What are fluids and electrolytes?

Structure and Functions

Humans live in a wide variety of environmental conditions. Some days are hot and wet, others are cold and dry, and most are somewhere between. At the same time, as foods and liquid are taken in, the body is exposed to a variety of chemical substances over a wide range of concentrations. Amid these widely changing circumstances, the internal environment, to which the body’s cells are exposed, remains essentially unchanged. This regulation of the internal environment, which is called homeostasis, is necessary for the correct functioning of the body. Essentially, all the organs and tissues of the body play roles in the homeostatic processes, and the main control mechanism operates through the movement of body fluids.

There are several different body fluids, but they are all solutions of solutes in water. The identity of the solutes and their concentrations differentiates one body fluid from another. Among the solutes, two categories exist. Some solutes dissociate into electrically charged particles when they dissolve and are thus called electrolytes. Others remain as neutral particles dissolved in the water and are nonelectrolytes. Both types of solutes play important roles in the correct physiological functioning of the body, but it is the electrolytes that draw the most attention. This is the case because the fluids and the electrolytes are interdependent and because imbalances of these factors are associated with a vast array of illnesses.

Although subject to some variation with age, gender, and physical condition, the body is composed of about 60 percent water by weight. For purposes of classification, this water is considered to be present in compartments. It is important to recognize that this terminology is conceptual only and does not refer to the existence of any real, separate, water-containing compartments in the body. Approximately twenty-five cubic decimeters of water are contained within the body’s cells; this is the intracellular fluid. Most of the remaining fluid, about twelve cubic decimeters, is termed extracellular and exists in the regions exterior to cells. The
extracellular fluid is further subdivided into the categories of interstitial fluid, which surrounds the cells; intravascular fluid, which is located within the blood vessels; and transcellular fluid, which includes the fluid found in the spinal column, the region of the lungs, the area surrounding the heart, the sinuses, and the eyes, along with sweat and digestive secretions. These subcategories are listed in order of the amount of fluid present. Of all these types, only the intravascular fluid is directly affected when a person drinks or eliminates fluid. Alterations in the other regions occur in response to that change, however, and there is a continual dynamic exchange of fluid among all compartments. The balance of conditions created by this exchange determines the state of health of the individual.

The solutes that are electrolytes generate positively charged ions called cations and negatively charged ions called anions. The amount of positive charge present in a solution is always equal to the amount of negative charge. The major cations present are hydrogen, sodium, potassium, calcium, and magnesium. The most important anions are chloride, hydrogen carbonate, hydrogen phosphate, sulfate, and those derived from organic acids such as acetic acid. Several other ions of both types are present at very low levels. The nonelectrolytes present include urea, creatinine, bilirubin, and glucose. All these solutes are involved with particular biological changes in the body, so their presence at the correct concentration is vital.

The fluid and its solutes move within the body by means of several transport mechanisms, some of which move solutes through the fluid and some of which move either water or the solutes from one side of a cell membrane to the other. The mechanisms available are diffusion, active transport, filtration, and osmosis. Diffusion is the movement of particles through a solution from a region in which the concentration of the particles is high to a region in which it is lower. The energy that drives this motion is thermal energy, and the transport rate is increased by increasing the temperature, which increases the concentration difference from point to point and is faster for smaller particles. Cell walls are a barrier to this type of transport unless the solute particles are small enough to pass through pores in the wall or are soluble in the cell wall itself. Active transport provides another means of moving solutes across cell walls. The energy for such movement is provided by a series of chemical reactions involving adenosine triphosphate. The movement of sodium out of and potassium into cells, as well as the transport of amino acids into cells,
occurs in this manner. Filtration is a means by which both water and some solutes are transported through a porous membrane. The solutes transported are those that are small enough to pass through the pores in the membrane. The driving force for filtration is provided by a difference in pressure on the two sides of the membrane, and the motion occurs from the high-pressure side to the low-pressure side. The pressure in this case results from gravity and from the pumping action of the heart. Osmosis is a process by which water is moved across a semipermeable membrane as the result of the influence of a different type of pressure. When two solutions of different concentrations of solute particles are separated by a semipermeable membrane, an osmotic pressure develops that acts as the driving force to move water from the side of the membrane where the concentration of solute particles is lower to the side where the solute particle concentration is higher.

A solute’s concentration in the body fluid has a great effect on the transport of materials and thus on the body’s health. Concentrations in body fluids are expressed in several ways. Electrolyte concentration is often expressed in terms of milliequivalents of solute per cubic decimeter of solution. This is a measure of the amount of change, positive or negative, provided by that solute. A solution with twice the number of milliequivalents per cubic decimeter will have twice the concentration of change. This also measures the solute’s combining power, because one milliequivalent of cations will chemically combine with one milliequivalent of anions. Osmolality, osmolarity, and tonicity refer to a solution’s ability to provide an osmotic pressure. Osmolality and osmolarity are proportional to the number of particles of solute present in the solution. When solutions of different osmolalities or osmolarities are separated by a semipermeable membrane, there will be a flow of solvent across the membrane. Isotonic solutions have equal osmotic effects. Tonicity is a way of comparing the osmotic potential of solutions by referring to one as being hypotonic, isotonic, or hypertonic to the other.

Disorders and Diseases

There are two ways to approach thinking about the health role of body fluids and electrolytes. One is to consider one particular fluid component, such as sodium, that is out of balance and proceed to trace possible causes of the imbalance and appropriate treatment modes. It must be noted, however, that there are many possible illnesses that could cause any particular imbalance. The second approach is to consider a representative number of specific diseases and to look at their effect on the fluid and electrolyte balance and how such effects may be treated.

The first of these two approaches is adopted here because it highlights the fluids and electrolytes themselves rather than the diseases. Two imbalances will be considered as examples of the types of effects seen. First to be considered is the volume of fluid itself. Second, the balance of calcium will be given attention because of the connection of calcium deficiency with the bone brittleness that often occurs during aging.

Volume imbalance that is larger than the system’s normal regulatory ability to control may occur in either the intracellular or extracellular fluid or both and may be in the direction of too little fluid (dehydration) or too much (overhydration). Both of these effects may result from a number of underlying illnesses, but each is, by itself, life-threatening and requires direct treatment. Often, this treatment precedes the diagnosis of the root cause.

The body apparently senses fluid volume imbalance with receptors near the heart, and several coping responses are triggered. Dehydration can be the result of vomiting, diarrhea, excessive perspiration, or blood loss. In such cases, the body’s responses are in the direction of maintaining the flow of blood to vital organs. Vessels at the extremities are constricted, and those in the regions of the vital organs are dilated. Kidney function is greatly slowed, the reabsorption of sodium is increased, and the production of urine is markedly decreased, ensuring water retention. Centers in the hypothalamus respond and cause the individual to become thirsty. Thus, the body acts to protect its most important functions while at the same time stimulating actions from the individual that will bring additional fluid volume into the system. The manner in which the individual responds to being thirsty will determine other bodily changes. If plain water is used to quench the thirst, the extracellular fluid becomes less concentrated in electrolytes than is the intracellular fluid, causing an
osmotic pressure imbalance that the body regulates by transporting more water into the cells, producing overhydration there and aggravating the original dehydration in the extracellular fluid. Notice that this means that drinking large amounts of water can, strange though it may seem, cause dehydration. If saltwater is ingested, the reverse occurs, with a resulting dehydration of the cells that in turn triggers extreme thirst but few cardiovascular problems. Proper volume replacement thus requires that the water brought into the system be of the same electrolyte concentration as the cellular fluids—that is, that they be isotonic. In that case, the osmotic pressure remains balanced and the fluid volumes in both of the major compartments can be built up.

Overhydration is a less common occurrence that is usually associated with cardiovascular disease, severe malnutrition and kidney disease, or surgical stress. When the heart is not able to act as an effective pump, a back pressure builds in the circulatory system that causes fluid to be filtered through the walls of the vessels and that results in the accumulation of fluid in the interstitial regions around the heart and lungs. A decrease in proteins in the bloodstream, resulting from either malnutrition or kidney malfunction, lowers the osmotic pressure in the blood and causes water retention in the interstitial spaces. Accumulation of excess fluid in the interstitial spaces is called edema. This same end condition also arises when the kidney excessively filters fluid from the bloodstream into the interstitial spaces. The treatment of overhydration takes the form of fluid intake restriction, restriction of dietary sodium, and the use of diuretic therapy to stimulate urine production.

Calcium, much of which comes from milk and milk products, is the fifth most abundant ion in the body and is involved with the formation of the mineral component of teeth and bones, the contraction of muscles, proper blood clotting, and the maintenance of cell wall permeability. Calcium is added to extracellular fluid as a result of the intestinal absorption of dietary calcium and bone resorption. It is lost from the extracellular fluid via secretion into the intestinal tract, urinary excretion, and deposition in bone. The maintenance of a proper calcium level mainly depends on processes occurring in the intestinal tract. Only a very small part of the body’s total calcium is in fluids. Both hypocalcemia and hypercalcemia, the shortage and the overabundance of calcium in the fluids, may occur. Unlike the case of water shortage or excess, however, there are few direct visual consequences of a calcium imbalance; one must rely on laboratory testing of the fluid and on indirect physical assessment.

Hypocalcemia in the blood is associated with reduced intake, increased loss, or altered regulation, as in hypoparathyroidism. Bone, a living material, continually absorbs and desorbs calcium. The parathyroid gland secretes a hormone that regulates bone resorption and thus can raise the calcium level in the extracellular fluid at the expense of decreasing the amount of bone. Obviously, this cannot be a long-term mechanism to provide calcium. The same hormone also regulates the absorption of calcium from the intestines and the kidneys. Vitamin D is an essential, although indirect, factor in permitting the absorption of calcium from the intestine. A deficiency of this vitamin is a major cause of hypocalcemia. When the calcium level in the extracellular fluid falls below normal, the nervous system becomes increasingly excited. If the level continues to fall, the nerve fibers begin to discharge spontaneously, passing impulses to the peripheral skeletal muscles,
where they cause a contractive spasm. Often, this is first seen in a contracting of the fingers. Generalized muscular spasming can be lethal if the calcium imbalance is not corrected quickly. Immediate calcium deficiency is treated with the administration of either oral or intravenous calcium compounds, with vitamin D therapy, and with the inclusion of foods of high calcium content in the diet. In the longer term, treatment of the underlying illness is necessary.

The opposite imbalance, hypercalcemia, can occur as a result of an excessive intake of calcium supplements and vitamin D, in conjunction with a high-calcium diet. Calcium excess is also associated with some tumors and with kidney or glandular diseases. It has also been found to be caused by prolonged immobility, in which case the bones resorb because of the lack of bone stress. This latter effect has been of major concern in the space program. Too high a level of calcium in the intercellular fluid causes a depression of the nervous system and a slowing of reflexes. Lack of appetite and constipation are also common results. At very high levels, calcium salts may precipitate in the blood system, an effect that can be rapidly lethal. Again, in the long term, the underlying cause of the imbalance must be corrected, but treatments do exist for more immediate alleviation. As long as the kidneys are functioning correctly, intravenous treatment with saline serves as a means of flushing out excess calcium. Calcium also
can be bound to phosphate that is delivered intravenously, but there is a risk of causing soft tissue precipitation of the calcium phosphate compound. Dietary control is used, at times in concert with steroid therapy, to counter high calcium levels. If resorption is the cause of the excess, there are therapies, both chemical and physical, that are effective in increasing bone deposition.

Perspective and Prospects

From the earliest times, those concerned with the treatment of illnesses have had their attention drawn to the fluids present in or exuded by the human body. The color, smell, and texture of fluids being given off by a sick or injured person provided clues to the nature of the illness or injury. Bleeding was commonly practiced as a means of venting the illness so that health could be restored. Lancing of ulcerative conditions was also practiced by early healers. These early attempts at understanding and of treatment have been greatly refined, and the search for better understanding and improved treatment modes continues.

This concern with fluids and electrolytes is easy to understand. The fluids and their components constitute both the external and the internal environment for all the body’s tissues and cells. Any abnormality in the cells or tissues is reflected in a variation from normal conditions in the fluids. All major illnesses and many minor ones have associated with them a fluid and electrolyte disorder. Fluids are more readily accessible for study than are tissues from deep within the body; hence, a considerable effort has been directed at measuring fluid constituents and interpreting the findings. The testing of fluids has evolved from highly labor intensive measurements of a few components to highly automated testing procedures applied to dozens of components. The reliability and precision of the measurements continue to increase, and the scope of measurements continues to expand.

Not all that is to be known about fluids and electrolytes, however, depends on laboratory testing. Some knowledge can be collected from close observation of the patient. Although the resulting measurements are not precise, they are nevertheless important because they are much more immediately available. Physical symptoms that carry information about fluids and electrolytes include the following: sudden weight gain or loss; changes in abdominal girth; changes in either the intake or output of fluids; body temperature; depth of respiration; heart rate; blood pressure; skin moisture, color, and temperature; the skin’s ability to relax to normal after being pinched; the swelling of tissue; the condition of the tongue; the appearance of visible veins; reflexive responses; apparent mental state; and thirst. Each of these observations, and more, is readily available to one who is monitoring the health of an individual.

As is the case with most testing and data-gathering situations, interpreting the test and observation results is the critical step. Any one measure, by itself, points to a vast array of possible illnesses. Only by considering the whole and recognizing the existence of patterns in the information can a health professional narrow the possibilities. It is this recognition of patterns that develops with education and experience, and it is this step that relies on judgment that makes medicine an art as well as a science.

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