Chapter 21
I.
INTRODUCTION
A. One main focus of this chapter
considers hemodynamics, 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 are surveyed.
II. STRUCTURE AND FUNCTION OF BLOOD
VESSELS
A. Blood vessels form a closed system of tubes that
carry blood away from the heart, transport it to the tissues of the body, and
then return it to the heart.
1. Arteries carry blood from the heart to the
tissues.
2. Arterioles are small arteries that connect to
capillaries.
3. Capillaries are the site of substance exchange
between the blood and body tissues.
4. Venules connect capillaries to larger veins.
5. Veins convey blood from the tissues back
to the heart.
6. Vasa vasorum are small blood vessels that supply
blood to the cells of the walls of the arteries and veins.
B. 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 a tunic externa. You should know the histology of these three
layers.
b. The functional properties of arteries
are elasticity and contractility.
1) Elasticity, due to the elastic tissue in the
tunica internal and media, allows arteries to accept blood under great pressure
from the contraction of the ventricles and to send it on through the system.
2) Contractility, due to the smooth muscle in the
tunica media, allows arteries to increase or decrease lumen size and to limit
bleeding from wounds.
2.
Elastic
Arteries
a. Large arteries with more elastic
fibers and less smooth muscle are called elastic arteries and are able
to receive blood under pressure and propel it onward.
b. They are also called conducting
arteries because they conduct blood from the heart to medium sized muscular
arteries.
c. They function as a pressure reservoir.
3. Muscular arteries have a large amount of smooth muscle
in their walls and distribute blood to various parts of the body.
C. 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.
D. Capillaries are microscopic vessels that usually
connect arterioles and venules.
1. The flow of blood through the
capillaries is called the microcirculation.
2. Capillaries are found near almost
every cell in the body, but their distribution varies with the metabolic
activity of the tissue.
3. The primary function of capillaries
is to permit the exchange of nutrients and wastes between the blood and tissue
cells through interstitial fluid.
4. Capillary walls are composed of only
a single layer of cells (endothelium) and a basement membrane.
5. 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. You should know what metarterioles, thoroughfare channels and true capillaries are.
d. There are three types of capillaries:
continuous, fenestrated, and sinusoids
E. 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.
F. 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. They
contain valves to prevent the backflow of blood.
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.
3. Weak valves can lead to varicose
veins. (Clinical Application)
G. Anastomoses
1. Anastomoses are the union of the branches of two or more arteries
supplying the same region.
2. They provide alternate routes for
blood to reach a tissue or organ.
3. The collateral circulation is
the alternate flow of blood to a body part through an anastomosis.
4. Arteries that do not anastomose are known as end arteries. Occlusion of
an end artery interrupts the blood supply to a whole segment of an organ,
producing necrosis (death) of that segment.
5. Anastomoses can also consist of two veins or a
vein and an artery.
H. Blood Distribution
1. 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.
2. 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, transcytosis,
and bulk flow (filtration and absorption).
B. The most important method of
capillary exchange is simple diffusion.
1. Substances such as O2, CO2,
glucose, amino acids, hormones, and others diffuse down their concentration
gradients.
2. All plasma solutes, except larger
proteins, pass freely across most capillary walls.
3. The prime exception of diffusion of
water-soluble materials across capillary walls is in the brain where the
blood-brain barrier exists.
C. Some materials cross the capillary
membrane by transcytosis, the enclosing of
substances within tiny vesicles that enter cells by endocytosis.
D. Bulk Flow: Filtration and Reabsorption
1. Whereas diffusion is more important
for solute exchange between plasma and interstitial fluid, bulk flow is
more important for regulation of the relative volumes of blood and interstitial
fluid.
2. The movement of water and dissolved
substances (except proteins) through capillaries is dependent upon hydrostatic
and osmotic pressures.
3. 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. You should know the formula for net filtration pressure
and what each of the pressures represents. Understand what is occurring at the
venous and arteriole ends of the capillary in terms of fluid movement and its
significance to homeostasis. The function
of the lymph capillaries is to return any excess fluid to the blood that has
not been reabsorbed at the venous end of the blood capillary. See
lecture notes for further detail.
4. 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 including
increased blood hydrostatic pressure in capillaries due to an increase in
venous pressure, decreased concentration of plasma proteins that lower blood
colloid osmotic pressure, increased
permeability of capillaries which allows greater amounts of plasma
proteins to leave the blood and enter tissue fluid, increased extracellular
fluid volume as a result of fluid retention, and blockage of lymphatic vessels
postoperatively or due to filarial worm infection.
IV. HEMODYNAMICS: FACTORS AFFECTING BLOOD FLOW
A. The distribution of cardiac output to
various tissues depends on the interplay of the pressure difference that drives
the blood flow and the resistance to blood flow.
B. Blood pressure (BP) is the pressure exerted on the
walls of a blood vessel; in clinical use, BP refers to pressure in arteries.
1. Cardiac output (CO) equals mean
aortic blood pressure (MABP) divided by total resistance (R). MABP = diastolic
BP + pulse pressure/3
(Remember that pulse pressure =
systolic BP – diastolic BP)
2. Factors that affect blood pressure
include cardiac output, blood volume, viscosity, resistance, and elasticity of
arteries. See lecture notes for further
detail.
3. As blood leaves the aorta and flows
through systemic circulation, its pressure progressively falls to 0 mm Hg by
the time it reaches the right atrium.
C. Resistance refers to the opposition to blood
flow as a result of friction between blood and the walls of the blood vessels.
1. Vascular resistance depends on the
diameter of the blood vessel, blood viscosity, and total blood vessel length. See lecture notes for further detail.
2. Systemic vascular resistance (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.
D. Venous Return
1. Venous return occurs because 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.
See lecture notes for further detail.
E. 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.
V. CONTROL OF BLOOD PRESSURE AND BLOOD
FLOW
A. Role of the Cardiovascular Center
1. The cardiovascular center (CV)
is a group of neurons in the medulla that regulates heart rate, contractility,
and blood vessel diameter.
2. The CV receives input from higher
brain regions and sensory receptors (baroreceptors
and chemoreceptors).
3. Output from the CV flows along
sympathetic and parasympathetic fibers.
a. Sympathetic impulses along cardioaccelerator nerves increase heart rate and
contractility.
b. Parasympathetic impulses along vagus 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. Since there is no
parasympathetic nerves going to blood vessel smooth muscle, vasodilation occurs by inhibiting sympathetic stimulation
via vasomotor nerves.
B. Neural Regulation of Blood Pressure
1. Baroreceptors 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.
2. Receptors sensitive to chemicals are
called chemoreceptors.
a. These receptors are located close to
the baroreceptors of the carotid sinus and arch of
the aorta. They are called carotid bodies and aortic bodies.
b. They monitor blood levels of oxygen,
carbon dioxide, and hydrogen ion concentration. See lecture notes for further detail.
C. Hormonal Regulation
1. Hormones such as angiotensin
II, epinephrine, norepinephrine, antidiuretic
hormone, and atrial natriuretic
peptide affect blood pressure and blood flow by altering cardiac output,
changing systemic vascular resistance by vasoconstriction (angiotensinII,
epinephrine, norepinephrine, antidiuretic
hormone, and vasodilation (atrial
natriuretic peptide), or adjusting the total blood
volume.
D. Autoregulation of Blood Pressure
1. The ability of a tissue to
automatically adjust its own blood flow to match its metabolic demand for
supply of O2 and nutrients and removal of wastes is called autoregulation.
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. See lecture notes for further detail.
VI. 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 depicts the most common pulse points.
2. A normal resting pulse (heart) rate
is between 70 to 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
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 is the force of blood recorded
during ventricular contraction.
b. Diastolic blood pressure 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 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
routes that deliver blood throughout the body.
2. The largest circulatory route is the
systemic circulation.
3. 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 caveae
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. The pulmonary and systemic
circulations differ from each other in several more ways.
a. Blood in the pulmonary circulation is
not pumped so far as in the systemic circulation and the pulmonary arteries
have a larger diameter, thinner walls, and less elastic tissue. As a result,
resistance to blood flow is very low meaning that less pressure is needed to
move blood through the lungs.
b. Because resistance in the pulmonary
circulation is low, normal pulmonary capillary hydrostatic pressure is lower
than systemic capillary hydrostatic pressure which tends to prevent pulmonary
edema.
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. Blood passes from the fetus to the
placenta via two umbilical arteries and returns from the placenta via a single
umbilical vein.
4. At birth, when pulmonary, digestive,
and liver functions are established, the special structures of fetal
circulation are no longer needed.
a. The ductus
arteriosus becomes the ligamentum
arteriosum shortly after birth.
b. The foramen ovale
becomes the fossa ovalis
shortly after birth.
c. The ductus
venosus becomes the ligamentum
venosum shortly after birth.
d. The umbilical arteries become
the medial umbilical ligaments.
e. The umbilical vein becomes the
ligamentum teres
(round ligament).