Answer
Structurally, capillaries and postcapillary venules are alike in that they have very thin walls --primarily, a simple endothelial layer on basement membrane-- and comparatively small diameters. But capillaries are of three basic types, and are more variable in structure than postcapillary venules.
Work Step by Step
The three types of capillaries arr continuous capillaries, fenestrated capillaries, and sinusoid capillaries
Capillaries are small blood vessels without tunica externa or tunica media and smooth muscles. They are basically small short tubes with walls of endothelial cells on a basement membrane . Pericytes congregate and wrap around some capillaries Capillary walls are about 0.3 um in thickness, and their diameters 5 um to 2um. The tight junctions between these endothelial cells are not very efficient in fenestrated and sinusoid capillaries, Capillaries have intercellular clefts and pores or fenestrations which facilitate exchange of substances with the interstitial fluid and tissues. The continuous capillaries are not not very permeable, and these are the type found in the blood brain barrier where leakage of plasma into ISF is not desirable. Fenestrated capillaries have intercellular clefts as well as areas with many large pores ( fenestrations). These structural features facilitate filtration of large molecules (in addition to fluids and small molecules), but exclude very large molecules and cells. Fenestrated capillaries are found in small intestine, and kidneys where rapid filtration is necessary. Sinusoidal capillaries are irregular in shape; they are found in bone marrow, liver and and spleen. Sinusoids have wider intercellular clefts, and no basal lamina beneath endothelial cells. This permits large molecules like albumins and clotting factors to pass through. Blood cells from lymphatic organs and bone marrow can also enter the blood stream through the walls of sinusoidal capillaries.
Most of the capillaries of the body form capillary networks or capillary beds of 10 to 100 capillaries . Capillaries empty into thoroughfare channels or into postcapillary venule.
Postcapillary venules are the smallest veins and they link the arterial and venous sides of the blood vasculature . They have very thin walls of endothelial cells surrounded by pericytes. The average wall thickness is 0.5-1.0 um. The diameter of these tubes range between 8-100 um. Like the walls of capillaries the walls of post capillary venules are very porous. Fluid and blood cells can pass through the walls of postcapillary venules. . In cases of inflammation leukocytes pass freely through venules into the aeas of inflammation
