Anatomy & Physiology

Anatomy & Physiology Study Guide • Water, ions, and tiny organic molecules, such as glucose, amino acids, and urea, can usually enter or leave the bloodstream by diffusion between adjacent endothelial cells or through the pores of fenestrated capillaries. • Many ions, including potassium, calcium, sodium, and chloride, can diffuse across endothelial cells by passing through channels in plasma membranes. • Large water-soluble compounds are unable to leave or enter the bloodstream except at fenestrated capillaries, such as those of the kidneys, the hypothalamus, many other endocrine organs, and the intestinal tract. • Lipids, such as fatty acids and steroids, and lipid-soluble materials, including soluble gases like oxygen and carbon dioxide, can cross capillary walls by diffusion through the endothelial plasma membranes. • Plasma proteins are normally unable to cross the endothelial lining anywhere except in sinusoids, such as those of the liver, where plasma proteins enter the bloodstream. Filtration As a solution flows across a porous membrane, some solutes are removed by filtration; solutes too big to go through the pores are filtered out of the solution. The source of power for filtration is hydrostatic pressure, which pushes water from an area of higher pressure to an area of lower pressure. In capillary filtration, water and small solutes are forced across a capillary wall, leaving larger solutes and proteins in the bloodstream. The solute molecules leaving the bloodstream are those small enough to pass between adjacent endothelial cells or through the pores in a fenestrated capillary. Filtration occurs at a capillary’s arterial end, where capillary hydrostatic pressure (CHP) is highest. Reabsorption Reabsorption occurs as the result of osmosis. Osmosis is a special term used to refer to the diffusion of water across a selectively permeable membrane separating two solutions of different solute concentrations. Water molecules tend to diffuse across a membrane toward the solution containing the higher solute concentration. The osmotic pressure (OP) of a solution is an indication of the force of osmotic water movement, or in other words, the pressure that has to be applied to prevent osmotic movement across a membrane. The higher the solute concentration, the greater the osmotic pressure of the solution. The presence of suspended proteins unable to cross capillary walls creates an osmotic pressure called blood colloid osmotic pressure (BCOP). Clinicians often use the term oncotic pressure (onkos, a swelling) when referring to the colloid osmotic pressure of body fluids. The two terms are equivalent. Osmotic water movement will continue until either the solute concentrations are equalized, or the movement is prevented by opposing hydrostatic pressure.

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