Problem 16 - Entrance Test

Strenuous exercise leads to increased lactic acid production, causing metabolic acidosis (drop in blood pH). The body has multiple buffering systems to maintain pH homeostasis. 1. Bicarbonate Buffer System: This is the most important buffer system in the extracellular fluid (including blood plasma). It involves carbonic acid (H2CO3) and bicarbonate ions (HCO3-). When H+ ions (from lactic acid) increase, they react with HCO3- to form H2CO3. H2CO3 then dissociates into CO2 and H2O. The increased CO2 stimulates chemoreceptors. 2. Respiratory Compensation: The increased CO2 and decreased pH are detected by peripheral chemoreceptors and (to a lesser extent) central chemoreceptors. This leads to an increased depth and rate of breathing (hyperventilation). Hyperventilation expels more CO2 from the body, thereby shifting the bicarbonate buffer equation to the left (removing H2CO3) and reducing H+ concentration, effectively raising pH. 3. Hemoglobin Buffer System: Hemoglobin in red blood cells can also bind H+ ions, acting as an intracellular buffer. 4. Phosphate Buffer System: Important in intracellular fluid and renal tubules, but less significant in buffering sudden changes in extracellular fluid pH compared to the bicarbonate system. 5. Renal Compensation: Kidneys can excrete H+ and reabsorb HCO3-, but this is a slower, long-term mechanism (hours to days), not an immediate response to sudden acidosis from exercise. Let's evaluate the options: A. Increased renal excretion of H+ ions and reabsorption of HCO3-: This is a renal compensatory mechanism, which is effective for chronic acid-base imbalances but too slow for the immediate buffering of lactic acid during acute exercise. B. Rapid increase in the activity of the phosphate buffer system in extracellular fluid: While a buffer, the phosphate system is less abundant and less important in the extracellular fluid compared to the bicarbonate system for acute changes. C. The bicarbonate buffer system, coupled with increased ventilation rate: This is the most accurate description of the primary immediate response. The bicarbonate system buffers the H+ ions, producing CO2. The subsequent increase in ventilation (respiratory compensation) quickly expels this CO2, effectively removing acid from the body and restoring pH. This coordinated action is crucial. D. Decreased hemoglobin affinity for CO2, enhancing CO2 transport: The Haldane effect describes how oxygenation decreases hemoglobin's affinity for CO2, facilitating CO2 release in the lungs. While important for CO2 transport, this doesn't directly address the buffering of lactic acid (H+ ions) from muscle, but rather the removal of CO2 produced as a result of buffering. The Bohr effect (H+ binding to Hb leading to O2 release) is more relevant for Hb as a buffer.