Water Balance

Objectives

When you have completed this section, you should be able to

• name the major fluid compartments and explain how water moves from one to another;
• list the body's sources of water and routes of water loss;
• describe the mechanisms of regulating water intake and output; and
• describe some conditions in which the body has a deficiency or excess of water or an improper distribution of water among the fluid compartments.

Think About It

Some tumors of the brain, pancreas, and small intestine secrete ADH. What type of water imbalance would this produce? Explain why.

Key Point Review

1. Name the two major fluid compartments and the subcategories of one of these. Approximately what fraction of total body water does each compartment contain?

2. List five routes of water loss. Which one accounts for the greatest loss? Which one is most controllable?

3. Explain why even a severely dehydrated person inevitably experiences further fluid loss.

4. Suppose there were no mechanisms to stop the sense of thirst until the blood became sufficiently hydrated. Explain why we would routinely suffer hypotonic hydration.

Electrolyte Balance

Objectives

When you have completed this section, you should be able to

• state the physiological roles of sodium, potassium, calcium, chloride, and phosphate;
• describe the hormonal and renal mechanisms that regulate the concentrations of these electrolytes; and
• state the term for an excess or deficiency of each electrolyte and describe the consequences of these imbalances.

Think About It

Some tumors of the adrenal cortex secrete excess aldosterone and may cause paralysis. Explain this effect and identify the electrolyte and fluid imbalances you would expect to observe in such a case.

Key Point Review

5. Which of these do you think would have the most serious effect, and why–a 5 mEq/L increase in the plasma concentration of sodium, potassium, chloride, or calcium?

6. Answer the same question for a 5 mEq/L decrease.

7. Explain why ADH is more likely than aldosterone to change the osmolarity of the blood plasma.

8. Explain why aldosterone hyposecretion could cause hypochloremia.

9. Why are more phosphate ions required in the ICF than in the ECF? How does this affect the distribution of calcium ions between these fluid compartments?

Acid-Base Balance

Objectives

When you have completed this section, you should be able to

• define buffer and write chemical equations for the bicarbonate, phosphate, and protein buffer systems;
• discuss the relationship between pulmonary ventilation, pH of the extracellular fluids, and the bicarbonate buffer system;
• explain how the kidneys secrete hydrogen ions and how these ions are buffered in the tubular fluid;
• identify some causes of respiratory and metabolic acidosis and alkalosis, and describe the effects of these pH imbalances; and
• explain how the respiratory and urinary systems correct acidosis and alkalosis, and compare the effectiveness and limitations of the two systems.

Think About It

In the systemic circulation, arterial blood has a pH of 7.40 and venous blood has a pH of 7.35. What do you think causes this difference?

Think About It

What protein do you think is the most important buffer in blood plasma? In erythrocytes?

Think About It

Suppose you measured the pH and ammonium chloride concentration of urine from a person with emphysema and urine from a healthy individual. How would you expect the two to differ, and why?

Key Point Review

10. Write two chemical equations that show how the bicarbonate buffer system compensates for acidosis and alkalosis and two equations that show how the phosphate buffer system compensates for these imbalances.

11. Why are phosphate buffers more effective in the cytoplasm than in the blood plasma?

12. Renal tubules cannot reabsorb HCO_3^-, and yet HCO_3^- concentration in the tubular fluid falls while in the blood plasma it rises. Explain this apparent contradiction.

13. In acidosis, the renal tubules secrete more ammonia. Why?

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