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Cardiovascular System

Cardiovascular System: The Heart

1. The heart, located in the thorax, is flanked laterally by the lungs and enclosed in a pericardium.

2. 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.

3. 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).

4. 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.

5. 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.

6. The time and events occurring from one heartbeat to the next is the cardiac cycle.

7. 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).

8. 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).

9. 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.

10. 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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Cardiovascular System: Blood Vessels

1. 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.

2. 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.

3. 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.

4. 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).

5. 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.

6. 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.

7. 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 (Figure 11.5).

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8. 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.

9. 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.

10. 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.

11. 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.

12. Many factors influence blood pressure. Some of these factors are the activity of the sympathetic nerves and kidneys, drugs, and diet.

13. Hypertension, which reflects an increase in peripheral resistance, strains the heart and damages blood vessels. In most cases, the precise cause is unknown.

14. 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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Developmental Aspects of the Cardiovascular System

1. The heart begins as a tubelike structure that is beating and pumping blood by the fourth week of embryonic development,

2. Congenital heart defects account for half of all infant deaths resulting from congenital problems.

3. 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.

4. 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.

5. Modifications in diet (decreased fats, cholesterol, and salt), stopping smoking, and regular aerobic exercise may help to reverse the atherosclerotic process and prolong life.