What does an increase in plasma lactic acid indicate during exercise?

• A. Decreased oxygen demand
• B. Increased metabolic activity in muscles
• C. Increased oxygen affinity of hemoglobin
• D. Decreased glycolysis rate

Answer: (B)

An increase in plasma lactic acid during exercise indicates that there is increased metabolic activity in the muscles. When muscles are engaged in strenuous activity, the demand for energy rises and, as a result, the glycolytic pathway is upregulated to meet this need. In the absence of adequate oxygen (as can occur in high-intensity exercise), cells switch to anaerobic metabolism, which converts glucose to lactic acid instead of completely oxidizing it to carbon dioxide and water.

This shift to anaerobic metabolism leads to the accumulation of lactic acid in the muscles and subsequently in the bloodstream. The presence of lactic acid serves as a marker for this anaerobic metabolic activity, reflecting that the muscles are working harder than can be sustained through aerobic respiration alone. Hence, the increase in plasma lactic acid is directly tied to the heightened energy demands of the exercising muscles.