Capillaries are the exchange vessels of the blood circulatory system. Their primary function is to exchange nutrients, metabolic waste , water and gases with the interstitial fluid and tissues. Large conducting and distributing arteries and arterioles have walls comprising three layers or tunics. These are the tunica externa, the tunica, media and the tunica intima. Capillaries need to take blood close to tissues cells,,so capillary walls usually have just one tunic, that is the tunica intima of endothelial tissue. Some capillaries are continuous, as in the capillaries of the blood brain barrier(BBB). The tight junctions of these continuous capillaries serve the function of protecting the vulnerable brain tissue from toxins. The exchange function of capillaries is facilitated by the structure of the fenestrated capillaries and of sinusoids. Even in continuous capillaries (outside the brain ) the tunica intima has covered intercellular clefts which permit the passage of small molecules like glucose, Proteins and other large molecules, are nevertheless excluded. Fenestrated capillaries have pores in their endothelial cells. These capillaries also allow small molecules including and small proteins (but not large molecules) to pass through their walls. Sinusoids are also considered a type of capillary . These are large irregularly-shaped spaces, with no basal lamina attached to the tunica intima; they also have wide gaps between endothelial cells. These structures, sinusoids, are found in liver, red bone marrow tissue, kidneys, and adrenal medulla. .Large protein molecules--albumins, clotting factors--can pass out through sinusoidal walls. Also, newly minted erythrocytes can pass into sinusoids from liver and red bone marrow tissue. Capillary beds are networks of scores of capillaries that function in organs as a unit. Blood flows from arteries to arterioles , through metarterioles into capillary networks. Passage of blood through a metarteriole into the capillary bed is controlled by smooth muscle cells that form precapillary sphincters around the entries to capillaries. Contraction of a precapillarry sphincter restricts blood from the capillary bed and diverts it into a thoroughfare channel that takes the blood out of the capillary bed into venules and veins.This mechanism enables capillary beds to alter the blood supply to areas such as the digestive organs ( after a meal) or to skeletal muscles during vigorous exercise.. The blood flow in the capillaries is slow and the blood pressure is low ( about 30-25 mmHg). This is well, because to facilitate exchange capillary walls are very thin-walled and would rupture if exposed to high blood pressures of around (90-100 mmHg.)
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Capillaries are very small vessels that at the terminal end o the arterial side of the blood circulatory system. The main function of arterioles is to serve as conduits vessels that exchange substances --nutrients , metabolic wastes, gases , water with the interstitial fluid and thus with cells of tissues. Unlike larger conducting and distributive vessels capillaries have only one thin layer of endothelial tissue as the major component of their walls. This is the tunica media of endothelial tissue. The thinness of he walls brings the blood close to tissue cells and facilitates exchange of substances Other structural modification also enhance the exchange capacities of capillaries. These include intercapillary spaces in continuous capillaries and pores in fenestrated capillaries. These structures(openings) allow molecule of various sizes to leave and enter capillaries. Sinusoids constitute a modified capillary structure that admits cells (RBCs and leukocytes directly into the blood stream and permits large protein molecules ( antibodies and clotting factors to leave. Capillary beds or capillary networks also have structural modifications that allow them to control blood flow in such a way as to facilitate absorption. These modifications also allow diversion of blood supply on a basis of primary or urgent physiological need. These structures are the precapillary sphincters of metarterioles that can contract to divert blood from the capillary bed and into the thoroughfare channel, or relax to fill the capillary bed and enhance absorption as in the case of capillary beds in digestive organs after a meal.