Capillaries

Capillaries are the smallest and most numerous blood vessels in the body, forming dense networks (capillary beds) that reach nearly every cell. They are the functional endpoint of the circulation: the entire purpose of the heart, arteries, and veins is to bring blood to these vessels so that exchange with tissues can occur.

Structure

A capillary is built for diffusion, not pressure. Its diameter is only about 4 to 10 micrometers, just wide enough for red blood cells to pass through in single file. Unlike arteries and veins, a capillary wall has essentially one layer: a single sheet of endothelial cells resting on a thin basement membrane, with no muscular tunica media. This extreme thinness allows substances to pass easily between the blood and surrounding tissue. Blood flow into capillary beds is regulated upstream by arterioles and by ring-like precapillary sphincters.

Types and Function

There are three structural types suited to different organs. Continuous capillaries, the most common, have a tight, unbroken endothelial lining and are found in muscle, skin, and the nervous system. Fenestrated capillaries have small pores that permit faster filtration, found in the kidneys, intestines, and endocrine glands. Sinusoidal (discontinuous) capillaries have large gaps that let even cells and large molecules pass, found in the liver, spleen, and bone marrow. Across all types, capillaries are where oxygen and nutrients move from blood into tissues and where carbon dioxide and metabolic wastes move back into the blood, driven by diffusion and the balance of hydrostatic and osmotic pressures.

Clinical Relevance

Capillary health is central to many conditions. In diabetes, chronic high blood sugar damages tiny vessels, contributing to retinopathy, kidney disease, and nerve damage. Leaky or inflamed capillaries contribute to edema (tissue swelling) and play a role in shock and sepsis. This content is for education only and is not medical advice.