Explain how the formation and absorption of tissue fluid occurs, referring to the processes of hydrostatic and osmotic pressure in a capillary bed.
Although the circulatory system transports oxygen, metabolites and solutes to all the tissues in the human body, the blood never directly comes into contact with any of the tissues other than the interiors of the cardiovascular system (except in case of trauma). Instead, the tissues are bathed in tissue fluid (sometimes called interstitial fluid), formed from, and very similar in composition to, the plasma of the blood.
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The tissue fluid is formed by the action of a variety of forces. Fluid is forced out of the blood vessels and capillaries into the tissue fluid by hydrostatic pressure – the great increase in blood pressure is produced every time the heart beats. This pressure forces water out of the capillaries through the semi-permeable membranes of the cells that line them (and in some vessels, through intercellular clefts, tiny membrane-sealed gaps between cells). These cells are also involved in facilitated diffusion of a wide variety of solutes and other molecules between the two fluid bodies. Metabolites diffuse from the plasma to the tissue fluid, and cellular waste products diffuse in the opposite direction.
Fluid is returned to the blood vessels by osmotic pressure. Large macromolecules (notably serum albumin) are unable to pass across the cellular membranes that line the vessels, and so they remain in circulation, lowering the osmotic pressure in the blood vessels (water flows across osmotic gradients from high to low osmotic pressure). Interaction between the two forces causes more fluid to leave at the arterial end of the capillary bed, and return at the venous end, as described by the Starling Equation.
Excess tissue fluid is removed by the lymphatic system, a series of vessels that carry fluid towards the heart, where it drains into the left subclavian vein. Failure of effective lymph transport results in a build-up of tissue fluid known as oedema.
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