Two types of pressure interact to drive each of these movements: hydrostatic pressure and osmotic pressure. ![]() In contrast, the movement of fluid from an area of higher pressure in the tissues into an area of lower pressure in the capillaries is reabsorption. This movement, often referred to as bulk flow, involves two pressure-driven mechanisms: Volumes of fluid move from an area of higher pressure in a capillary bed to an area of lower pressure in the tissues via filtration. The mass movement of fluids into and out of capillary beds requires a transport mechanism far more efficient than mere diffusion. Some large proteins in blood plasma can move into and out of the endothelial cells packaged within vesicles by endocytosis and exocytosis. Larger molecules can pass through the pores of fenestrated capillaries, and even large plasma proteins can pass through the great gaps in the sinusoids. Glucose, ions, and larger molecules may also leave the blood through intercellular clefts. Glucose, amino acids, and ions-including sodium, potassium, calcium, and chloride-use transporters to move through specific channels in the membrane by facilitated diffusion. Small molecules, such as gases, lipids, and lipid-soluble molecules, can diffuse directly through the membranes of the endothelial cells of the capillary wall. ![]() ![]() The primary purpose of the cardiovascular system is to circulate gases, nutrients, wastes, and other substances to and from the cells of the body.
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