Model Answer
0 min readIntroduction
Acid-base balance, maintaining a stable pH (typically 7.35-7.45) in body fluids, is crucial for optimal enzymatic function and cellular metabolism. Disruptions, leading to acidosis (low pH) or alkalosis (high pH), can have severe physiological consequences. The kidneys play a vital role alongside the lungs in regulating this delicate balance. Unlike the lungs, which primarily regulate volatile acids (CO2), the kidneys handle non-volatile acids, primarily metabolic acids. This answer will explore the intricate mechanisms employed by the kidneys to maintain acid-base homeostasis.
Kidney’s Role in Acid-Base Balance: A Physiological Perspective
The kidneys contribute to acid-base balance through several mechanisms, primarily involving the regulation of bicarbonate (HCO3-) and the excretion of hydrogen ions (H+) and ammonium (NH4+). These processes occur in the proximal and distal tubules, as well as the collecting ducts.
1. Bicarbonate Reabsorption and Generation
Bicarbonate is the primary buffer in the extracellular fluid. The kidneys reabsorb approximately 90-95% of filtered bicarbonate under normal conditions. This process is complex:
- Proximal Tubule: Most bicarbonate reabsorption occurs here. The reaction involves carbonic anhydrase, which catalyzes the conversion of CO2 and H2O to carbonic acid (H2CO3), which then dissociates into H+ and HCO3-. The H+ is secreted into the tubular lumen, and bicarbonate is reabsorbed.
- Distal Tubule and Collecting Duct: When plasma bicarbonate levels are low, the kidneys generate new bicarbonate. This involves buffering the H+ secreted in the distal tubule and collecting duct with urinary buffers like phosphate and ammonia.
2. Ammonium (NH4+) Excretion
The kidneys excrete excess acid as ammonium. This process, known as the ammonium shuttle, is particularly important during periods of chronic metabolic acidosis.
The reaction involves the following steps:
- Glutamine is metabolized in the proximal tubule, producing ammonium (NH4+) and bicarbonate (HCO3-).
- NH4+ is secreted into the tubular lumen in exchange for H+.
- The newly generated HCO3- is then reabsorbed into the bloodstream.
3. Phosphate Buffering
Phosphate, like ammonium, can act as an intracellular buffer in the proximal tubule. It binds to H+, preventing it from entering the tubular lumen and facilitating bicarbonate reabsorption.
| Mechanism | Location | Process |
|---|---|---|
| Bicarbonate Reabsorption | Proximal Tubule | CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3- (catalyzed by carbonic anhydrase) |
| Ammonium Excretion | Proximal/Distal Tubule | Glutamine metabolism → NH4+ + HCO3-; NH4+ secretion in exchange for H+ |
| Phosphate Buffering | Proximal Tubule | H+ + HPO42- ⇌ H2PO4- |
Clinical Significance
Kidney dysfunction can severely impair acid-base balance. Acute kidney injury or chronic kidney disease can lead to metabolic acidosis due to reduced bicarbonate reabsorption and impaired ammonium excretion. Certain medications, such as carbonic anhydrase inhibitors, can also affect kidney function and disrupt acid-base balance. For example, patients with diabetic ketoacidosis often require bicarbonate administration to correct the severe acidosis.
Conclusion
In conclusion, the kidneys are indispensable in maintaining acid-base balance through intricate mechanisms involving bicarbonate reabsorption/generation and ammonium excretion. These processes are vital for overall physiological homeostasis. Dysfunction of the kidneys can lead to significant acid-base disturbances, highlighting the importance of renal health in maintaining overall well-being. Further research into targeted therapies for acid-base disorders remains a crucial area of medical advancement.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.