As mentioned previously, arterioles have only one or two layers, and a small artery may have as many as eight layers of smooth muscle in the tunica media (Fig. 13.19) and (Plate 35, page 438). Typically, the tunica intima of a small artery has an internal elastic membrane, whereas this layer may or may not be present in the arteriole. The endothelium in both is essentially similar to endothelium in other arteries, except that at the electron microscope level, gap junctions may be found between endothelial cells and the smooth muscle cells of the tunica media. Last, the tunica adventitia is a thin, ill-defined sheath of connective tissue that blends with the connective tissue in which these vessels travel.
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 dilate 60% to 100% from their resting diameter, and they can maintain as much as 40% constriction for a long time. Therefore, a large decrease or increase in vascular resistance has a direct effect on distribution of 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
FIGURE 13.19 • Electron micrograph and photomicrograph of arterioles. a. This electron micrograph shows a cross section of an arteriole. The tunica intima 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. 6,000. b. Photomicrograph of arteriole and venule in the dermis. One arteriole is seen in longitudinal section, and 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. 320. 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. 600.
ﬂow to skeletal muscle is increased by dilation of arterioles, and blood ﬂow to the intestine is reduced by arteriolar constriction. After ingestion of a large meal, however, the reverse is true.
Capillaries are the smallest diameter blood vessels, often smaller than the diameter of an erythrocyte.