Basic Transport Equations According to Thermodynamics, Molecular Diffusion, Mechanisms in Membranes, and Multiphasic Structure

Abstract

Now we shall consider the movement of water and other fluids in our bodies, especially the exchange of fluid between blood and the extravascular tissues. Red blood cells cannot leave the blood vessel; but water, ions, and some white blood cells can. The fluid in the extravascular space moves and exchanges matter with the cells in the body. The ionic composition of the fluid in the cells is quite different from that in the extracellular space. Extracellular fluid is rich in Na+, Cl−, HCO −3 , whereas the intracellular fluid is rich in K+ Mg++, phosphates, proteins, and organic phosphates. The composition of blood plasma is fairly similar to that of the extravascular fluid, except that the plasma has some 14 mEq/L of proteins while extracellular fluid has essentially none. See Table 8.1:1. To talk about mass transport in the body we must explain how this difference in composition comes about.