Fluid and electrolyte balance
Regulation of fluid by the body is necessary to maintain homeostasis. If the water or electrolyte equilibrium is affected, many body functions fail to proceed at normal rates.
Water
Water is a major constituent of all living things. Most of the ions and molecules that make up living matter have chemical and physical relationships with water.
The total body water content varies among different species, age, sex, nutritional factors etc. Water content is highest in the new born animal and declines as age advances.
An adult contains 60% of water by weight depending upon age and the amount of body fat. Body fat is inversely proportional to the body water content. For example, a very lean animal will contain 70% of water whereas very fat animal will have only 40% of total body water.
Body fluid Compartments
Body fluid is present in three different compartments namely, intracellular fluid and extracellular fluid, which in turn is divided into interstitial fluid and plasma.
In a lean animal, 50% of water is present intracellular, 15% in the interstitial spaces and 5% in the blood plasma. Apart from this, water is also present in the transcellular fluids such as in CSF, aqueous humor of the eye, synovial fluids, urine, bile etc.
Water molecules can rapidly penetrate most of the cell membrane. If an osmotic or hydrostatic pressure gradient exists between body fluid compartments, a shift of water will occur. Addition of an isotonic NaCl solution to the ECF causes equal distribution of water extracellularly and intracellularly.
If a hypertonic NaCl solution is added, water would begin to shift into the plasma, while, an addition of hypotonic NaCl solution shifts the water into the cell.
Water balance
The total amount of water in the body almost remains relatively constant. The body gains water either by ingestion or as end product of cellular metabolism. Similarily loss of water occurs in urine, from the skin, expired gases, faeces etc.