Hiussuonet

Hiussuonet, or capillaries, are the smallest blood vessels in the circulatory system. They form an extensive network that connects arterioles to venules and serves as the primary site of exchange between blood and tissues. In humans and other vertebrates, capillary networks permeate almost all tissues, with density and structure adapted to local metabolic needs.

Anatomically, capillary walls are typically one endothelial cell thick, surrounded by a basal lamina. The lumen is only about 5 to 10 micrometers across, often allowing red blood cells to transit in single file. Pericytes may wrap the capillary wall in many beds and contribute to stability and regulation of blood flow. Endothelial cells can form tight junctions or fenestrations depending on tissue type.

Capillaries are traditionally categorized into three main types: continuous capillaries, which have uninterrupted endothelium and are common in muscle and skin; fenestrated capillaries, which have small pores and are found in kidneys, intestinal mucosa, and endocrine glands; and sinusoidal (discontinuous) capillaries, which have larger openings and are typical of the liver, spleen, and bone marrow, enabling passage of larger molecules and cells.

Exchange across the capillary walls occurs primarily by diffusion of gases and nutrients, with water and small solutes moving through interendothelial gaps or fenestrations; larger molecules may require transcytosis. Capillaries also participate in fluid balance and leukocyte trafficking, and their permeability can be modulated by inflammation.

Capillary density and function are influenced by age, activity, and disease. In the brain, specialized continuous capillaries form the blood-brain barrier. In diabetes and hypertension, capillary function and structure can be altered, contributing to microvascular complications.