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Chapter 24: Water, Electrolyte, and Acid-Base Balance

Chapter 24: Water, Electrolyte, and Acid-Base Balance

1. Blood loss produces a drop in blood pressure. This stimulates the production of angiotensin II, which stimulates the thirst center.

2. The baby was suffering from hypotonic hydration and hyponatremia because it was losing sodium and other electrolytes in the ordinary course of urination, while the mother was giving it sodium-free water. (Infant kidneys are also less able than mature kidneys to conserve sodium.) Volume excess results in edema, as exhibited by this baby, and cerebral edema can produce seizures. Volume excess also produces acidosis because there is less Na⁺ reabsorption by the renal tubules, therefore less H⁺ secretion (which is linked to Na⁺ reabsorption by the Na⁺-H⁺ antiport). (This question is based on a case report in the U.S. Public Health Service publication, Morbidity and Mortality Weekly Reports, 9 September 1994.)

3. At the pH of the ECF, the reaction HCO_3^- + H⁺ -> H₂CO₃ + CO₂ would not proceed very far to the right in vitro, and would not buffer very much acid. In the body, however, the respiratory and urinary systems continually eliminate the CO₂. By the law of mass action, this keeps the reaction proceeding toward the right and neutralizing more acid.

4.	Cause	Effect	Explanation
a.	# H₂O	Ł Na⁺	Water dilutes the ECF, causing relative hyponatremia.
b.	# Na⁺	# Cl^-	Cl^- passively follows Na⁺ when Na⁺ is absorbed from the intestine or reabsorbed by a nephron.
c.	Ł K⁺	Ł H⁺	Hypokalemia causes K⁺ to diffuse out of the cells down its concentration gradient. This alters the electrical gradient across the plasma membrane and causes H⁺ to diffuse in to replace the K⁺. Thus the H⁺ concentration of the ECF drops.
d.	Ł H⁺	Ł K⁺	H⁺ dissociates from intracellular proteins and diffuses out of the cells down its concentration gradient. This creates a steeper electrical gradient from the outside to the inside of the cell (leaving the ICF more negative). K⁺ diffuses into the cells along this electrical gradient, leaving less of it in the ECF.
e.	# Ca²⁺	Ł PO₄^3-	As the kidneys reabsorb Ca²⁺, the excrete PO₄^3-. This keeps the Ca²⁺ ions in solution, because if Ca²⁺ and PO₄^3- concentrations both rise, they reach the solubility product at which they are deposited in the bones or other tissues instead of remaining in solution.

5. Chronic diarrhea induced by pathogens in contaminated drinking water (in cholera, for example) can result in hypovolemia and hypotension, which in turn can lead to cardiac arrest. Hypokalemia also develops as diarrhea flushes K⁺ from the body, especially when the dietary intake of K⁺ is inadequate. Hypokalemia often results in cardiac arrhythmia.

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