During moderate aerobic exercise

Options

A. arterial pH decreases
B. blood lactate level in decreases
C. alveolar ventilation increases
D. All of the above

Answer

### Step-by-Step Solution:

1. **Understanding the Context of Moderate Aerobic Exercise**:
- During moderate aerobic exercise, the body requires more energy to support the increased activity levels. This energy is primarily derived from the breakdown of glucose through aerobic respiration, which necessitates oxygen.

2. **Oxygen Demand**:
- As the skeletal muscles contract during exercise, their demand for oxygen increases. This is because oxygen is essential for the production of ATP (adenosine triphosphate), the energy currency of the cell.

3. **Gaseous Exchange**:
- The body obtains oxygen from the atmosphere through the process of gaseous exchange. This occurs in the alveoli of the lungs, where oxygen is absorbed into the bloodstream, and carbon dioxide (a by-product of metabolism) is expelled.

4. **Changes in Blood Chemistry**:
- With increased exercise, carbon dioxide levels in the blood rise due to enhanced metabolic activity. This leads to a decrease in blood pH (increased acidity) because carbon dioxide reacts with water to form carbonic acid, which dissociates into bicarbonate and hydrogen ions.

5. **Ventilation Rate**:
- To meet the increased oxygen demand and to eliminate the excess carbon dioxide, the body responds by increasing the rate of breathing. This is referred to as increased alveolar ventilation. The respiratory rhythm is regulated to ensure that sufficient oxygen is supplied and carbon dioxide is expelled efficiently.

6. **Lactic Acid Levels**:
- During moderate aerobic exercise, the levels of lactate (lactic acid) do not significantly change. Lactic acid is produced during anaerobic respiration, which occurs when oxygen levels are insufficient. However, in moderate exercise, aerobic metabolism is sufficient to meet energy demands, thus preventing a significant accumulation of lactic acid.

7. **Conclusion**:
- Therefore, the primary physiological change during moderate aerobic exercise is an increase in the rate of alveolar ventilation (breathing rate) to accommodate the higher oxygen demand and to expel carbon dioxide effectively.

### Final Answer:
The correct answer is that during moderate aerobic exercise, the **alveolar ventilation increases**.

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