7.1 Body Fluids

The body fluid consists of water and substances dissolved in them. There are two types of body fluids, the intracellular fluid present inside the cells and the extracellular fluid present outside the cells. The three types of extracellular fluids are the interstitial fluid or tissue fluid (surrounds the cell), the plasma (fluid component of the blood) and lymph. The blood flowing into the capillary from an arteriole has a high hydrostatic pressure.

This pressure is brought about by the pumping action of the blood and it tends to force water and small molecules out through the permeable walls of the capillary into the tissue fluid. The volume of fluid which leaves the capillary to form tissue fluid is the result of two pressure (hydrostatic pressure and Oncotic pressure). At the anterior end of the capillary bed, the water potential is lesser than hydrostatic pressure inside the capilary bed which is enough to push fluid into the tissues. The tissue fluid has low concentration of protien than that of plasma.

At the venous end of the capillary bed, the water potential is greater than the hydrostatic pressure and the fluid from the tissues flows into the capillary and water is drawn back into the blood, taking with it waste products produced by the cells.