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Blood vessels called capillaries are the sites of fluid, gas, nutrient, and waste exchange between the blood and body tissues. This exchange occurs by the process of diffusion. Capillary walls contain small pores that allow certain substances to pass into and out of the blood vessel. Fluid exchange is controlled by blood pressure within the capillary vessel (hydrostatic pressure) and osmotic pressure of the blood within the vessel. The osmotic pressure is produced by high concentrations of salts and plasma proteins in the blood. The capillary walls allow water and small solutes to pass through its pores but does not allow proteins to pass through.
As blood enters the capillary bed on the arteriole end, the blood pressure in the capillary vessel is greater than the osmotic pressure of the blood in the vessel. The net result is that fluid moves from the vessel to the body tissue. On the venule end of the capillary bed, blood pressure in the vessel is less than the osmotic pressure of the blood in the vessel. The net result is that fluid, carbon dioxide and wastes are drawn from the body tissue into the capillary vessel.
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