- Key People:
- Marcello Malpighi
- August Krogh
capillary
What is the primary function of capillaries?
How do capillaries connect arteries and veins?
What roles do capillaries play besides substance exchange?
capillary, in human physiology, any of the minute blood vessels that form networks throughout the bodily tissues. It is through the capillaries that oxygen, nutrients, and wastes are exchanged between the blood and the tissues. The capillary networks are the ultimate destination of arterial blood from the heart and are the starting point for flow of venous blood back to the heart. Between the smallest arteries, or arterioles, and the capillaries are intermediate vessels called precapillaries, or metarterioles, that, unlike the capillaries, have muscle fibers that permit them to contract. Thus, the precapillaries are able to control the emptying and filling of the capillaries.
Capillaries, despite their very small size, collectively form the most significant part of the circulatory system. Stretched out from end to end, the capillaries that are found in the human body reach across a distance measuring in the tens of thousands of kilometers. The remarkable surface area of all the capillaries combined allows for the very efficient exchange of nutrients and wastes that are critical for the healthy functioning of just about every organ and tissue.
Most capillaries are about 8 to 10 µm (micrometer;1 µm = 0.001 mm) in diameter, which is only large enough to allow red blood cells to travel single file. Capillary walls are made up of a thin layer of endothelial cells. Gases and other substances are able to readily diffuse across this very thin barrier, with the exchange of molecules driven largely by differences in concentration and pressure.
There are different types of capillaries, including continuous, fenestrated, and sinusoidal. Continuous capillaries notably occur in muscle, skin, and the brain; in these tissues, precise control of the movement of substances across the capillary barrier is critical. The tiny pores found in fenestrated capillaries provide opportunities for rapid nutrient and waste exchange; such capillaries are common in organs such as the kidneys and intestines. The larger pores that characterize sinusoidal capillaries allow for the movement of molecules of greater diameter across the barrier; these capillaries occur in the liver, spleen, and bone marrow.
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Networks of capillaries are especially dense in organs that require significant amounts of oxygen and nutrients; examples include the lungs, kidneys, and active muscles. By contrast, capillary networks are less dense in connective tissues such as tendons and ligaments, contributing to slower healing after injury.
Capillaries further serve roles in moderating body temperature, immune responses, and fluid balance. In the presence of inflammation, the permeability of capillary walls in damaged tissues increases to permit the entry of immune cells and proteins. The movement of fluid between the bloodstream and tissues is altered by even small increases in permeability, which is relevant in conditions such as edema, which is characterized by excess fluid accumulation in tissues.
The lymphatic system contains very small vessels that are also called capillaries; likewise, in the liver, the minute channels through which bile flows are referred to as capillaries.