Major Minerals: Water and Electrolytes

Water loss through respiration is about 350 ml per day, varying with climate. About 150 to 200 ml is lost through feces.

Control of Water Distribution

(cellcontrols) Cellular Controls

For reasons of chemistry, water molecules tend to desire equality, passing from a more dilute solution with high water and low solute concentrations to a less dilute environment with low water and high solute concentration and thereby bringing the solutions to near conformity. This phenomenon of water movement is called osmosis. The bodies of animals and humans maintain proper fluid balance by taking advantage of this natural tendency of water through the manipulation of solutes. The body shifts the solutes, and the water, whether ECF or ICF, moves to follow and equalize the concentration. The movement of solutes in the body occurs in two separate ways. Diffusion is a passive process, requiring no energy, in which particles in solution spread throughout the solution and cross membranes to an area of lesser solute concentration. Active transport involves energy from the cell, but allows transport of particles across membranes from solutions with low concentrations to solutions with high concentrations.

There are three main types of solutes: electrolytes, plasma proteins, and small organic compounds.

Electrolytes are chemical elements such as acid, alkali, or salt dissociated into ions. Sodium comprises about 45% of the total electrolyte concentration. The sodium cation in ECF is the primary osmotic force in maintaining maintains the necessary water volume for cells. Chloride, the main anion in ECF, provides balance to sodium. The ICF contains potassium and phosphate. The concentration of electrolytes in a solution is based on the number of particles in solution. This concentration is measured in milliequivalents (mEq).
Plasma proteins are substances with large molecular weight that influence the shift of water from ECF to ICF, or vice versa. These are referred to as colloids since they form colloidal complexes that do not pass through membranes very well. These proteins, primarily albumin, remain in the blood vessels and maintain the integrity of blood volume by exerting a colloidal osmotic pressure (COP) that maintains the proper ration of water by pulling fluids and solutes from interstitial spaces into the blood circulation. Hydrostatic pressure, the pressure exerted by a liquid on the surfaces of walls that contain the liquid, serves to balance COP; it causes fluid to be pushed out of the capillary and into the interstitial fluid.
Small organic compounds such as glucose, urea, and amino acids, flow freely through membranes. They only affect water balance if they occur in unusually high concentrations.

Organismal Controls

On a broader, less localized level, the kidney is primarily responsible for maintaining water and electrolyte balance in the body. The kidney is alternately triggered to action by the hormones vasopressin and aldosterone.

Vasopressin, also called antidiuretic hormone (ADH), is secreted by the pituitary gland and stimulates the reabsorption of water. ADH secretion can be stimulated by a loss of body water, whether it is an actual loss or the result of a shift of water from plasma to interstitial ECF spaces as occurs in congestive heart failure. Aldosterone, secreted by the adrenal gland, acts primarily to conserve sodium, but in doing so has the affect of controlling water loss.
The mechanism of action of aldosterone is referred to as the renin- angiotensin-aldosterone mechanism. Renin is an enzyme secreted by the renal cortex into the blood under the circumstances of decreased sodium intake, sodium loss, or decreased fluid volume. Renin interacts with a serum globulin from the liver to form angiotensin I and angiotensin II in the blood. Angiotensin II increases the force of the heart beat, constricts arterioloes, and diminishes renal blood flow. This triggers the release of aldosterone. Aldosterone causes the kidneys to retain and reabsorb sodium. This action, in turn, conserves water and results in loss of potassium.