Capillaries are tiny blood containing structures that connect arterioles to venules. They are the smallest and most abundant form of blood vessel in the body. Capillaries are small enough to penetrate body tissues, allowing oxygen, nutrients, and waste products to be exchanged between tissues and the blood. This occurs via passive diffusion and pinocytosis (ingestion of fluid by cells). White blood cells also enter tissues via the capillaries, attacking infections and repairing damage.
Most capillaries are 3 to 4 µm (micrometers) in diameter, but some can be as large as 40 µm. They are composed of a thin layer of epithelial cells and a basal lamina, or basement membrane, known as the tunica intima. There is also an incomplete layer of cells, that partially encircles the epithelial cells, known as pericytes. Microvascular pericytes regulate blood pressure in the capillaries through contraction. This improves the efficiency of exchange between the blood in the capillary and the tissue surrounding it. Blood flow into the capillaries is controlled by precapillary sphincters, smooth muscle bands that wrap around metarterioles.
There are 3 types of capillary in the body; continuous, fenestrated, and sinusoidal.
As their name suggests, continuous capillaries have a continuous endothelial lining. They have tight junctions between their endothelial cells along with intercellular clefts through which small molecules, like ions, can pass. Continuous capillaries are generally found in the nervous system, as well as in fat and muscle tissue. Within nervous tissue, the continuous endothelial cells form a blood brain barrier, limiting the movement of cells and large molecules between the blood and the interstitial fluid surrounding the brain.
These capillaries can be found in tissues where a large amount of molecular exchange occurs, such as the kidneys, endocrine glands, and small intestine. They are particularly important in the glomeruli of the kidneys, as they are involved in filtration of the blood during the formation of urine.
The capillaries have small openings in their endothelium known as fenestrae or fenestra, which are 80 to 100 nm in diameter. Fenestra have a non-membraneous, permeable membrane, which is diaphragm-like and spanned with fibrils. This arrangement allows quick movement of macromolecules in and out of the capillary. The basement membrane of the epithelial cells in the lining remains unbroken by the fenestra.
Sinusoidal capillaries, sometimes referred to as sinusoids, or discontinuous capillaries, have endothelial linings with multiple fenestrations (openings), that are around 30 to 40 nm in diameter. These have no diaphragm and either a discontinuous or non-existent basal lamina. This allows blood cells and serum proteins to pass through the capillary wall as if it were a colander.
Sinusoidal capillaries are mainly found in the liver, between epithelial cells and hepatocytes. They can also be found in the sinusoids of the spleen where they are involved in the filtration of blood to remove antigens, defective red blood cells, and microorganisms. Sinusoidal capillaries can also be found in the lymph nodes, bone marrow and some of the glands of the endocrine system.