Figure 129

Electron micrograph and photomicrograph of arterioles, a. This electron micrograph shows a cross section of an arteriole. The tunica in-tima of the vessel is composed of an endothelium and a very thin layer of subendothelial connective tissue (collagen fibrils and ground substance). The arrows indicate the site of junction between adjoining endothelial cells. The tunica media consists of a single layer of smooth muscle cells (SM). The tunica adventitia is composed of collagen fibrils and several layers of fibroblasts (F) with extremely attenuated processes. Red blood cells are visible in the lumen, x6,000. b. Photomicrograph of arteriole and venule in the dermis. One arteriole is

seen in longitudinal section, while another is seen in cross section. The round and ovoid nuclei in the wall of the longitudinally sectioned arteriole belong to the smooth muscle cells of the tunica media. Their round to ovoid shape indicates that these cells have been cut in cross section. The elongated nuclei (arrows) belong to endothelial cells. x320. Inset. The cross-sectioned arteriole is shown here at higher magnification and reveals the endothelial cell nuclei bulging into the lumen (arrows). They reflect a cross-sectional cut. The nuclei of the smooth muscle cells in the tunica media appear as elongate profiles reflecting their circular pattern around the vessel. X600.

Hypertension, or high blood pressure, occurs in about 25% of the population and is defined by a sustained diastolic pressure greater than 90 mm Hg or a sustained systolic pressure in excess of 140 mm Hg. Hypertension is often associated with atherosclerotic vascular disease and with an increased risk of cardiovascular disorders such as stroke and angina pectoris. In most cases of hypertension, the size of the lumen of the small muscular arteries and arterioles is reduced, which leads to increased vascular resistance. Restriction in the luminal size may also result from active contraction of the smooth muscle in the vessel wall, an increase in the amount of smooth muscle in the wall, or both.

In individuals with hypertension, multiplication of smooth muscle cells occurs. The additional smooth muscle then adds to the thickness of the tunica media. Concomitantly, some of the smooth muscle cells accumulate lipid. This is one reason why hypertension is a major risk factor for atherosclerosis. In fat-fed animals, hypertension accelerates the rate of lipid accumulation in the vessel wall. In the absence of a fatty diet, hypertension increases the rate of intimai thickening that occurs naturally with age.

Cardiac muscle is also affected by chronic hypertension. Ventricular hypertrophy, caused by an increase in number and size of cardiac muscle cells, is a common manifestation of hypertension. Ventricular hypertrophy makes the wall less elastic and the heart must then work harder to pump blood. Recent studies have shown, however, that prolonged reduction of blood pressure in patients with ventricular hypertrophy due to chronic hypertension can actually reduce the degree of hypertrophy.

Arterioles control blood flow to capillary networks by contraction of the smooth muscle cells

Arterioles serve as flow regulators for the capillary beds. In the normal relationship between an arteriole and a capillary network, contraction of the smooth muscle in the wall of an arteriole increases the vascular resistance and reduces or shuts off the blood going to the capillaries. The slight thickening of the smooth muscle at the origin of a capillary bed from an arteriole is called the precapillary sphincter. Most arterioles can di-

late 60 to 100% from their resting diameter, and they can maintain up to 40% constriction for a long time. Therefore, a large decrease or increase in vascular resistance has a direct effect on blood flow and systemic arterial pressure. This regulation directs blood to where it may be most needed. For instance, during strenuous physical exertion, such as running, blood flow to skeletal muscle is increased by dilation of arterioles, and blood flow to the intestine is reduced by arteriole constriction. After ingestion of a large meal, however, the reverse is true.

Atherosclerotic lesions are the most common acquired abnormality of blood vessels. More than half of the annual deaths in the United States are related to complications of atherosclerotic disease, which includes ischemic heart disease, myocardial infarction, stroke, and gangrene of the limbs. The lesions develop in the intima and consist of a thick layer of fibrous connective tissue containing scattered smooth muscle cells, macrophages, foam cells, lymphocytes, cholesterol crystals, and cell debris. It is believed that both macrophages and smooth muscle cells accumulate lipid, par-

Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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