Water and electrolyte metabolism

The human body is comprised of 60% water. Two-thirds of the body’s water are inside cells, or intracellular, and one-third is extracellular or outside the cells. Of this, 25% is inside the blood vessels and 75% is interstitial fluids. 

The fluid is kept inside and outside the cell compartments by osmotic forces which are contributed by positive and negative ions. Potassium is the major positive ion in the intracellular fluid and Sodium is the major cation in the extracellular fluid. 

What we normally measure from the blood is what is seen in the serum. It is extracellular fluid. Normal sodium in the cell is 12 meq. Normal sodium in the extracellular fluid is 140 meq. Normal potassium intracellularly is 140 meq, and extracellularly is 3.5-5 meq.

Osmotic forces exerted by these ions control the fluid in and out of the cells. Concentration of combined solute in water is called osmolality. This is the amount of solute per liter of water. Plasma osmolality can be measured and or calculated by formula. Normal body osmolality is 270-290 milliosmals per kilogram. Sodium is the major determinant of osmolality. 

Water crosses freely across the membranes from areas of low solute concentration to areas of high solute concentration. The passage of electrolytes, like sodium and potassium, are restricted across the cell membranes by osmotic pressure. Body fluids and electrolytes are normally maintained within a narrow range. The kidneys are the main organs which control body fluids.