The capillary bed is an interwoven network of capillaries that supplies an organ—the more metabolically active the cells, more capillaries required to supply nutrients and carry away waste products.

A capillary bed can consist of two types of vessels: true capillaries, which branch mainly from arterioles and provide an exchange between cells and the circulation, and vascular shunts, short vessels that directly connect arterioles and venules at opposite ends of the bed, allowing for bypass. Capillaries are important in the exchange of fluids and solutes between the circulatory system and the cells.

There are three main types of capillaries:

Continuous: an uninterrupted lining, only allowing small molecules like water and ions to diffuse through.

Fenestrated: Fenestrated capillaries have pores in the endothelial cells (60-80 nanometers in diameter) that allow small molecules and limited amounts of protein to diffuse.

Sinusoidal: Sinusoidal capillaries are a special type of fenestrated capillaries that have larger openings (30–40 μm in diameter) in the endothelium. These types of blood vessels allow red and white blood cells (7.5μm–25μm diameter) and various serum proteins through.

The exchange of gases and solutes occurs in the capillaries by the mass movement of fluids into and out of capillary beds. This movement often referred to as bulk flow, involves two pressure-driven mechanisms: Volumes of fluid move from an area of higher pressure in a capillary bed to an area of lower pressure in the tissues via filtration. In contrast, the movement of fluid from an area of higher pressure in the tissues into an area of lower pressure in the capillaries is reabsorption. Two types of pressure interact to drive each of these movements: hydrostatic pressure and osmotic pressure.

Blood hydrostatic pressure is the force exerted by the blood confined within blood vessels or heart chambers. This is the force that drives fluid out of capillaries and into the tissues. As fluid exits, a capillary and moves into tissues, the hydrostatic pressure in the interstitial fluid correspondingly rises. This opposing hydrostatic pressure is called the interstitial fluid hydrostatic pressure. Generally, the pressure originating from the arterial pathways is considerably higher than the interstitial pressure, because lymphatic vessels are continually absorbing excess fluid from the tissues. Thus, fluid generally moves out of the capillary and into the interstitial fluid. This process is called filtration. Reabsorption occurs due to the movement of fluid from the interstitial fluid back into the capillaries due to osmotic pressure. Whereas hydrostatic pressure forces fluid out of the capillary, osmotic pressure draws fluid back in.

Capillaries also play a part in thermoregulation and heat loss. Warmblood from the body’s core typically loses heat to the environment as it passes near the skin in capillary beds by radiation. Capillary beds have a large surface area and so can be a good source of heat loss to assist thermoregulation, they cannot vasoconstrict or dilate, however.