That portion of the extra cellular fluid found among the cells is called the tissue fluid, or interstitial fluid. Tissue fluid originates primarily with a fluid portion of the blood that escapes into the tissues from the capillaries. Part of this escaped fluid enters the venous vessels. However, a large percentage of the tissue fluid is picked up by another segment of circulatory system, the lymphatic system. Thus, there is a continuous flow of the fluids throughout the body. In addition, the intracellular fluid and the immediate extra cellular fluid are continually being exchanged.
Exchange between Intracellular and Extra cellular Compartments:
The ICF and the ECF are separated by the membranes of the body cells. The protein components of these membranes give them substantial permeability to water while carefully controlling the permeability of selected ions. Cell membranes are flexible. If water flows into the cell, it expands; whereas they contracts when water flows out from them. This movement of water follows principle of osmosis. Osmosis occurs when there is a gradient of impermeable solute across a water-permeable membrane. In cells, osmotic flow would occur if there were an osmotic gradient between the intracellular and extra cellular fluids. In the body, these two compartments are always in osmotic equilibrium, even though the composition of the fluids in them is very different.
The addition or subtraction of water or solutes from one or more of the body's fluid compartments will result in water exchange between the ICF and the ECF. Isotonic fluids have similar osmolal concentration (280 milliosmole/kg of water) as that of blood plasma and cells retains their shape in them (A). Cells placed in hypotonic solution (less than 280 milliosmole/kg of water) absorb water into them and subsequently swell in shape (C). Cells placed in hypertonic solution (more than 280 milliosmole/kg of water) loose water from them and subsequently shrink in shape (B).
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