4.1 Capillaries are the smallest and most numerous of the blood vessels, and connect the arterioles which carry blood away from the heart with the venules which carry it back towards the heart (Applegate, 2000, 261). They are a continuation of the smallest arterioles, but the walls of capillaries consist of only a single layer of endothelial cells, overlying a basement membrane. This thin wall permits the exchange of materials between the blood in the capillary and the tissue cells. This is the primary function of the capillaries - an exchange system to bring nutrients to the tissues and take away waste products. Areas of the body with high metabolic activity such as the skeletal muscles, liver, and kidneys, are richly endowed with capillaries because they need an abundant supply of oxygen and nutrients. Tissues with less demand for oxygen and nutrients, such as connective tissues, have fewer capillaries.
Capillary beds play a vital role in the exchange of gasses, nutrients, and metabolic wastes between the blood and the tissue cells (Applegate, 2000, 263). Tissue cells are surrounded by interstitial fluid, which is formed from fluid which has left the capillaries, Substances pass through the interstitial fluid when moving between the capillaries and tissue cells, and the interstitial fluid plays an intermediary role in the exchange process. Substances which leave the capillaries do so by three methods: diffusion, filtration, and osmosis.
Oxygen, carbon dioxide, and glucose move across the capillary wall down a concentration gradient by diffusion (Applegate, 2000, 263). The hydrostatic pressure of the blood (blood pressure) forces water molecules and certain dissolved substances through the capillary walls by the blood pressure generated by capillary contraction. More filtration will occur in the arteriolar end of the capillary because the pressure is higher in the arteriole than in the venule. Large protein molecu...