Osmosis in living organisms

Osmosis is the mechanism for transferring fluids and nutrients across membranes. It occurs in plants and animals and maintains the proper balance between extracellular and intracellular fluids.   

Although osmotic pressure is measured in the laboratory and has been referred to as a driving force for osmosis     Osmotic pressure only occurs in laboratory experiments and calculations.   Osmotic pressure is exerted on a bulk fluid.  Application of osmotic pressure to the wine side of the osmosis experiment will stop the flow of pure water into the wine.  It raises the water vapor pressure of the wine.      A significant increase in osmotic pressure will cause only a small increase in vapor pressure.    Our bodies do not produce significant increases in bulk pressures.  Osmotic pressure is a secondary variable in understanding osmotic flow in our bodies.   

As noted previously, vapor pressure is the driving force for osmosis flow across a membrane.    Flow proceeds from a higher vapor pressure to a lower vapor pressure region (from a higher concentration to a lower concentration area).   Free molecules that exist in liquids provide the actual flow mechanism for flow across a membrane and these free molecules also produce vapor pressure.  

Vapor pressure is primarily influenced by concentration of chemicals in body fluids and by temperature.     Increased chemical solutes in the blood will tend to lower water vapor pressure.   This will draw water from the cells.     Energy generation (metabolism) in cells will increase cell temperature and produce water.  Increased cell temperature will increase vapor pressure and cause water to leave the cell.    The net effect of metabolism on cell water content will depend on many factors including the health of the organism.     

We conclude that vapor pressure is the driving force for osmosis. The net flow of small, free water molecules passes through a membrane from the high water vapor pressure side to the lower vapor pressure.

Note:    Our discussion of osmosis is introduced to clarify the osmosis process only.   The marvelous processes and interactions with in living organisms are amazing and beyond the scope of this discussion.   Osmosis is simply the process by which water, nutrients, wastes, etc. are transferred across cell membranes.  Osmosis is the process by which flow proceeds from a high vapor pressure region through a membrane to a lower vapor pressure region.   Understanding this will help with understanding the processes in living organisms.  

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