What happens to arterial PO2, PCO2, and H+ concentration during moderate excercise? How and why does this stimulate an increase in breathing rate?

During moderate exercise, several physiological changes occur in the body to meet the increased demand for oxygen and removal of carbon dioxide, which leads to adjustments in arterial PO2 (partial pressure of oxygen), PCO2 (partial pressure of carbon dioxide), and H+ concentration.

1. Arterial PO2: During exercise, the demand for oxygen by working muscles increases. To maintain an adequate oxygen supply, the body enhances oxygen uptake in the lungs and blood flow to the muscles. This results in a higher arterial PO2, as more oxygen is being delivered to the tissues.

2. Arterial PCO2: As exercise increases, the metabolic activity in muscles also increases. This leads to an increased production of carbon dioxide as a byproduct of energy production. The elevated PCO2 in the tissues drives the diffusion of carbon dioxide into the bloodstream and further into the alveoli of the lungs, where it is exhaled. This helps to maintain an appropriate arterial PCO2 level.

3. H+ Concentration: During exercise, the metabolic breakdown of glucose produces lactic acid, which increases the concentration of H+ ions in the muscles and blood. This creates an acidic environment. To maintain the body's pH within a narrow range, the respiratory center in the brain responds by increasing breathing rate to eliminate excess CO2 (which converts to H+ ions) and restore pH balance.

The increase in breathing rate during moderate exercise is primarily stimulated by the detection of increased H+ concentration in the arterial blood by chemoreceptors located in the central chemoreceptor region of the brainstem. These chemoreceptors are sensitive to changes in blood pH (H+ concentration). As H+ ions increase, the chemoreceptors send signals to the respiratory center in the brain, triggering an increase in the rate and depth of breathing. This enhanced ventilation helps to remove excess CO2 and maintain pH homeostasis.

In summary, during moderate exercise, arterial PO2 increases to supply more oxygen to the working muscles, arterial PCO2 increases due to heightened metabolic activity, and H+ concentration increases. The increase in H+ concentration stimulates an increase in breathing rate to remove excess CO2 and restore pH balance.