Student Question

How do hydrostatic and osmotic pressure determine fluid movement across the walls of capillaries?

Expert Answers

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Hydrostatic and osmotic pressure determine fluid movement across capillaries by raising and lowering the pressure within cells.  This directly influences whether water will be able to cross the selectively permeable membrane most cells have.  Osmosis is the diffusion of water across the cell membrane in an effort to equalize the concentration of water content both inside the cell and outside the cell.  If there is a higher concentration of sugar inside the cell, for example, water will diffuse across the membrane from the outside of the cell to equalize it.  This is the very reason one can not drink salt water from the ocean on a continued basis, because the water in the persons cells will diffuse out to try to equalize the water content in the salt water.  This will cause the person to become very dehydrated, and ultimately die, if not rehydrated within reason.  Hydrostatic pressure is the passing of pressure equally among all molecules of fluids.  If there is less pressure on the interior of the capillary, the greater pressure on the outside of the exterior of the capillary will influence water to diffuse inside.

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