Fluid Movement - Physiology
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Water in the Body
Water is a very important substance in the body making up aroximately 70% of it. Over 50% of that water is found in cells refered to as the intracellular fluid. The remainder makes up the base of the fluid surrounding the cell which is refered to as the extracellular fluid or ECF.
Extracellular Fluid
The extracellular fluid is subdivided into three parts:
- The plasma of the blood
- Tissue fluid (interstitial fluid)
- Lymph fluid
Water Movement
Water is able to move between the various compartments easily. The movement is determined by the following:
- Differances in hydrostatic pressure
- Differances in osmotic pressure
These forces play differant roles depending on the structure being crossed. Both influence passage across capillary walls however only osmotic pressure influences that movement across cell membranes. The reason hydrostatic pressure has little influence over the transport across cell membranes is that the hydrostatic pressure of the intracellular and interstitial fluid is relatively constant.
The Effects of Osmolarity
The osmolarity of a solution is inversely proportional to the water concentration. Water tends to flow from an area of lower osmolarity to an area of higher osmolarity. Until the osmolarity of both is equal. If the osmolarity of the ECF is changed say after a meal high in salt then because the ECF now has a higher osmolarity than the interstitial fluid water moves from the interstitium into the ECF until a neq equilibrium is reached. Water in turn flows out of the intracellular fluid into the interstitium as this now has a higher osmolarity. Thus cell volume changes.