Page 402 - Atlas of Histology with Functional Correlations

Vasa Vasorum

The walls of medium and large arteries and veins are too thick to provide
nourishment to the cells by direct diffusion from their lumina. As a result, these
walls are supplied by their own small blood vessels from adjacent small arteries
called the vasa vasorum (blood vessels of the larger blood vessel). The vasa

vasorum allows for the exchange of nutrients and metabolites with cells in the
tunica adventitia and the deeper tunica media. The vessels of vasa vasorum are
much more extensive in the walls of the veins than in the arteries because of the
poor oxygen content of venous blood.

Types of Capillaries

Capillaries are the smallest blood vessels. Their average diameter is about 8
μm, which is about the size of an erythrocyte (red blood cell [RBC]). Each

capillary consists of a thin endothelium, an underlying basal lamina, and a few
randomly scattered pericytes. These cells surround the capillaries with
branching cytoplasm and are enclosed by a basal lamina that also encloses the

capillary endothelium. There are three types of capillaries: continuous
capillaries, fenestrated capillaries, and sinusoids (Fig. 10.3). These structural
variations in capillaries allow for different types of metabolic exchange between
blood and the surrounding tissues.

FIGURE 10.3 ■ Three types of capillaries (transverse sections).

Continuous capillaries are the most common. They are found in muscle,
connective tissue, nervous tissue, skin, respiratory organs, and exocrine glands.
In these capillaries, the endothelial cells are joined and form an uninterrupted,

solid endothelial lining. Tight junctions, desmosomes, and gap junctions are seen
in these capillaries.

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