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Gondor Design Biology - Osmoregulation
Relevant, clear, basic discussion of osmoregulation in human physiology.

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Biological Membranes

The Urinary System

Osmosis is the tendency for water to move through biological membranes. Water moves from regions of high concentration of osmotically active substances to regions of low concentration. Osmotic balance is achieved through the regulation of osmolytes, ions, sugars, proteins, etc. Salt balance is most often managed through the activity of sodium potassium pumps. Through their activity, potassium concentrations are higher in the intracellular fluid while sodium concentrations are high in the extracellular fluid. By pumping three sodium ions out for every two potassium ions moved into the cell, the net activity increases the extracellular osmotic pressure, drawing water out of the cell. In human physiology, control of the composition of the urine is a primary strategy for osmoregulation.

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