i Electron micrograph and schematic diagram of a fenestrated capillary. The cytoplasm of the endothelial cells contains numerous fenestrations (small arrows). In some of the thicker regions of the endothelial cell where the fenestrations are absent, pinocytotic vesicles are present. Part of a pericyte is seen on the left side of the electron micrograph, including its nucleus in the upper left corner of the micrograph. x21,500. The inset shows to advantage the fenestrations and the diaphragm that spans the openings (largearrows). x55,000.

when no absorption is occurring. When absorption takes place, the walls thin, and the number of pinocytotic vesicles and fenestrations increases rapidly. The ionic changes in the perivascular connective tissue, caused by the absorbed solutes, stimulate pinocytosis. These observations support the suggested mode of formation of the fenestrations described above.

Discontinuous capillaries (sinusoidal capillaries, or sinusoids) are typically found in the liver, spleen, and bone marrow. They are larger in diameter and more irregularly shaped than other capillaries. Structural features of these capillaries vary from organ to organ and include specialized cells. Stellate sinusoidal macrophages (Kupffer cells) and vitamin A-storing hepatic stellate cells (Ito cells) in the liver occur in association with the endothelial cells. In the spleen, endothelial cells exhibit a unique spindle shape with gaps between the neighboring cells; the basal lamina underlying the endothelium may be partially or even completely absent.

Functional Aspects of Capillaries

To understand capillary function, two important points, blood flow and extent or richness of the capillary network, should be considered. Blood flow is controlled through local and systemic signals. In response to vasodilating agents (e.g., EDRFs, NO, low 02 tension), the smooth muscle in the walls of the arterioles relaxes, resulting in vasodilation and increased blood flow through the capillary system. Pressure within the capillaries increases, and much of the plasma fluid is driven into the tissue. This process occurs in peripheral edema. Systemic signals carried by the autonomic nervous system and release of norepinephrine by the adrenal gland cause the smooth muscle of the arterioles to contract (vasoconstriction), resulting in decreased blood flow through the capillary bed. In this condition, capillary pressure can decrease and greatly increase absorption of tissue fluid. This situation occurs during loss of blood volume and can add approximately I L of fluid into the blood, preventing hypovolemic shock.

The richness of the capillary network is related to the metabolic activity of the tissue. The liver, kidney, cardiac muscle, and skeletal muscle have rich capillary networks. Dense connective tissue is less metabolically active and has less extensive capillary networks.

v arteriovenous shunts

Arteriovenous shunts allow blood to bypass capillaries by providing direct routes between arteries and veins

Generally, in a microvascular bed, arteries convey blood to the capillaries, and veins convey blood away from the capillaries. However, all the blood does not necessarily pass from arteries to capillaries and thence to veins. In many tissues, there are direct routes between the arteries and veins that divert blood from the capillaries. These routes are called arteriovenous (AV) anastomoses, or shunts (see Fig. 12.1). AV shunts are commonly found in the skin of the fingertips, nose, and lips and in the erectile tissue of the penis and clitoris. The arteriole of AV shunts is often coiled, has a relatively thick smooth muscle layer, is enclosed in a connective tissue capsule, and is richly innervated. Contrary to the ordinary precapillary sphincter, contraction of the arteriole smooth muscle of the AV shunt sends blood to a capillary bed; relaxation of the smooth muscle sends blood to a venule, bypassing the capillary bed. AV shunts serve in thermoregulation at the body surface. Closing an AV shunt in the skin causes blood to flow through the capillary bed, enhancing heat loss. Opening an AV shunt in the skin reduces the blood flow to the skin capillaries, conserving body heat. In erectile tissue, such as the penis, closing the AV shunt directs blood flow into the corpora cavernosa, initiating the erectile response (see page 711).

In addition, preferential thoroughfares, whose proximal segment is called a metarteriole (Fig. 12.13), also allow some blood to pass more directly from artery to vein. Capillaries arise from both arterioles and metarterioles. Although capillaries themselves have no smooth muscle in their walls, a sphincter of smooth muscle, called the precapillary sphincter, is located at their origin from either an arteriole or a metarteriole. These sphincters control the amount of blood passing through the capillary bed.

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