Water and Electrolytes
Water constitutes 50-55% of a woman's body weight and 55-60% of man's weight. Water gives structure and form to the body, helps maintain body temperature, and also creates the necessary environment for cell metabolism and
There are two compartments of body water, extracellular and intracellular fluid. Extracellular fluid (ECF) is water found outside of cells. Making up approximately 20% of total body weight, the ECF consists of blood plasma, interstitial fluid surrounding the cells, secretory fluid, which is water in transit, and dense tissue fluid, which is water located within dense connective tissue such as cartilage and bone. Intracellular fluid (ICF) is the water inside the cells. It makes up 35-40% of total body weight.
The maintenance of correct proportions of ECF and ICF is vital to proper functioning of the body, as can be seen below in Clinical Problems. The body maintains fluid balance by calibrating water intake, excretion, and internal processes controlling water distribution.
Water Intake (Thirst)
The thirst mechanism in animals is a complex interaction of control centers in the brain and hypothalamus; though the details of that mechanism go well beyond the scope of this SparkNote, it is important to know that the thirst mechanism is linked to water loss. When total body fluid volume decreases by 0.5-1.0%, the thirst mechanism asserts itself. Approximately 55% of water intake is derived directly from fluids, 35% from food, and 10% from water produced as a byproduct of metabolism.
Kidneys excrete approximately 1 to 2 liters of urine per day. Approximately 900 milliliters (ml) of this amount is obligatory water excretion that gets rid of solutes and is constant from day to day. The remainder is excreted according to the fluctuating needs of the body and the changing renal tubular reabsorption rate.
The process by which the body loses water through the skin is called insensible water loss. Approximately 350 ml of water is excreted by diffusion through the skin, while another 100 ml is lost through normal perspiration. Heavy perspiration may cause a greater loss.
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.
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.
Hypertonic Dehydration and Hypotonic Dehydration
Usually caused by excessive loss of water or low water intake, hypertonic dehydration refers to the bodily condition when the osmotic pressure of ECF is higher than that of ICF, causing water to shift from the cell into the ECF. In this scenario, water loss exceeds electrolyte losses. Symptoms of hypertonic dehydration include thirst, a hot and dry body, vomiting, disorientation, and low output of concentrated urine. There are a variety of possible reasons for inadequate water intake, including a defective thirst center, impaired consciousness, a lack of water, or an inability to drink water.
The opposite situation, in which water shifts from the ECF to the cell, is called hypotonic dehydration. This can be caused by excessive intake of water without an adequate amount of electrolytes. Symptoms include weakness without thirst or decreased urine output.
Imbalances Caused by Disease
Diseases can greatly affect the bodies ability to regulate its fluid balance, resulting in swelling, pain, and other symptoms. Listed below are the most common diseases affecting water balance.
- Gastrointestinal disorders may result in excess loss of water and electrolytes in secretions, giving rise to symptoms such as weakness, vomiting, and disorientation.
- Congestive heart failure causes cardiac edema, an excessive accumulation of fluids in the body resulting in swelling.
- Liver disease causes a decrease in plasma protein that can result in ascites, an accumulation of excessive fluid in the abdomen.
- Renal disease, such as nephrosis, also results in ascites.
- Decreased protein stores that can result from the malnutrition often associated with cancer can cause edema.
- Hypertension causes an increase in fluid volume associated with sodium retention or insufficient potassium.
Normal intake of water through fluids is approximately 1200 to 1500 ml per day. About 700 to 1000 ml are ingested daily in foods.