10.7 · Sodium & potassium imbalance

Importance of sodium

Water balance, conduction of nerve impulses, regulation of acid-base balance, biochemical reactions, and membrane transport

Sodium regulation

No sodium receptors have been discovered, so sodium levels are coupled with water levels and therefore with blood pressure and volume

Why sodium determines osmotic pressure

It has the highest concentration of any ion in the extracellular fluid

Hormones controlling sodium

Aldosterone, ADH, and natriuretic hormones, which control reabsorption of water and sodium in the kidneys

Hypernatremia

Excess sodium ions in the blood, above 145 mEq/L

Cellular effect of hypernatremia

Intracellular dehydration by osmosis, causing cells to shrink

Manifestations of hypernatremia

Thirst, alterations in membrane potential, and nervous system effects including confusion, coma, and convulsions

Causes of hypernatremia

Retention or infusion of sodium, increased loss of body fluids, or decreased water intake

Cause of sodium retention

Oversecretion of aldosterone or ACTH, or IV infusion such as sodium bicarbonate to treat acidosis

Hypernatremia due to sodium retention

Hypervolemia with increased blood pressure, bounding pulse, weight gain, pitting edema, and ascites

Hypernatremia due to fluid loss

Hypovolemia with low blood pressure, weak pulse, tachycardia, decreased urine output, and dehydration symptoms

Hyponatremia

Decreased concentration of sodium ions in the blood, below 135 mEq/L

Cellular effect of hyponatremia

Water moves into the cells by osmosis, causing cells to swell

Most common cause of hyponatremia

Dilutional hyponatremia from retention of water

Water intoxication

Replacement of body fluids with electrolyte-free fluids

Other causes of hyponatremia

SIADH, acute renal failure, severe congestive heart failure, loss of sodium from burns, vomiting, diarrhea, diuretics, or insufficient aldosterone, and gaining more water than sodium

Manifestations of hyponatremia

Lethargy, apprehension, confusion, depressed reflexes, seizures, coma, muscle twitching, and weakness

Importance of potassium

Fluid balance, nerve impulse conduction, and biochemical reactions; the ECF/ICF ratio directly affects the resting membrane potential

Renal regulation of potassium

Negative feedback release of aldosterone controls serum potassium through sodium-potassium exchange, and an H+/K+ transporter responds to increased serum potassium

Potassium and acid-base status

ECF potassium levels rise with acidosis and fall with alkalosis

Insulin and potassium

Insulin causes uptake of potassium into cells by stimulating the ATP pump

Hyperkalemia

Excess potassium ions in the blood, above 5.0 mEq/L

Causes of hyperkalemia

Renal disease, undersecretion of aldosterone, too-rapid IV intake, a shift from intracellular to extracellular from trauma, burns, hypoxia or acidosis, and insulin deficits

Manifestations of hyperkalemia

Decreased neuromuscular excitability with paresthesias progressing to loss of muscle tone and paralysis

Most serious effect of hyperkalemia

Slowed heart rate leading to ventricular fibrillation or cardiac arrest

Hypokalemia

Decreased concentration of potassium ions in the blood, below 3.5 mEq/L

Causes of hypokalemia

Excessive loss from diuretics or increased aldosterone and cortisol, inadequate intake such as fad diets, and increased entry into cells from drugs, excess insulin, or metabolic alkalosis

Manifestations of hypokalemia

Increased urine output with polyuria and thirst, muscle weakness, fatigue, cramps, and cardiac arrhythmias