They are composed of only the tunica intima (the innermost layer of an artery or vein), consisting of a thin wall of simple squamous endothelial cells.

[2] They are the site of the exchange of many substances from the surrounding interstitial fluid, and they convey blood from the smallest branches of the arteries (arterioles) to those of the veins (venules).

Capillaries and sinusoids are short vessels that directly connect the arterioles and venules at opposite ends of the beds.

They are spanned by a diaphragm of radially oriented fibrils that allows small molecules and limited amounts of protein to diffuse.

Both of these types of blood vessels have continuous basal laminae and are primarily located in the endocrine glands, intestines, pancreas, and the glomeruli of the kidney.

Sinusoids are irregular spaces filled with blood and are mainly found in the liver, bone marrow, spleen, and brain circumventricular organs.

[16] The term angiogenesis denotes the formation of new capillaries from pre-existing blood vessels and already-present endothelium which divides.

Molecules larger than 3 nm such as albumin and other large proteins pass through transcellular transport carried inside vesicles, a process which requires them to go through the cells that form the wall.

Molecules smaller than 3 nm such as water and gases cross the capillary wall through the space between cells in a process known as paracellular transport.

[20] Capillaries that form part of the blood–brain barrier only allow for transcellular transport as tight junctions between endothelial cells seal the paracellular space.

[22] In the lungs, special mechanisms have been adapted to meet the needs of increased necessity of blood flow during exercise.

[citation needed] Extreme exercise can make capillaries vulnerable, with a breaking point similar to that of collagen.

[23] Capillary permeability can be increased by the release of certain cytokines, anaphylatoxins, or other mediators (such as leukotrienes, prostaglandins, histamine, bradykinin, etc.)

[34][35] William Harvey did not explicitly predict the existence of capillaries, but he saw the need for some sort of connection between the arterial and venous systems.

[38] His 1922 estimate that total length of capillaries in a human body is as long as 100,000 km, had been widely adopted by textbooks and other secondary sources.