Cardiovascular System: The Heart
Cardiovascular System: Blood Vessels
Developmental Aspects of the Cardiovascular System
- The heart, located in the thorax, is flanked laterally by the lungs
and enclosed in a pericardium.
- The bulk of the heart (myocardium) is composed of cardiac muscle.
The heart has four hollow chambers: two atria (receiving chambers) and
two ventricles (discharging chambers), each lined with endocardium (Figure 11.1). The heart is divided longitudinally by a septum.
- The heart functions as a double pump. The right heart is the pulmonary
circuit (right heart to lungs to left heart). The left heart is the
systemic circuit (left heart to body tissues to right heart).
- Four valves prevent backflow of blood in the heart. The AV valves
(mitral and tricuspid) prevent backflow into the atria when the ventricles
are contracting. The semilunar valves prevent backflow into the ventricles
when the heart is relaxing. The valves open and close in response to
pressure changes in the heart.
- The myocardium is nourished by the coronary circulation, which
consists of the right and left coronary arteries and their branches,
and drained by the cardiac veins and the coronary sinus.
- The time and events occurring from one heartbeat to the next is
the cardiac cycle.
- As the heart beats, sounds resulting from the closing of the valves
("lub-dup") can be heard. Faulty valves reduce the efficiency of the
heart as a pump and result in abnormal heart sounds (murmurs).
- Cardiac muscle is able to initiate its own contraction in a regular
way, but its rate is influenced by both intrinsic and extrinsic factors.
The intrinsic conduction system increases the rate of heart contraction
and ensures that the heart beats as a unit. The SA node is the heart's
pacemaker (Figure 11.2).
- Cardiac output, the amount of blood pumped out by each ventricle
in one minute, is the product of heart rate (HR) X stroke volume (SV).
SV is the amount of blood ejected by a ventricle with each beat.
- SV rises or falls with the volume of venous return. HR is influenced
by the nerves of the autonomic nervous system, drugs (and other chemicals),
and ion levels in the blood.
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- Arteries, which transports blood away from the heart, and veins,
which carry blood back to the heart, are conducting vessels. Only capillaries
play a role in actual exchanges with tissue cells.
- Except for capillaries, blood vessels are composed of three tunics
(Figure 11.3): The tunica intima
forms a friction-reducing lining for the vessel. The tunica media is
the bulky middle layer of muscle and elastic tissue. The tunica externa
is the protective, outermost connective tissue layer. Capillary walls
are formed of the intima only.
- Artery walls are thick and strong to withstand pressure fluctuations.
They expand and recoil as the heart beats. Vein walls are thinner, their
lumens are larger, and they are equipped with valves. These modifications
reflect the low-pressure nature of veins.
- All the major arteries of the systemic circulation are branches
of the aorta, which leaves the left ventricle. They branch into smaller
arteries and then into the arterioles, which feed the capillary beds
of the body tissues. For the names and locations of the systemic arteries
see (Figure 11.4).
- The major veins of the systemic circulation ultimately converge
on one of the venae cavae. All veins above the diaphragm drain into
the superior vena cava, and those below the diaphragm drain into the
inferior vena cava. Both venae cavae enter the right atrium of the heart.
- The arterial circulation of the brain is formed by branches of
paired vertebral and internal carotid arteries. The circle of Willis
provides alternate routes for blood flow in case of a blockage in the
brain's arterial supply.
- The hepatic portal circulation is formed by veins draining the
digestive organs, which empty into the hepatic portal vein. The hepatic
portal vein carries the nutrient-rich blood to the liver, where it is
processed before the blood is allowed to enter the systemic circulation
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- The fetal circulation is a temporary circulation seen only in the
fetus. It consists primarily of three special vessels: The single umbilical
vein that carries nutrient- and oxygen-laden blood to the fetus from
the placenta, and the two umbilical arteries that carry carbon dioxide
and waste-laden blood from the fetus to the placenta. Shunts bypassing
the lungs and liver are also present.
- Blood pressure is the pressure that blood exerts on the walls of
the blood vessels. It is the force that causes blood to continue to
flow in the blood vessels. It is highest in the arteries, lower in the
capillaries, and lowest in the veins. Blood is forced along a descending
pressure gradient. Both systolic and diastolic pressures are recorded.
- The pulse is the alternate expansion and recoil of a blood vessel
wall (the pressure wave) that occurs as the heart beats. It may be felt
easily over any superficial artery; such sites are called pressure points.
- Arterial blood pressure is directly influenced by heart activity
(increased heart rate leads to increased blood pressure) and by resistance
to blood flow. The most important factors increasing the peripheral
resistance are a decrease in the diameter or stretchiness of the arteries
and arterioles, and an increase in blood viscosity.
- Many factors influence blood pressure. Some of these factors are
the activity of the sympathetic nerves and kidneys, drugs, and diet.
- Hypertension, which reflects an increase in peripheral resistance,
strains the heart and damages blood vessels. In most cases, the precise
cause is unknown.
- Substances move to and from the blood and tissue cells through
capillary walls. Some substances are transported in vesicles, but most
move by diffusion directly through the endothelial cell plasma membranes,
through intercellular clefts, or through fenestrations. Fluid is forced
from the bloodstream by blood pressure and drawn back into the blood
by osmotic pressure.
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- The heart begins as a tubelike structure that is beating and pumping
blood by the fourth week of embryonic development,
- Congenital heart defects account for half of all infant deaths
resulting from congenital problems.
- Varicose veins, a structural defect due to incompetent valves,
is a common vascular problem, especially in the obese and people who
stand for long hours. It is a predisposing factor for thrombophlebitis.
- Arteriosclerosis is an expected consequence of aging. Gradual loss
of elasticity in the arteries leads to hypertension and hypertensive
heart disease, and clogging of the vessels with fatty substances leads
to coronary artery disease and stroke. Cardiovascular disease is an
important cause of death in individuals over age 65.
- Modifications in diet (decreased fats, cholesterol, and salt),
stopping smoking, and regular aerobic exercise may help to reverse the
atherosclerotic process and prolong life.