Endothelial cells form a continuous ring around the lumen of capillary except at the gaps (intercellular clefts) between endothelial cells . Clefts (gaps at inter-endothelial junctions) are usually 4 to 10 nm (less than 15 nm) in diameter. There is no discontinuity in endothelium of the capillaries . seen in skeletal and smooth muscles, connective tissues, and lungs . Clefts are absent in capillaries of cerebral blood vessels (blood-brain barrier) as the inter-endothelial junctions in these vessels are tight junctions.

endothelial cells are perforated by many fenestrations (pores), which are 20 to 100 nm in diameter. cytoplasm of endothelial cells is attenuated to form gaps, which are called fenestrations They allow passage of molecules having the molecular weight of up to 69,000 Seen in epithelia as in intestinal villi, choroid plexuses of ventricles in brain, ciliary processes of eyes, exocrine glands and parts of kidneys.

larger discontinuities between the endothelial cells. gaps between the endothelial cells are more than 400 nm in diameter Some gaps are 600 to 3000 nm in diameter they have multiple large fenestrae in their cytoplasm. seen in in liver, spleen and bone marrow

smooth muscle forms a cuff, called precapillary sphincter not innervated, but responds to various vasoconstrictor and dilator chemicals. The capillary circulation largely depends on the contraction or relaxation of the precapillary sphincter

change in their caliber occurs passively due to alteration in flow of blood through their lumen few contra ctile elements like myosin and actin filaments are present in the capillary walls . Rouget cells / myoepithelial cells / pericytes ( surrounds the endothelial cells of capillaries )are believed to be the primitive form of vascular smooth muscle and they respond to vasoconstrictor agents.They themselves also release vasoactive substances. Therefore, active capillary constriction and dilation do occur to some extent.

capillaries are weak vessels and surrounded by tissues. Therefore, if the capillary pressure decreases below the tissue pressure, capillaries collapse and the flow in capillaries ceases. The capillary pressure, below which the flow stops, is called critical closing pressure.

a few vessels that directly connect the arterioles and venules, bypassing the capillaries. resting tissue - mostly through thoroughfare vessels flow through true capillaries increases during exercise (active tissue)

because intermittent contraction and relaxation of the arterioles and metarterioles, which regulate flow through the capillaries usually occurs 5–10 times per minute) Vasomotion is partly contributed by the chemicals released by the endothelium of the blood vessels.

in most tissues is less than the diameter of RBCs. Therefore, red cells while passing through the lumen of capillaries come in very close contact with the capillary membrane. As such, the flow of blood in capillaries is very slow. These two factors, in addition to the thin capillary wall facilitate the exchange of gasses between capillaries and tissues

Though individual capillaries have small diameter and they provide high vascular resistance

Endothelial cells of capillaries contain many endocytic vesicles that contribute to transcytosis of water and water-soluble substances across the capillary wall. Some endothelial cells have fenestrations that run completely through the cells from capillary-interior to the interstitial space

difference between the hydrostatic pressure of vascular compartment (i.e. of blood) and the interstitial tissues compartment (i.e. of tissue fluid). In skeletal muscle the hydrostatic pressure of blood at the arteriolar end and at the venular end of capillaries is about 37 and 17 mm Hg respectively, and in the interstitial tissues space, it is negligible, which is about 1 mm Hg the hydrostatic pressure gradient favors filtration both at arteriolar and venular end of the capillaries. however , at arteriolar end, the gradient is more, i.e. 36 (37–1) mm Hg than at the venular end of the capillaries, which is 16 (17–1) mm Hg

difference between the oncotic pressure (the osmotic pressure of blood), which is 25 mm Hg, and the osmotic pressure in the interstitial tissue space, which is very negligible (almost zero). osmotic pressure of tissue space is nil, the osmotic pressure gradient along the capillary wall is always inward, which favors absorption of fluid from the interstitial tissue space into the capillary.

At the arteriolar end of the capillary: • The net filtration pressure is 11 mm Hg [(37 – 1) – 25] in outward direction At the venular end of the capillary: • The net filtration pressure is 9 mm Hg [25 – (17 – 1)] in the inward direction. Thus, at the arteriolar end, fluid moves out of the capillaries and at the venular end, fluid moves into the capillaries. About two units of fluid are left in the interstitial tissue space as the outward filtration at the arteriolar end is 2 mm Hg more than the inward filtration at the venular end. amount of fluid is usually taken up by the lymphatics in the interstitial space

depends on the integrity of the capillary endothelial membrane Capillary permeability is increased especially in inflammatory conditions.

1. Increased Hydrostatic Pressure of Capillaries

2.Decreased Oncotic Pressure

3.Increased Capillary Permeability -

decreased lymphatic drainage . can occur either due to diseases of the 1. lymphatics (lymphangitis) 2. surgery (radical mastectomy that removes lymphatic ducts) 3. by infection (filariasis) Edema due to lymphatic obstruction is usually non-pitting type