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Transcript
Chapter 18 / The Cardiovascular System: Blood Vessels and Circulation
I. INTRODUCTION
A. One main focus of this chapter considers hemodynarnics, the means by which
blood
flow is altered and distributed and by which blood pressure is regulated.
B. The histology of blood vessels and anatomy of the primary routes of arterial and
venous
systems is surveyed.
II. ANATOMY OF BLOOD VESSELS
A. Arteries
1. Arteries carry blood away from the heart to the tissues.
a. The wall of an artery consists of a tunica interna, tunica media (which
maintains
elasticity and contractility), and tunica extema.
b. Large arteries are referred to as elastic (conducting) arteries, and medium
sized
arteries are called muscular (distributing) arteries.
2. Many arteries anastomose--the distal ends of two or more vessels unite.
a. Anastomoses between arteries provide alternate routes for blood to reach a
tissue
or organ.
b. Thus, if a vessel is blocked by disease, injury, or surgery, circulation to a part
of
the body is not necessarily stopped.
1) An alternate blood route from an anastomosis is called collateral
circulation.
2) Arteries that do not anastomose are called end arteries. Occlusion of an end
artery interrupts the blood supply to a whole segment of an organ,
producing
necrosis (death) of that segment.
B. Arterioles
1. Arterioles are very small, almost microscopic, arteries that deliver blood to
capillaries.
2. Through vasoconstriction (decrease in the size of the lumen of a blood vessel) and
vasodilation (increase in the size of the lumen of a blood vessel), arterioles
assume a
key role in regulating blood flow from arteries into capillaries and in altering
arterial
blood pressure.
C. Capillaries
1. Capillaries are microscopic blood vessels through which materials are exchanged
between blood and tissue cells.
a. They usually connect arterioles and venules.
b. Capillary walls are composed of only a single layer of cells (endothelium) and
a
basement membrane.
2. Capillaries branch to form an extensive capillary network throughout the tissue.
This
network increases the surface area, allowing a rapid exchange of large quantities
of
materials.
a. The flow of blood through capillaries is regulated by vessels with smooth
muscle in
their walls.
b. Rings of smooth muscle fibers (cells) called precapillary sphincters regulate
blood
flow through true capillaries.
c. Some capillaries are continuous, whereas others are fenestrated .
d. Microscopic blood vessels in organs such as the liver, spleen, and bone marrow
are
called sinusoids ; they are wider than capillaries, more tortuous, and specialized
for
the functions of the specific organs.
D. Venules
1. Venules are small vessels that are formed from the union of several capillaries;
venules merge to form veins.
2. They drain blood from capillaries into veins.
E. Veins
1. Veins consist of the same three tunics as arteries but have a thinner tunica
interna
and media and a thicker tunica externa; they have less elastic tissue and smooth
muscle and are therefore thinner-walled than arteries .
a. They contain valves to prevent backflow of blood.
b. Weak valves can lead to varicose veins.
2. Vascular (venous) sinuses are veins with very thin walls with no smooth muscle
to
alter their diameters. Examples are the brain's superior sagittal sinus and the
coronary
sinus of the heart.
F. Blood Distribution
1. The volume of blood in various parts of the cardiovascular system varies
considerably.
2. At rest, the largest portion of the blood is in systemic veins and venules,
collectively
called blood reservoirs.
a. They store blood and, through venous vasoconstriction, can move blood to
other
parts of the body if the need arises.
b. In cases of hemorrhage, when blood pressure and volume decrease,
vasoconstriction of veins in venous reservoirs helps to compensate for the
blood
loss.
3. The principal reservoirs are the veins of the abdominal organs (liver and spleen)
and
skin.
III. CAPILLARY EXCHANGE
A. Substances enter and leave capillaries by diffusion, vesicular transport
(endocylosis and
exocytosis), and bulk flow (filtration and absorption).
B. The movement of water and dissolved substances (except proteins) through
capillaries
is dependent upon hydrostatic and osmotic pressures.
1. The near equilibrium at the arterial and venous ends of a capillary by which
fluids
exit and enter is called Starling's law of the capillaries
2. Occasionally, the balance of filtration and reabsorption between interstitial fluid
and plasma is disrupted, allowing an abnormal increase in interstitial fluid
called
edema. Edema may be caused by several factors:
a. Increased blood hydrostatic pressure in capillaries due to an increase in
venous
pressure.
b. Decreased concentration of plasma proteins that lower blood colloid osmotic
pressure.
c. Increased permeability of capillaries, allowing greater amounts of plasma
proteins
to leave the blood and enter tissue fluid.
d. Increased extracellular fluid volume as a result of fluid retention.
e. Blockage of lymphatic vessels postoperatively or due to filarial worm
infection.
IV. HEMODYNAMICS: PHYSIOLOGY OF CIRCULATION
A. Velocity of Blood Flow
1. The volume that flows through any tissue in a given period of time is blood
flow.
2. The velocity of blood flow is inversely related to the cross-sectional area of
blood
vessels; blood flows most slowly where cross-sectional area is greatest.
3. Blood flow decreases from the aorta to arteries to capillaries and increases as it
returns to the heart.
B. Volume of Blood Flow
1. Blood flow is determined by blood pressure and resistance.
2. Blood flows from regions of higher to lower pressure; the higher the resistance
the lower the blood flow.
C. Blood pressure (BP) is the pressure exerted on the walls of a blood vessel; in
clinical
use, BP refers to pressure in arteries.
1. Factors that affect blood pressure include cardiac output, blood volume,
viscosity,
resistance, and elasticity of arteries.
2. As blood leaves the aorta and flows through systemic circulation, its pressure
progressively falls to 0 mm Hg (Mercury) by the time it reaches the right
atrium.
D. Resistance refers to the opposition to blood flow as a result of friction between
blood
and the walls of the blood vessels.
1. Resistance depends on blood viscosity, blood vessel length, and blood vessel
radius.
2. Systemic vascular resistance (SVR) (also known as total peripheral resistance)
refers
to all of the vascular resistances offered by systemic blood vessels; most
resistance
is in arterioles, capillaries, and venules due to their small diameters.
E. Venous Return
1. A number of factors aid venous return, the volume of blood flowing back to the
heart from the systemic veins, by increasing the magnitude of the pressure
gradient
between the venules and the right atrium.
2. Blood return to the heart is maintained by several factors, including skeletal
muscular
contractions, valves in veins (especially in the extremities), and pressure
changes
associated with breathing.
V. CONTROL OF BLOOD PRESSURE AND BLOOD FLOW
A. Cardiovascular Center
1. The cardiovascular (CV) center is a group of neurons in the medulla that
regulates
heart rate, contractility, and blood vessel diameter.
2. The CV center receives input from higher brain regions and sensory receptors
(baroreceptors and chemoreceptors).
3. Output from the CV center flows along sympathetic and parasympathetic
fibers.
a. Sympathetic impulses along cardioaccelerator nerves increase heart rate and
contractility.
b. Parasympathetic impulses along vagus (X) nerves decrease heart rate.
c. The sympathetic division also continually sends impulses to smooth muscle
in blood vessel walls via vasomotor nerves. The result is a moderate state of
tonic contraction or vasoconstriction, called vasomotor tone.
B. Neural Regulation of Blood Pressure
1. Baroreceptors (pressoreceptors) are important pressure-sensitive sensory
neurons
that monitor stretching of the walls of blood vessels and the atria.
a. The cardiac sinus reflex is concerned with maintaining normal blood
pressure
in the brain and is initiated by baroreceptors in the wall of the carotid sinus.
b. The aortic reflex is concerned with general systemic blood pressure and is
initiated by baroreceptors in the wall of the arch of the aorta or attached to
the
arch.
c. If blood pressure falls, the baroreceptor reflexes accelerate heart rate,
increase
force of contraction, and promote vasoconstriction.
d. The right heart (atrial) reflex responds to increases in venous blood pressure
and
is initiated by baroreceptors in the right atrium and venae cavae.
2. Receptors sensitive to chemicals are called chemoreceptors.
a. They monitor blood levels of oxygen, carbon dioxide, and hydrogen ion
concentration.
C. Hormonal Regulation of Blood Pressure
1. Several hormones affect blood pressure and flow by acting on the heart,
altering
blood vessel diameter, or adjusting the total blood volume.
a. Among the hormones that help regulate blood pressure are epinephrine,
norepinephrine (NE), antidiuretic hormone (ADH), angiotensin, atrial
natriuretic
Deptide (ANP), histamine, and kinins.
D. Autoregulation of Blood Pressure
1. Autoregulation refers to local, automatic adjustments of blood flow in a given
region
to match the particular needs of the tissue.
2. In most body tissues, oxygen is the principal, though not direct, stimulus for
autoregulation.
3. Researchers have identified two general types of stimuli that cause
autoregulatory
changes in blood flow– physical and chemical.
VI. SHOCK AND HOMEOSTASIS
A. Shock is an inadequate cardiac output that results in failure of the cardiovascular
system
to deliver adequate amounts of oxygen and nutrients to meet the metabolic needs of
body cells. As a result, cellular membranes dysfunction, cellular metabolism is
abnormal,
and cellular death may eventually occur without proper treatment.
B. Signs and symptoms of shock include clammy, cool, pale skin; tachycardia; weak,
rapid
pulse; sweating; hypotension (systemic pressure < 90 mm Hg); altered mental
status;
decreased urinary output; thirst; and acidosis.
C. Stages of shock are all characterized by inadequate perfusion of tissues.
1. The stages of shock are described in the textbook as they apply to hypovolemic
shock,
since it has been studied so extensively. Hypovolemic shock refers to decreased
blood
volume resulting from loss of blood or plasma due to acute hemorrhage or
excessive
fluid loss (as in excess vomiting, diarrhea, sweating, dehydration, urine
production,
and burns).
2. The development of shock occurs in three principal stages, which merge with one
another.
a. Stage I is compensated (nonprogressive) shock, in which negative feedback
systems
restore homeostasis.
1) If the initiating cause does not get any worse, a full recovery follows.
2) Compensatory adjustments include activation of the sympathetic division of
the
ANS, the renin-angiotensin pathway, release of antidiuretic hormone (ADH),
and
release of vasodilator factors in response to hypoxia.
b. Stage II is decompensated (progressive) shock, in which positive feedback
cycles
intensify the shock and immediate medical intervention is required.
1) It occurs when there has been a reduction in blood volume of 15-25%.
2) Among the positive feedback cycles that contribute to decreased cardiac
output
and blood pressure are depression of cardiac activity, depression of
vasoconstriction, increased permeability of capillaries, intravascular clotting,
cellular
destruction, and acidosis.
c. Stage III is irreversible shock, in which there is rapid deterioration of the
cardiovascular system than cannot be helped by compensatory mechanisms or
medical intervention.
VII. CHECKING CIRCULATION
A. Pulse
1. Pulse is the alternate expansion and elastic recoil of an artery wall with each
heartbeat.
It may be felt in any artery that lies near the surface or over a hard tissue, and is
strongest in the arteries closest to the heart; the radial artery is most commonly
used,
to feel the pulse.
2. A normal resting pulse (heart) rate is between 70 and 80 beats per minute.
a. Tachycardia means a rapid resting heart or pulse rate (> 100 beats/min).
b. Bradycardia indicates a slow resting heart or pulse rate (< 60 beats/min).
B. Measurement of Blood Pressure (BP)
1. Blood pressure is the pressure exerted by blood on the wall of an artery when
the left ventricle undergoes systole and then diastole. It is measured by the use
of a sphygmomanometer, usually in one of the brachial arteries.
a. Systolic blood pressure (SBP) is the force of blood recorded during
ventricular
contraction.
b. Diastolic blood pressure (DBP) is the force of blood recorded during
ventricular
relaxation.
c. The various sounds that are heard while taking blood pressure are called
Korotkoff sounds.
d. The normal blood pressure of a young adult male is 120/80 mm Hg (8- 10
mm
Hg less in a young adult female). The range of average values varies with
many
factors.
2. Pulse pressure (PP) is the difference between systolic and diastolic pressure. It
normally is about 40 mm Hg and provides information about the condition of
the
arteries.
VIII. CIRCULATORY ROUTES
A. Introduction
1. The blood vessels are organized into parallel routes that deliver blood throughout
the
body.
2. The largest circulatory route is the systemic circulation.
3. Two of the several subdivisions of the systemic circulation are coronary (cardiac)
circulation, which supplies the myocardiurn of the heart; the cerebral circulation,
which
supplies the brain; and the hepatic portal circulation, which extends from the
gastrointestinal tract to the liver.
4. Other routes include pulmonary circulation and fetal circulation.
B. Systemic Circulation
1. The systemic circulation takes oxygenated blood from the left ventricle through
the
aorta to all parts of the body, including some lung tissue (but does not supply the
air
sacs of the lungs) and returns the deoxygenated blood to the right atrium.
a. The aorta is divided into the ascending aorta, arch of the aorta, and the
descending
aorta.
b. Each section gives off arteries that branch to supply the whole body.
2. Blood returns to the heart through the systemic veins. All the veins of the
systemic
circulation flow into the superior or inferior venae cavae or the coronary sinus,
which
in turn empty into the right atrium.
C. Hepatic Portal Circulation
1. The hepatic portal circulation collects blood from the veins of the pancreas,
spleen,
stomach, intestines, and gallbladder and directs it into the hepatic portal vein of
the
liver before it returns to the heart.
2. A portal system carries blood between two capillary networks, in this case from
capillaries of the gastrointestinal tract to sinusoids of the liver.
3. This circulation enables nutrient utilization and blood detoxification by the liver.
D. Pulmonary Circulation
1. The pulmonary circulation takes deoxygenated blood from the right ventricle to
the air
sacs of the lungs and returns oxygenated blood from the lungs to the left atrium.
2. It allows blood to be oxygenated for systemic circulation.
E. Fetal Circulation
1. The fetal circulation involves the exchange of materials between fetus and
mother.
2. The fetus derives its oxygen and nutrients and eliminates its carbon dioxide and
wastes through the maternal blood supply by means of a structure called the
placenta.
3. At birth, when pulmonary (lung), digestive, and liver functions are established,
the
special structures of fetal circulation are no longer needed.
IX. AGING AND THE CARDIOVASCULAR SYSTEM
A. General changes associated with aging and the cardiovascular system include loss
of compliance (extensibility) of the aorta, reduction in cardiac muscle fiber (cell)
size,
progressive loss of cardiac muscular strength, reduced cardiac output, a decline in
maximum heart rate, and increased systolic blood pressure.
B. The incidence of coronary artery disease (CAD), congestive heart failure (CHF),
and atherosclerosis increases with age.
X. DEVELOPMENTAL ANATOMY OF BLOOD VESSELS AND BLOOD
A. Blood vessels develop from isolated masses of mesenchyme in the mesoderm
called
blood islands.
1. Spaces soon appear in the islands and become the lumens of the blood vessels.
2. Some of the mesenchymal cells immediately around the spaces give rise to the
endothelial lining of the blood vessels.
B. Blood plasma and blood cells are produced by the endothelial cells of blood
vessels,
a function later assumed by the liver, spleen, bone marrow, and lymph nodes.
XI. DISORDERS: HOMEOSTATIC IMBALANCES
A. Hypertension, or high blood pressure, is the most common disease affecting the
heart
and blood vessels. It is classified as primary (essential) or secondary.
1. Primary (essential) hypertension (approximately 90-95% of all hypertension
cases)
is a persistently elevated blood pressure that cannot be attributed to any
particular
organic cause.
2. Only secondary hypertension has causes that are identifiable. Two of these are
kidney
disease and adrenal hypersecretion.
3. A number of treatment strategies are effective in lowering blood pressure.
B. An aneurysm is a thin, weakened section of the wall of an artery or a vein that
bulges
outward, forming a balloonlike sac of the blood vessel. Left untreated, it may
burst,
causing massive hemorrhage with shock, severe pain, cardiovascular accident
(CVA,
or stroke), or death. Some may be surgically repaired.
C. Coronary artery disease (CAD) is a condition in which the heart muscle receives
inadequate oxygen due to blockage of its blood flow. It may be caused by
atherosclerosis, coronary artery spasm, and other factors.
D. Deep-venous thrombosis (DVT) is a blood clot (thrombus) in a deep vein,
especially
in the lower extremities. It has two serious complications: pulmonary embolism
and
postphlebitic syndrome.