Electrolyte Balance

Electrolyte Balance

When the quantities of electrolytes the body gains equal those it loses, electrolyte balance exists. This is also maintained by homeostasis. The most import-ant electrolytes needed for cellular functions are sodium, potassium, calcium, magnesium, chloride, sulfate, phosphate, bicarbonate, and hydrogen ions. These are mostly provided in food but also are pres-ent in water and other beverages and as byproducts of metabolic reactions. Of all the electrolytes, sodium imbalance is most significant. A severe deficiency of electrolytes may produce a desire to eat salty foods known as salt craving. Salts help to control fluid move-ment in the body and provide needed minerals for excitability, membrane permeability, and secretory activities. Potassium and calcium are also among the most important electrolytes.

More electrolytes are lost by sweating on warm days and during strenuous exercise. Additional amounts are lost in the feces, but the greatest elec-trolyte output occurs because of kidney function and urine production. The kidneys control electrolyte out-put to maintain balance. Positive ions such as calcium, potassium, and sodium are essential for maintenance of cell membrane potential, muscle fiber contrac-tion, and nerve impulse conduction. Nearly 90% of positively charged ions in the extracellular fluids are sodium ions, which are regulated by the kidneys and the hormone aldosterone. Electrolyte imbalances are summarized in TABLE 23-2.

Sodium Balance

Salts primarily enter the body in foods and fluids,­ but lesser amounts are due to metabolic activity­. During catabolism of bone matrix and nucleic acids, phosphates are liberated. It is not difficult for us to obtain the amounts of electrolytes we need, how-ever. Most Americans eat much more sodium than they actually need. Natural foods contain plenty of sodium, whereas processed foods contain too much sodium. Table salt or sodium chloride is used to excess.

Salts are mostly lost from the body via sweating, vomiting, and in the urine and feces. Sweat is usually hypotonic, but large amounts of salt can be lost when sweating becomes profuse. Large losses of salt in vomit or feces may be linked to disorders of the gastrointes-tinal tract. To balance salts in the body, the kidneys must be healthy.

Potassium Balance

The intracellular fluid contains nearly 98% of the body’s potassium. This electrolyte diffuses out of the cellular cytoplasm into the extracellular fluid and therefore requires the cells to expend energy to recover its ions. The potassium ion concentration in the extracellular fluid is based on a balance between the rate at which the ions are gained across the diges-tive epithelium and the rate at which they are lost into the urine. The actions of the ion pumps in the distal parts of nephrons and the collection system regulate potassium loss in the urine. When a sodium ion is reabsorbed from the tubular fluid, there is usually an exchange between it and a cation, most commonly potassium, from the peritubular fluid.

Normally, between 50 and 150 mEq of urinary potassium ions are lost, whereas the same amount is absorbed across the digestive epithelium. There is only a small amount of potassium lost in the perspi-ration and feces. In the extracellular fluid, potassium ion concentration is controlled by regulating active secretion rates along the distal convoluted tubule and nephron collecting system.

Three factors relate to how the rate of tubular secretion of potassium ions varies: changes in the potassium ion concentration of the extracellular fluid, changes in pH, and aldosterone levels. Basically, the higher the concentration of potassium in the extracel-lular fluid, the higher the rate of secretion. When pH falls in the extracellular fluid, the pH of the peritubu-lar fluid also falls. There is then a decline in the rate of potassium secretion. This is because hydrogen ions, not potassium ions, are secreted as part of an exchange with sodium ions in the tubular fluid.

Aldosterone greatly affects the rate at which potassium ions are lost in the urine. This results from the ion pumps being sensitive to aldosterone and therefore reabsorbing sodium ions from the filtrate, exchanged for potassium ions from the peritubular fluid. Angiotensin II stimulates aldosterone secre-tion as part of blood volume regulation. Aldosterone secretion is also directly stimulated by high plasma potassium ion concentrations. The ways aldosterone influences the amounts of conserved sodium and the amounts of potassium excreted via the urine are closely related. Once plasma concentrations of potas-sium fall below 3.5 mEq/L hypokalemia develops, with extensive muscular weakness being followed by paralysis. Hypokalemia may cause death by affecting ­normal cardiac function.

Calcium Balance

There is more calcium in the body than any other mineral,­ and 99% of body calcium is deposited in the skeleton. This makes up 1 to 2 kg, or 2.2–4.4 pounds, of body calcium. Calcium is vital for controlling muscular and neural activities, for blood clotting, for ­forming the crystalline components of bones, as a cofactor for enzymatic reactions, and because of its second messenger functions. Calcium homeostasis is maintained in the extracellular fluid by parathyroid hormone and calcitriol but also by calcitonin to a smaller degree. Calcium ion concentrations are raised by parathyroid hormone and calcitriol, whereas calci-tonin opposes their actions. Calcitriol is produced by the kidneys.

Although a small amount of calcium is lost every day in the bile, only tiny amounts are lost via the urine or feces. Therefore, an adult must absorb only 0.8–1.2 g/day of calcium, which is only approx-imately 0.03% of the amount of calcium stored in the skeleton. Calcium absorption is stimulated by parathyroid hormone and calcitriol. It is absorbed in the digestive tract and reabsorbed in the distal convoluted tubule.

Hypercalcemia is present when the extracel-lular fluid calcium ion concentration is higher than 5.3 mEq/L. In adults, it is usually caused by hyperparathyroidism­, which is over-secretion of parathyroid hormone. Additional causes include malignant cancers of the breast, kidneys, bone marrow,­ or lungs. Excessive use of supplements containing­ calcium or vitamin D may also cause hypercalcemia. Hypercalcemia is considered severe when calcium ion concentration exceeds 12–13 mEq/L in the extracel-lular fluid. Signs and symptoms include confusion, fatigue, calcification of soft tissues such as the kidneys, and cardiac arrhythmias.

The opposite condition is hypocalcemia, in which there is a calcium ion concentration under 4.3 mEq/L. Much less common than hypercalcemia, this is ­usually caused by hypoparathyroidism, which is under­-­secretion of parathyroid hormone, chronic renal ­failure, or vitamin D deficiency. Signs and symptoms include osteoporosis, weak heartbeat, muscle spasms that may be accompanied by generalized convulsions, and cardiac arrhythmias.

Phosphate Balance

Phosphate ions are essential for the mineralization of bones. The mineral salts of the skeleton store approximately 740 g of phosphate ions. Phosphate most significantly affects the intracellular fluid, where it helps to activate enzymes, form high-­energy compounds, and synthesize nucleic acids. In the plasma the normal concentration of phosphate ions is 1.8–3.0 mEq/L. It is reabsorbed from the tubular fluid in the proximal convoluted tubule. This reabsorption is stimulated by calcitriol. Via the urine and feces, approximately 30–45 mEq, or 0.8–1.2 g of phosphate, is lost every day.

Chloride Balance

Chloride ions are the most common ions found in the extracellular fluid, with normal plasma concentrations between 100 and 108 mEq/L. Chloride is usually very low in the intracellular fluid, approximately 3 mEq/L. Chloride is absorbed across the digestive tract along with sodium. In the renal tubules, chloride and sodium ions are absorbed by several carrier proteins. Very little loss of chloride ions occurs via the urine and perspiration. Therefore, only 48–146 mEq, or 1.7–5.1 g/day, are required to maintain a chloride ion balance.

1. Define the terms electrolytes, water balance, and electrolyte balance.

2. List the causes and symptoms of dehydration.

3. Describe how salts are lost and gained by the body.