Capillaries (capillus = hair) are networks of microscopic endothelial tubes interposed between the metarterioles and venules. The true capillaries (without any smooth muscle cell) begin after a transition zone of 50-100 micron beyond the precapillary sphincters. The capillaries are replaced by dilated spaces in the sex organs, splenic pulp and placenta.
Size of capillaries:
The average diameter of a capillary is 6—8 micron, just sufficient to permit the red blood cells to pass through in ‘single file’. But the size varies from organ to organ. It is smallest in the brain and intestines, and largest in the skin and bone marrow.
Types and Structure of capillaries:
The capillaries are classified as continuous and fenestrated according to the type of junctions between the endothelial cells.
Continuous capillaries are found in the skin, connective tissue, skeletal and smooth muscles, lung and brain. These allow the small molecules to pass across their walls.
Fenestrated capillaries are found in the renal glomeruli, intestinal mucosa, endocrine glands and pancreas. These allow passage across their walls of larger molecules.
The capillary bed and postcapillary venules form an enormous area for the exchange of nutrients, gases, metabolites and water, between the blood and interstitial fluid. Capillaries also allow migration of leucocytes out of the vessels.