A. Atrial natriuretic peptide, which decreases blood pressure, is produced mainly by car- diac muscle cells of the right atrium. It inhibits the release of renin and causes the kidneys to decrease the resorption of sodium and water (see Chapter 11IA1b).

D. The muscular artery possesses a distinct internal elastic lamina. Elastic arteries possess an incomplete internal elastic lamina, whereas capillaries, arterioles, metarterioles, and veins do not possess an internal elastic lamina (see Table 11.1 and Chapter 11 ID).

B. The pericardium is a fibroserous sac that encloses the heart. The innermost layer of the peri- cardium, the epicardium, is also known as the visceral pericardium (see Chapter 11 I A 1).

C. Vasa vasorum are the small blood vessels that serve the walls of elastic and muscular arter- ies with oxygen and nutrients just as the coronary arteries provide for the walls of the heart (see Chapter 11 1 B 1).

4. C. The proximal portion of a central channel is known as a metarteriole, whereas its distal portion is the thoroughfare channel. Blood from metarterioles may enter the capillary bed if their precapillary sphincters are relaxed. If the precapillary sphincters of metarterioles are con- stricted, blood bypasses the capillary bed and flows directly into thoroughfare channels and from there into a venule (see Chapter 11 1 B 1 d).

A. Somatic (continuous) capillaries lack fenestrae, whereas visceral (fenestrated) capillaries are characterized by their presence. Both types of capillary possess a continuous basal lamina and are surrounded by occasional pericytes (see Chapter 11 IC 2).

D. Impulses are generated in the sinoatrial node, which is the pacemaker of the heart. They are then conducted to the AV node. The bundle of His and Purkinje fibers conduct impulses from the AV node to the cardiac muscle cells of the ventricles. Sympathetic nerves can increase the rate of the heartbeat but do not originate it (see Chapter 11 IA 4 a).

D. Capillaries in the CNS are of the continuous type and thus lack fenestrae but have a continu- ous basal lamina. In contrast to continuous capillaries in other parts of the body, they contain only a few pinocytic vesicles; this characteristic is thought to be partly responsible for the blood-brain barrier (see Chapter 11 IC 2).

. C. Acetylcholine stimulates the endothelial cells of a vessel to release nitric oxide (EDRF), which causes relaxation of smooth muscle cells. Thus, acetylcholine does not act directly on smooth muscle cells (see Chapter 111 B 2).

D. The smooth endothelial lining of intact, healthy capillaries inhibits clot formation. Capillaries do not control blood pressure (see Chapter 11 IC 2).

E. Ischemic heart disease is usually caused by coronary atherosclerosis, resulting in decreased blood flow to the myocardium. It may develop into angina pectoris, myocardial infarction, chronic ischemic cardiopathy, or sudden cardiac death. On the other hand, arteriosclerosis (hardening of the arteries) usually attacks renal arteries and is related to hypertension and diabetes mellitus (see Chapter 11 1 B Clinical Considerations).

A. A person who has had rheumatic fever as a child may develop heart valve disease later in life. Although the mitral valve is the one most commonly affected, the other valves may also be involved. The mitral valve becomes inflamed, fibrotic, and eventually incompetent or ste- notic. This condition leads to respiratory hypertension and edema, which restricts respiratory function. The key is a history of rheumatic fever, because those individuals are predisposed to developing heart valve diseases (see Chapter 11 IA 3 Clinical Considerations).

B. Diagnostic tests for ischemic heart disease reveal atherosclerosis of the coronary vessels. Over time, excessive plaque composed of cholesterol and fats is layered beneath the intima of these vessels, thus restricting blood flow to the myocardium of the heart, leading to angina pectoris, an infarction, or perhaps sudden death (see Chapter 11 IB 1 Clinical Considerations).