Capillaries contain only one component, endothelium. This provides a very thin membrane between the blood and the cells, allowing for efficient diffusion. The presence of only endothelium in these vessels is consistent with their exchange function.
The large veins have much collagen. Collagen is stiffer than elastin by several orders of magnitude, and is functionally analogous to the metal wires in automobile tires. The collagen acts as a safety net in the large vessels which often bear high pressures, and prevents bursting when they experience high hydrostatic pressures. Clearly, the major components of the blood vessels fit their respective physiological functions.
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