The graph shows the lactic acid concentration in the blood during exercise and during recovery after exercise.
Which process accounts for the shape of the graph at Z?
A) aerobic respiration of lactic acid in the kidney
B) aerobic respiration of lactic acid in the liver
C) anaerobic respiration of lactic acid in the kidney
D) anaerobic respiration of lactic acid in the liver
▶️ Answer/Explanation
Ans: B
The decrease in lactic acid concentration at Z indicates its removal from the blood. This occurs primarily in the liver, where lactic acid is converted back into glucose via aerobic respiration (the Cori cycle).
Anaerobic respiration (C, D) produces lactic acid but does not remove it. The kidney (A) plays a minor role in lactic acid metabolism, making B the correct answer.
Vigorous exercise can cause an oxygen debt.
Which process removes the oxygen debt?
A) aerobic respiration of lactic acid in the liver
B) a decrease in breathing rate
C) a decrease in heart rate
D) an increase in blood supply to the skin
▶️ Answer/Explanation
Ans: A
During vigorous exercise, muscles resort to anaerobic respiration, producing lactic acid as a byproduct. The oxygen debt is repaid through aerobic respiration in liver cells, where about 20% of lactic acid is completely oxidized to CO₂ and H₂O, while the remaining 80% is converted back to glucose via the Cori cycle.
Options B and C are incorrect because they describe reduced physiological responses that would actually prevent oxygen debt repayment. Option D relates to thermoregulation rather than metabolic recovery. The correct process (A) requires increased oxygen supply to convert the accumulated lactic acid (C₃H₆O₃) through the equation:
C₃H₆O₃ + 3O₂ → 3CO₂ + 3H₂O + energy
This explains why we continue breathing heavily after exercise stops – to supply the oxygen needed for these metabolic processes.