The density of the capillary network determines the total surface area available for exchange between the blood and tissue. It 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.
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 AV shunts (see Fig. 13.11). 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.
FIGURE 13.22 • 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 cells 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. 21,500. The inset shows to advantage the fenestrations and the diaphragm that spans the openings (large arrows). 55,000.
Preferential thoroughfares, whose proximal segment is called a metarteriole (Fig. 13.23), 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.
The tunics of veins are not as distinct or well defined as the tunics of arteries. Traditionally, veins are divided into four types on the basis of size.
 Venules are further subclassified as postcapillary and muscular venules. They receive blood from capillaries and have a diameter as small as 0.1 mm.
 Small veins are less then 1 mm in diameter and are continuous with muscular venules.
 Medium veins represent most of the named veins in this category. They usually are accompanied by arteries and have a diameter of as much as 10 mm.
 Large veins usually have a diameter greater than 10 mm. Examples of such veins include the superior and inferior vena cava and hepatic portal vein.