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The blood vascular system is a continuous series of hollow tubes that carry blood from the heart to the tissues and back again. Blood exits the heart through the aorta that gives rise to a series of diverging, progressively narrower muscular arteries and arterioles, delivering blood to all organs of the body. The terminal arterioles arborize into an interconnecting network of microvessels, mostly capillaries that nourish and cleanse the peripheral tissues. The capillary network empties into a series of converging venules and progressively larger veins that ultimately return the blood to the heart. All segments of this vascular system are lined by a one-cell-thick layer of epithelium-like cells called endothelium (Fig. 162-1). All vascular endothelial cells (ECs) share common features and functions, and hence may be collectively described as one cell type. However, ECs from one segment of the vascular system may differ in significant ways from the ECs at other anatomic sites. Blood vessel ECs differ from lymphatic ECs, which are not discussed in this chapter.
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Blood vessel ECs must perform three important constitutive functions. First, vascular ECs must maintain homeostasis ...