Electrolytes and Their Functions
Electrolytes are important constituents of body fluids. These compounds separate into positively and negatively charged ions in solution. Positively charged ions are called cations; negatively charged ions are called anions. Electrolytes are so-named because they conduct an electrical current in solution. A few of the most important ions are reviewed next:
* Positive ions (cations):
* Sodium is chiefly responsible for maintaining osmotic balance and body fluid volume. It is the main positive ion in extracellular fluids. Sodium is required for nerve impulse conduction and is important in maintaining acid-base balance.
* Potassium is also important in the transmission of nerve impulses and is the major positive ion in intracellular fluids. Potassium is involved in cellular enzyme activities, and it helps regulate the chemical reactions by which carbohydrate is converted to energy and amino acids are converted to protein.
* Calcium is required for bone formation, muscle contraction, nerve impulse transmission, and blood clotting.
* Negative ions (anions):
* Phosphate is essential in carbohydrate metabolism, bone formation, and acid-base balance. Phosphates are found in plasma membranes, nucleic acids (DNA and RNA) and ATP.
* Chloride is essential for the formation of hydrochloric acid in the stomach. It also helps to regulate fluid balance and pH. It is the most abundant anion in extracellular fluids.
Figure 17-1 Main fluid compartments showing relative percentage by weight of body fluid. Fluid percentages vary but total about 60% of body weight. Fluids are constantly exchanged among compartments, and each day fluids are lost and replaced.
The body must keep electrolytes in the proper concentration in both intracellular and extracellular fluids. The maintenance of water and electrolyte balance is one of the most difficult problems for health workers in caring for patients. Although some electrolytes are lost in the feces and through the skin as sweat, the job of balancing electrolytes is left mainly to the kidneys.
The Role of Hormones Several hormones are involved in balancing electrolytes. Aldosterone, produced by the adrenal cortex, promotes the reabsorption of sodium (and water) and the elimination of potassium. In Addison disease, in which the adrenal cortex does not produce enough aldosterone, there is a loss of sodium and water and an excess of potassium. When the blood concentration of sodium rises above the normal range, the pituitary secretes more antidiuretic hormone (ADH). This hormone increases water reabsorption in the kidney to dilute the excess sodium. Hormones from the parathyroid and thyroid glands regulate calcium and phosphate levels.
Parathyroid hormone increases blood calcium levels by causing the bones to release calcium and the kidneys to reabsorb calcium. The thyroid hormone calcitonin lowers blood calcium by causing calcium to be deposited in the bones.
The pH scale is a measure of how acidic or basic (alkaline) a solution is. As described here, the pH scale measures the hydrogen ion (H+) concentration in a solution. Body fluids are slightly alkaline in a pH range of 7.35 to 7.45. These fluids must be kept within a narrow range of pH, or damage, even death, will result. A shift in either direction by three tenths of a point on the pH scale, to7.0 or 7.7, is fatal.
Regulation of pH
The body constantly produces acids in the course of metabolism. Catabolism of fats yields fatty acids and other acidic byproducts; cellular respiration yields pyruvic acid and, under anaerobic conditions, lactic acid; carbon dioxide dissolves in the blood and yields carbonic acid. Conversely, a few abnormal conditions may result in alkaline shifts in pH. Several systems act together to counteract these changes and maintain acidbase balance:
* Buffer systems. Buffers are substances that prevent sharp changes in hydrogen ion (H) concentration and thus maintain a relatively constant pH. Buffers work by accepting or releasing these ions as needed to keep the pH steady. The main buffer systems in the body are bicarbonate buffers, phosphate buffers, and proteins, such as hemoglobin in red blood cells and plasma proteins.
* Respiration. The role of respiration in controlling pH was described here. Recall that carbon dioxide release from the lungs makes the blood more alkaline by reducing the amount of carbonic acid formed. In contrast, carbon dioxide retention makes the blood more acidic. Respiratory rate can adjust pH for shortterm regulation.
* Kidney function. The kidneys regulate pH by reabsorbing or eliminating hydrogen ions as needed. The kidneys are responsible for long-term pH regulation.
If shifts in pH cannot be controlled, either acidosis or alkalosis results. Acidosis is a condition produced by a drop in the pH of body fluids to less than pH 7.35. This condition depresses the nervous system, leading to mental confusion and ultimately coma. Acidosis may result from a respiratory obstruction or any lung disease which prevents the release of CO2. It may also arise from kidney failure or prolonged diarrhea, which drains the alkaline contents of the intestine. Long-term excessive exercise under anaerobic conditions can produce lactic acidosis. Acidosis may also result from inadequate carbohydrate metabolism, as occurs in diabetes mellitus, ingestion of a low-carbohydrate diet, or starvation. In these cases, the body metabolizes too much fat and protein from food or body materials, leading to the production of excess acid. When acidosis results from the accumulation of ketone bodies, as in the case of diabetes, the condition is more accurately described as ketoacidosis. Alkalosis results from an increase in pH to greater than 7.45. This abnormality excites the nervous system to produce tingling sensations, muscle twitches, and eventually paralysis. The possible causes of alkalosis include hyperventilation (the release of too much carbon dioxide), ingestion of too much antacid, and prolonged vomiting with loss of stomach acids. It is convenient to categorize acidosis and alkalosis as having either respiratory or metabolic origins. Respiratory acidosis or alkalosis results from either an increase or a decrease in blood CO2. Metabolic acidosis or alkalosis results from unregulated increases or decreases in any other acids.