Anatomy and Physiology / Unit 10: The Urinary and Reproductive Systems
10.4 Fluid, Electrolyte and Acid-Base Balance
Notes
Total body water is about 60% of body weight in adults; it is divided between intracellular fluid (about two-thirds) and extracellular fluid (plasma and interstitial fluid).
- Water intake comes from drinking, food and metabolism; output is urine, sweat, breathing and feces. Thirst is triggered by the hypothalamus when blood is more concentrated.
- Sodium is the main extracellular cation; potassium is the main intracellular cation. Sodium controls water distribution.
- Blood pH is 7.35 to 7.45. Acidosis is pH below 7.35; alkalosis is above 7.45.
- Three defenses: chemical buffers (such as the bicarbonate system, immediate), the respiratory system (breathing out CO2, within minutes), and the kidneys (excrete hydrogen ions and conserve bicarbonate, slowest but most powerful).
- Example: in prolonged hypoventilation CO2 builds up, so pH drops (respiratory acidosis).
Practice
Blood pH is measured at 7.30. This condition is:
Normal blood pH is 7.35 to 7.45, so 7.30 is acidosis.
Which is the fastest response to a change in blood pH?
Buffers act immediately, then breathing adjusts, and the kidneys work last.
Which ion is the major cation of the extracellular fluid?
Sodium is the main extracellular cation and controls water distribution.
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