows backward toward the heart, it fills the cusps and closes
the valves.
HOMEOSTATIC IMBALANCE Heart valves—like any
mechanical pump—can function with “leaky”
valves as long as the impairment is not too great.
An incompetent valve forces the heart to repump
the same blood over and over because the valve
does not close properly and blood backflows.
In valvular stenosis (“narrowing”), the valve flaps
become stiff and constrict the opening. This
stiffness compels the heart to contract more
forcibly than normal.
In both instances, the heart’s workload increases
and, ultimately, the heart may be severely
weakened. Under such conditions, the faulty valve
(most often the mitral valve) is replaced .
Cardiac muscle structure
Heart Physiology
-The ability of cardiac muscle to depolarize and contract is
intrinsic; that is, it is a property of heart muscle and does
not depend on the nervous system. Even if all nerve
connections to the heart are severed, the heart continues
to beat rhythmically (as demonstrated by transplanted
hearts).
-Nevertheless,the healthy heart is amply supplied with
autonomic nerve fibers that can alter the basic rhythm of
heart activity set by intrinsic factors.
Setting the Basic Rhythm: The Intrinsic Conduction System
The independent activity consists of noncontractile cardiac
cells specialized to initiate and distribute impulses
throughout the heart in an orderly, sequential manner.
Thus, the heart beats as a coordinated unit.
Sequence of Excitation
1-Sinoatrial node. The crescent-shaped sinoatrial (SA)
node is located in the right atrial wall, just inferior
to the entrance of the superior vena cava. A minute
cell mass with a mammoth job, the SA node
typically generates impulses about 75 times every
minute. (However, its inherent rate in the absence
of extrinsic neural and hormonal factors is closer to
100 times per minute.) Because no other region of
the conduction system or the myocardium has a
faster depolarization rate, the SA is the heart’s
pacemaker, and its characteristic rhythm, called
sinus rhythm, determines heart rate.
2-Atrioventricular node.
From the SA node, the depolarization wave
spreads to the atrioventricular (AV) node, located
in the inferior portion of the interatrial septum
immediately above the tricuspid valve. At the AV
node, the impulse is delayed allowing the atria to
respond and complete their contraction before the
ventricles contract.
Once through the AV node, the signaling impulse
passes rapidly through the rest of the system.
3-Atrioventricular bundle. From the AV node, the
impulse sweeps to the atrioventricular (AV) bundle
(also called the bundle of His) in the superior part
of the interventricular septum.
4- Right and left bundle branches. The AV bundle
persists only briefly before splitting into two
pathways—the right and left bundle branches,
which course along the interventricular septum
toward the heart apex.
5- Purkinje fibers complete the pathway through the
interventricular septum, penetrate into the heart
apex, and then turn superiorly into the ventricular
walls.
HOMEOSTATIC IMBALANCE
Defects in the intrinsic conduction system can cause
irregular heart rhythms, or arrhythmias (ah-rith′meahz):
fibrillation, a condition caused by ischemia . It is a
rapid and irregular contractions in which control of
heart rhythm is taken away from the SA node by rapid
activity in other heart regions. The heart in fibrillation
has been compared with a squirming bag of worms.
Fibrillating ventricles are useless as pumps; and
unless the heart is defibrillated quickly, circulation
stops and brain death occurs.
Defibrillation is accomplished by electrically shocking
the heart.The hope is that the SA node will begin to
function normally and sinus rhythm will be
reestablished.
• Because the only route for impulse transmission
from atria to ventricles is through the AV node, any
damage to the AV node, referred to as a heart
block.
-In total heart block no impulses get through and
the ventricles beat at their intrinsic rate, which is
too slow(20-40/min) to maintain adequate
circulation.
-In partial heart block, only some of the atrial
impulses reach the ventricles.
In both cases, pacemakers are used to recouple the
atria to the ventricles as necessary.
Modifying the Basic Rhythm: Extrinsic
Innervation of the Heart
• The sympathetic nervous system (the
“accelerator”) increases both the rate and the
force of heartbeat.
• The Parasympathetic activation (the “brakes”)
slows the heart. It sends inhibitory impulses to
the heart via branches of the vagus nerves.
The cardiac centers are located in the medulla
oblongata (cardioacceleratory center and
cardioinhibitory center )
• They sends impulses to the SA and AV nodes.
Electrocardiography
The electrical currents generated in and transmitted
through the heart spread throughout the body and
can be detected with an electrocardiograph. A
graphic record of heart activity is called an
electrocardiogram .An ECG is a composite of all the
action potentials generated by nodal and contractile
cells at a given time and not, as sometimes
assumed, a tracing of a single action potential.
A typical ECG has three waves :
1-The first, the small P wave, lasts about 0.08 s and
results from movement of the depolarization wave
from the SA node through the atria.
2-The large QRS complex results from
ventricular depolarization. It has a
complicated shape because the paths of the
depolarization waves through the ventricular
walls change continuously, producing
corresponding changes in current direction.
3- The T wave is caused by ventricular
repolarization.
Because atrial repolarization takes place during
the period of ventricular excitation, the wave
representing atrial repolarization is normally
obscured by the large QRS complex being
recorded at the same time.
Heart Sounds
During each heartbeat, two sounds can be distinguished
when the thorax is auscultated (listened to) with a
stethoscope. These heart sounds, often described as lub-dup,
are associated with closing of heart valves.
The basic rhythm of the heart sounds is lub-dup, pause,
lub-dup, pause, and so on.
• The first sound, which occurs as the AV valves close, signifies
the point when ventricular pressure rises above atrial
pressure .The first sound tends to be louder, longer, and more
resonant than
• the second sound, which is a short and sharp sound heard as
the SL valves close at the beginning of ventricular relaxation
(diastole).
HOMEOSTATIC IMBALANCE
Blood flows silently as long as the flow is smooth and
uninterrupted. If it strikes obstructions, however, its
flow becomes turbulent and generates heart murmurs
that can be heard with a stethoscope.
• Heart murmurs are fairly common in young children
(and some elderly people) with perfectly healthy
hearts, probably because their heart walls are relatively
thin and vibrate with rushing blood.
• Most often, however, murmurs indicate valve
problems. If a valve is incompetent, a murmur is heard
as the blood backflows or regurgitates through the
valve.
• A stenotic valve, in which the valvular opening is
narrowed, restricts blood flow through the valve.
Mechanical Events: The Cardiac Cycle
The heart alternately contracts, forcing blood out of
its chambers, and then relaxes, allowing its
chambers to refill with blood. The terms systole
(sis′to-le) and diastole (di-as′to-le) refer respectively
to these contraction and relaxation periods. The
cardiac cycle includes atrial systole and diastole
followed by ventricular systole and diastole`(one
complete heart beat). These mechanical events
always follow the electrical events seen in the ECG.
The average heart beats approximately 75 times
per minute , so the length of the cardiac cycle is
normally about 0.8 seconds.
Cardiac output (CO)
is the amount of blood pumped out by each ventricle in 1
minute. It is the product of heart rate (HR) and stroke
volume (SV).
Stroke volume is defined as the volume of blood pumped out
by one ventricle with each beat. In general, stroke volume
is correlated with the force of ventricular contraction.
Using normal resting values for heart rate (75 beats/min)
and stroke volume (70 ml/beat), the average adult cardiac
output is about 5 L.(heart rate x stroke volume)
Thus, the entire blood supply passes through each side of the
heart once each minute.
Notice that cardiac output varies directly with SV and HR.
Thus CO increases when the stroke volume increases or the
heart beats faster or both, and decreases when either or
both of these factors decrease.
Regulation of Heart Rate
1- When blood volume drops sharply or when
the heart is seriously weakened, SV declines
and CO is maintained by increasing HR and
contractility.
2 -Temporary stressors can also influence HR—
and consequently CO—by acting through
homeostatic mechanisms induced neurally,
chemically, and physically.
a-Autonomic Nervous System Regulation The most
important extrinsic controls affecting heart rate.
-When the sympathetic nervous system is activated by
emotional or physical stressors, such as fright, anxiety,
or exercise ,sympathetic nerve fibers release
norepinephrine. As a result, the pacemaker fires more
rapidly and the heart responds by beating faster.
-The parasympathetic division opposes sympathetic
effects and effectively reduces heart rate when a
stressful situation has passed. Cutting the vagal nerves
results in an almost immediate increase in heart rate of
about 25 beats/min, reflecting the inherent rate (100
beats/min) of the pacemaking SA node.
b-Chemical Regulation
1. Hormones.
-Epinephrine, liberated by the adrenal medulla
during sympathetic nervous system activation,
enhances heart rate and contractility.
-Thyroxine is a thyroid gland hormone that
increases metabolic rate and body heat
production and heart rate.
2. Ions. Plasma electrolyte imbalances pose real
dangers to the heart.
- Low calcium(hypocalcemia) depress the heart
- Excessive K+ (hyperkalemia) may lead to heart
block and cardiac arrest
c-Other Factors
-age,, and also influence HR. Resting heart rate is
fastest in the fetus (140–160 beats/min) and gradually
declines throughout life.
- gender,, average heart rate is faster in females (72–80
beats/min) than in males (64–72 beats/min).
- Exercise raises HR by acting through the sympathetic
nervous system . However, resting HR in the physically
fit tends to be as slow as 40 beats/min.
- body temperature increases HR by enhancing the
metabolic rate of cardiac cells. This explains the rapid,
pounding heartbeat you feel when you have a high
fever and also accounts, in part, for the effect of
exercise on HR (remember, working muscles generate
heat). Cold directly decreases heart rate.
HOMEOSTATIC IMBALANCE Although HR varies with
changes in activity, marked and persistent rate
changes usually signal cardiovascular disease.
• Tachycardia (take-kar′de-ah; “heart hurry”) is an
abnormally fast heart rate (more than 100 beats/min)
that may result from elevated body temperature,
stress, certain drugs, or heart disease. Because
tachycardia occasionally promotes fibrillation,
persistent tachycardia is considered pathological.
• Bradycardia (brade-kar′de-ah; brady = slow) is a heart
rate slower than 60 beats/min. It may result from low
body temperature, certain drugs, or parasympathetic
nervous activation. It is a known, and desirable,
consequence of endurance training.
Homeostatic Imbalance of Cardiac Output
-The heart’s pumping action ordinarily maintains
a balance between cardiac output and venous
return. Were this not so, blood congestion would
occur in the veins returning blood to the heart.
- When the pumping efficiency (CO) of the heart
is so low that blood circulation is inadequate to
meet tissue needs, the heart is said to be in
congestive heart failure (CHF). It occurs in:
1. Coronary atherosclerosis
2. Persistent high blood pressure
3. Multiple myocardial infarcts
Because the heart is a double pump, each side can
initially fail independently of the other.
- If the left side fails, The right side continues to
propel blood to the lungs, but the left side does not
adequately eject the returning blood into the
systemic circulation. causing pulmonary edema. If
the congestion is untreated, the person suffocates
- If the right side of the heart fails, peripheral
congestion occurs. Blood stagnates in body organs,
and pooled in the extremities (feet, ankles, and
fingers).
Failure of one side of the heart puts a greater
strain on the other side, and ultimately the whole
heart fails.
OVERVIEW OF BLOOD VESSEL STRUCTURE AND FUNCTION
The blood vessels of the body form a closed delivery system
that begins and ends at the heart.
The three major types of blood vessels are :
1- the arteries, capillaries, and veins. As the heart contracts,
it forces blood into the large arteries leaving the ventricles.
The blood then moves into successively smaller arteries,
finally reaching their smallest branches, the arterioles (arte′re-ōlz; “little arteries”), which feed into
2-the capillary beds of body organs and tissues. Blood drains
from the capillaries into
3-the venules (ven′ūlz), the smallest veins, and then on into
larger and larger veins that merge to form the large veins that
ultimately empty into the heart.
- Altogether, the blood vessels in the adult human stretch for
about 100,000 km (60,000 miles) .
Structure of Blood Vessel Walls
•
•
•
•
•
The walls of all blood vessels, except the very smallest, have
three distinct layers, or tunics :
The walls of arteries and veins are composed of the tunica
intima (endothelium underlain by loose connective tissue),
the tunica media (smooth muscle cells and elastic fibers), and
the tunica externa (largely collagen fibers).
Capillaries are composed of only of simple squamous
epithelium on a basement membrane.
Depending on the body’s needs at any given moment, either
vasoconstriction (reduction in lumen diameter as the smooth
muscle contracts) or vasodilation (increase in lumen diameter
as the smooth muscle relaxes) can be effected.
The arterial wall is thicker , more circular , with less diameter
than the corresponding vein.
Large vein also has valves and large arteries are able to
expand and recoil.
venous return
Venous pressure is normally too low to promote
adequate venous return. Hence, three factors are
critically important to venous return:
1-the respiratory “pump.” As we inhale, abdominal
pressure increases, squeezing the local veins and
forcing blood toward the heart. At the same time, the
pressure in the chest decreases, allowing thoracic
veins to expand and speeding blood entry into the
right atrium.
2- the muscular “pump.” Skeletal muscle activity, or
the so-called muscular pump, is the more important
pumping mechanism. As the skeletal muscles
surrounding the deep veins contract and relax, they
“milk” blood toward the heart, and once blood
passes each successive valve, it cannot flow back.
3- the valves in the large veins.
HOMEOSTATIC IMBALANCE
• Varicose veins are veins that have become
tortuous and dilated because of incompetent
(leaky) valves. More than 15% of adults suffer from
varicose veins, usually in the lower limbs. Several
factors contribute, including heredity ,prolonged
standing, obesity, or pregnancy. Consequently,
blood pools in the lower limbs, and the venous
walls stretch and become floppy.
• A serious complication is thrombophlebitis,
inflammation of a vein that results when a clot
forms in a vessel with poor circulation .
• The clot may detach and pulmonary embolism may
occur.
The muscular
pump. When
contracting skeletal
muscles press
against a vein, the
valves proximal to
the area of
contraction are
forced open and
blood is propelled
toward the heart.
The valves distal to
the area of
contraction are
closed by the back
flowing blood.
ANASTOMOSES
• An anastomosis is a connection, or joining, of vessels,
that is, artery to artery or vein to vein. The general
purpose of these connections is to provide alternate
pathways for the flow of blood if one vessel becomes
obstructed. An arterial anastomosis helps ensure that
blood will get to the capillaries of an organ to deliver
oxygen and nutrients and to remove waste products.
• There are arterial anastomoses, for example, between
some of the coronary arteries that supply blood to the
myocardium.
• A venous anastomosis helps ensure that blood will be
able to return to the heart in order to be pumped
again. Venous anastomoses are most numerous among
the veins of the legs, where the possibility of
obstruction increases as a person gets older.
CAPILLARIES
Capillaries carry blood from arterioles to venules. Their walls
are only one cell in thickness; capillaries are actually the
extension of the endothelium, the simple squamous lining, of
arteries and veins.
-Some tissues do not have capillaries; these are the
epidermis, cartilage, and the lens and cornea of the eye.
- Most tissues, however, have extensive capillary networks.
The quantity or volume of capillary networks in an organ
reflects the metabolic activity of the organ. The functioning of
the kidneys, for example, depends upon a good blood supply.
The vessels in kidneys are dense, most of which are
capillaries. In contrast, a tendon such as the Achilles tendon
at the heel or the patellar tendon at the knee would have far
fewer vessels, because fibrous connective tissue is far less
metabolically active.
Blood flow into capillary networks
-Is regulated by smooth muscle cells called precapillary
sphincters, found at the beginning of each network.
Precapillary sphincters constrict or dilate depending on the
needs of the tissues. Because there is not enough blood in
the body to fill all of the capillaries, precapillary sphincters
are usually slightly constricted.
-In an active tissue that requires more oxygen, such as
exercising muscle, the precapillary sphincters dilate to
increase blood flow. These automatic responses ensure that
blood, the volume of which is constant, will circulate where
it is needed most.
-Some organs have another type of capillary called sinusoids,
which are larger and more permeable than are other
capillaries. Sinusoids are found in the red bone marrow and
spleen, where blood cells enter or leave the blood, and in
organs such as the liver and pituitary gland, which produce
and secrete proteins into the blood.
EXCHANGES IN CAPILLARIES
Capillaries are the sites of exchanges of materials .
Some of these substances move from the blood to
tissue fluid, and others move from tissue fluid to the
blood. They move by diffusion, that is, from their area
of greater concentration to their area of lesser
concentration. Oxygen, therefore, diffuses from the
blood in systemic capillaries to the tissue fluid, and
carbon dioxide diffuses from tissue fluid to the blood to
be brought to the lungs and exhaled.
Blood pressure here(the pushing power) is about 30 to
35 mmHg, and the pressure of the surrounding tissue
fluid is much lower, about 2 mmHg. Because the
capillary blood pressure is higher, the process of
filtration occurs, which forces plasma contents (except
cells and albumin) out of the capillaries and into tissue
fluid.
Arrows shows the direction of movement. Filtration takes place at the
arterial end of the capillary. Osmosis takes place at the venous end.
Blood pressure decreases as blood reaches the
venous end of capillaries, to become 15mmHg .
Albumin contributes to the osmotic pressure of
blood; this is an “attracting” pressure, or “pulling”
rather than a “pushing” pressure and constant all
through the capillary which is about 25mmHg. At
the arterial end of capillaries, the pulling power of
albumin is less than pushing power of blood
pressure so plasma goes out the capillaries.Blood
pressure at the venous end is less than osmotic
pressure so fluid comes back to circulation.
Pathways of Circulation
Pulmonary circulation: Right ventricle  pulmonary artery  pulmonary
capillaries (exchange of gases)  pulmonary veins  left atrium
.
Systemic circulation: Left ventricle  aorta  capillaries in body
tissues  superior and inferior caval veins  right atrium.
Hepatic portal circulation: Blood from the digestive organs and spleen flows
through the portal vein to the liver where it divides into capillaries then it
joins into hepatic vein before returning to the heart. The purpose is that the
liver stores some nutrients or regulates their blood levels and detoxifies
potential poisons before blood enters the rest of peripheral circulation.
FETAL CIRCULATION
The fetus depends upon the mother for oxygen and nutrients and for the removal of carbon
dioxide and wastes.
• Because the fetal lungs are deflated and do not provide for
gas exchange, blood is shunted away from the lungs. The
foramen ovale is an opening in the interatrial septum that
permits some blood to flow from the right atrium to the
left atrium. The blood that does enter the right ventricle is
pumped into the pulmonary artery.
• The ductus arteriosus is a short vessel that diverts most
of the blood in the pulmonary artery to the aorta, to the
body. Both the foramen ovale and the ductus arteriosus
permit blood to bypass the fetal lungs .
• Just after birth, the baby breathes and expands its lungs,
which pulls more blood into the pulmonary circulation.
• More blood then returns to the left atrium,and a flap on
the left side of the foramen ovale is closed. The ductus
arteriosus constricts, probably in response to the higher
oxygen content of the blood.
BLOOD PRESSURE
Blood pressure is the force the blood exerts against the walls of the
blood vessels
.
-The dynamics of blood flow in blood vessels is similar
to any fluid driven by a pump and the nearer the fluid
is to the pump, the greater the pressure exerted on
the fluid. The blood flows through the blood vessels
along a pressure gradient always moving from higherto lower-pressure areas.
-Systemic blood pressure is highest in the aorta and
declines throughout the pathway to finally reach 0 mm
Hg in the right atrium.
-The steepest drop in blood pressure occurs in the
arterioles, which offer the greatest resistance to blood
flow.
Blood pressure in various blood vessels of the systemic circulation.
Quest ion: In which class of blood vessels does the greatest drop in blood pressure
occur?
Answer: Arterioles, because this is the site of greatest resistance. Arterioles control
the distribution of blood to the tissues by changing their resistance.
Arterial blood pressure
Reflects two factors: (1) how much the elastic
arteries close to the heart can be stretched, and
(2) the volume of blood forced into them.
If the amounts of blood entering and leaving the
elastic arteries in a given period were equal,
arterial pressure would be constant.
Instead, blood pressure rises and falls in a regular
fashion in the elastic arteries near the heart.As the
left ventricle contracts and expels blood into the
aorta, it stretches the elastic aorta and large
arteries. This pressure peak, called the systolic
pressure, averages 120 mm Hg in healthy adults.
• Blood moves forward into the arterial bed because
the pressure in the aorta is higher than the
pressure in the more distal vessels.
• During diastole, the aortic valve closes, preventing
blood from flowing back into the heart, and the
walls of the aorta (and other elastic arteries)
recoil, maintaining sufficient pressure to keep the
blood flowing forward into the smaller vessels.
During this time, aortic pressure drops to its
lowest level (approximately 70 to 80 mm Hg in
healthy adults), called the diastolic pressure.
• The difference between the systolic and diastolic
pressures is called the pulse pressure. It is felt as a
throbbing pulsation in an artery (pulse) during systole,
as the elastic arteries are expanded by the blood being
forced into them by ventricular contraction.
• Increased stroke volume and faster blood ejection
from the heart (a result of increased contractility)
cause temporary increases in the pulse pressure.
Factors that increase BP
1-increased stroke volume.
2-increased heart rate.
3-increased blood viscosity .
4-increased peripheral resistance
• Peripheral resistance: is the friction force created
between the blood and the walls of the blood
vessels which hinders blood flow.
• Viscosity: The greater the viscosity the greater the
resistance to flowing. The presence of blood cells
and plasma proteins increases the viscosity of the
blood hence the greater the force is needed to
move it in the vascular system.
• Therefore, the BP rises as the blood viscosity
increases and drops as viscosity decreases.
• Hypertension is a resting systemic pressure above the
normal range. Clinicians now consider:
• from125 to 139/85 to 89 mmHg to be pre-hypertension.
• A systolic reading of 140 to 159 mmHg or a diastolic reading
of 90 to 99 mmHg may be called stage 1 hypertension, and
a systolic reading above 160 mmHg or a diastolic reading
above 100 mmHg may be called stage 2 hypertension.
• The term “primary or essential hypertension” means that
no specific cause can be determined; most cases are in this
category. For some people, however, the cause of their
hypertension is known ,it is called secondary hypertension.
• Although hypertension often produces no symptoms, the
long-term consequences may be very serious on the heart
and may be fatal(silent killer).
• Although the walls of arteries are strong,
hypertension weakens them and arteries may rupture
or develop aneurysms, which may in turn lead to a
cerebro-vascular accident (CVA) or kidney damage.
• Hypertension let the left ventricle works harder and,
like any other muscle, enlarges as more work is
demanded; this is called left ventricular hypertrophy.
• This abnormal growth of the myocardium, however, is
not accompanied by a corresponding growth in
coronary capillaries, and the blood supply of the left
ventricle may not be adequate for all situations.
• Exercise, for example, puts further demands on the
heart, and the person may experience angina due to
a lack of oxygen or a myocardial infarction if there is a
severe oxygen deficiency.
• Although several different kinds of medications (diuretics,
vasodilators) are used to treat hypertension, people with
moderate hypertension may limit their dependence on
medications by following certain guidelines:
• Don’t smoke, because nicotine stimulates vasoconstriction,
which raises BP. Smoking also damages arteries, contributing to
arteriosclerosis.
• Lose weight if overweight. A weight loss of as little as 10 pounds
can lower BP. A diet high in fruits and vegetables may, for some
people, contribute to lower BP. Saturated fat, and cholesterol
increase the possibilities of hypertension.
• Cut salt intake in half. Although salt consumption may not be the
cause of hypertension, reducing salt intake may help lower blood
pressure by decreasing blood volume.
• Exercise on a regular basis. A moderate amount of aerobic
exercise (such as a half hour walk every day) is beneficial for the
entire cardiovascular system and may also contribute to weight
loss.
• Avoid Stress
• Secondary hypertension, which accounts for 10% of cases,
is due to identifiable disorders, such as obstruction of the
renal arteries, kidney disease, arteriosclerosis, and
endocrine disorders such as hyperthyroidism and Cushing’s
disease. Treatment for secondary hypertension is directed
toward correcting the causative problem.
• The rennin and angiotensin mechanism
1. Decreased blood pressure stimulates the kidneys to secrete
renin.
2. Renin splits the plasma protein angiotensinogen
(synthesized by the liver) to angiotensin I.
3. Angiotensin I is converted to angiotensin II by an enzyme
(called converting enzyme) secreted by lung tissue and
vascular endothelium.
4. Angiotensin II:
-causes vasoconstriction
-stimulates the adrenal cortex to secrete aldosterone
• Hypotension is a systolic pressure below 100 mm Hg. Low
blood pressure is often associated with old age free of
cardiovascular illness.
• Elderly people are prone to orthostatic hypotension—
temporary low blood pressure and dizziness when they rise
suddenly. Because the aging sympathetic nervous system
blood pools briefly in the lower limbs, reducing blood delivery
to the brain. Making postural changes slowly usually prevents
this problem.
• Chronic hypotension may indicate poor nutrition because
poorly nourished people are often anemic and have
inadequate levels of blood proteins so blood viscosity is low.
Chronic hypotension also warns of Addison’s disease
(inadequate adrenal cortex function), hypothyroidism, or
severe tissue wasting.
• Acute hypotension is one of the most important signs of
circulatory shock and a threat to patients undergoing surgery
and those in intensive care units.
1.
2.
3.
4.
Developmental Aspects of Blood Vessels
The fetal vasculature is functioning in blood delivery
by the fourth week.
Fetal circulation differs from circulation after birth.
The pulmonary and hepatic shunts and special
umbilical vessels are normally occluded shortly after
birth.
Blood pressure is low in infants and rises to adult
values.
vascular problems include varicose veins,
hypertension, and atherosclerosis. Hypertension is
the most important cause of sudden cardiovascular
death in middle-aged men. Atherosclerosis is the
most important cause of cardiovascular disease in the
aged.
AGING AND THE VASCULAR SYSTEM
• It is believed that the aging of blood vessels, especially
arteries, begins in childhood, although the effects are
not apparent for decades.
• The cholesterol deposits of atherosclerosis are to be
expected with advancing age, with the most serious
consequences in the coronary arteries.
• The veins also deteriorate with age; their thin walls
weaken and stretch, making their valves incompetent.
This is most likely to occur in the veins of the legs; their
walls are subject to great pressure as blood is returned
to the heart against the force of gravity. Varicose veins
and phlebitis are more likely to occur among elderly
people.
HOMEOSTATIC IMBALANCE
• Each year about 30,000 infants are born in the U.S. with one or
more of 30 different congenital heart defects, making them the
most common of all birth defects.
• Most congenital heart problems are traceable to environmental
influences, such as maternal infection or drug intake during
month 2 when the major events of heart formation occur.
• The most prevalent abnormalities produce two basic kinds of
disorders in the newborn. They either :
(1) lead to mixing of oxygen-poor systemic blood with oxygenated
pulmonary blood (so that inadequately oxygenated blood reaches
the body tissues) as septal defects and patent ductus arteriosis.
(2) involve narrowed valves or vessels that greatly increase the
workload on the heart as coarctation of the aorta .
Modern surgical techniques can correct most of these heart
defects.