Tissue fluid: Encyclopedia II - Tissue fluid - Formation of tissue fluid

Tissue fluid - Formation of tissue fluid

In the capillaries, there are two forces acting on the movement of water and other aqueous substances; hydrostatic pressure and water potential.

The hydrostatic pressure is generated by the pumping force of the heart. It pushes water out of the capillaries.

The water potential is created due to the inability of large solutes to pass through the capillary walls. This buildup of solutes induces osmosis. The water passes from a high concentration (of water) to a low concentration in an attempt to reach an equilibrium. This draws water back into the vessels. Because the blood in the capillaries is constantly flowing, equilibrium is never reached.

The balance between the two forces is different at different points in the capillaries. At the atrial end of the vessel, the hydrostatic pressure is greater than the water potential, so the net movement (see net flux) favors water and other solutes being passed into the tissue fluid. At the venous end, the water potential is greater, so the net movement favours substances being passed back into the capillary. This difference is created by the direction of the flow of blood, and the imbalance in solutes created by the net movement of water favoring the tissue fluid.

Adapted from the Wikipedia article "Formation of tissue fluid", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki