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Capillary Exchange The ultimate goal of the cardiovascular and respiratory systems is to deliver oxygen-rich and nutrient-rich blood to the capillary networks. Although the amount of blood within the capillaries at any given time is only about 5 percent of the total volume, it is the most important blood in the body. It is at the level of the capillaries that capillary exchange occurs. The thin and porous construction of capillary walls provides an avenue for substances such as oxygen and glucose to move out of the blood plasma and into the interstitial fluids surrounding the tissue cells. The reverse process, uptake of substances such as carbon dioxide and wastes from the interstitial fluids into the plasma, occurs simultaneously. In addition to the back-and-forth exchange of solutes, water also moves between the bloodstream and interstitial fluid in both directions within the capillary networks. There are three mechanisms by which substances enter and leave capillaries:diffusion, transcytosis, and bulk flow. Diffusion Diffusion, the movement of a solute from an area of high concentration to an area of low concentration, is a passive process. Diffusion is the driving force for the exchange of solutes at the capillary tissue interface. Thus, the oxygen and glucose, whose concentrations are high in the blood, will move out of the vessels into the interstitial fluids. Carbon dioxide levels are high in the interstitial fluid due to diffusion from the tissue cells. Carbon dioxide will move across the capillary walls into the blood plasma, where the concentration is lower. The physical characteristics of each solute determine the route by which that solute will diffuse. Small water-soluble molecules, such as glucose and amino acids, diffuse primarily through the intercellular clefts between the endothelial cells in continuous capillaries or through the fenestrations of fenestrated capillaries. Lipid-soluble molecules, such as oxygen and steroid hormones, can diffuse through the plasma membranes of the endothelial cells. Larger, lipid-insoluble substances must move down their concentration gradients via transcytosis. Transcytosis Transcytosis is a combination of endocytosis and exocytosis that is necessary when the substance to be transported cannot be exposed to the cytoplasm of an intervening cell. Transcytosis is important in moving large lipid-insoluble molecules such as proteins across the capillary endothelial cells. A protein, for instance albumin or insulin, which is high in concentration in the blood plasma, will be enclosed in tiny vesicles formed from the endothelial plasma membrane. This process is considered pinocytosis because the molecules dissolved in the blood plasma are taken up randomly along with the fluid. These molecules are transported within the vesicles across the endothelial cell. At the opposite plasma membrane surface, the vesicle merges with the membrane, and the molecules are released into the interstitial fluid. In the developing fetus, certain plasma proteins and antibodies are moved from maternal to fetal circulation via transcytosis. Bulk Flow Bulk flow is the movement of fluids due to pressure gradients. Fluids will move en masse from an area of high pressure to an area of low pressure. While diffusion is most important in the transfer of larger solutes, bulk flow controls the movement of fluids plus the ions and other small particles that are dissolved within the fluid. The movement of fluids from the capillaries into the interstitial fluid at the arterial end of the capillary bed is called filtration. The movement of fluids from the interstitial fluid into the capillaries at the venous end of the capillary bed is called reabsorption.