Final answer:
The alveolar PO2 can increase with hyperventilation and higher atmospheric pressure, decrease due to pulmonary fibrosis and a slower respiratory rate, and external respiration depends on the partial pressure differences in the alveoli and pulmonary capillaries.
Step-by-step explanation:
The partial pressure of oxygen (PO2) in the alveoli is influenced by various factors which can cause an increase, decrease, or no change in alveolar PO2.
- Hyperventilation leads to an increase in alveolar PO2 because more air and consequently oxygen enters the alveoli.
- Pulmonary fibrosis typically results in a decrease in alveolar PO2 as scar tissue reduces the efficiency of gas exchange.
- An increase in atmospheric pressure, as might be experienced in a hyperbaric chamber, generally causes an increase in alveolar PO2 as the gradient for oxygen to enter the alveoli is increased.
- A decrease in respiratory rate leads to a decrease in alveolar PO2 because less fresh air containing oxygen is brought into the lungs.
External respiration relies on the difference in partial pressures of gases between the alveoli and blood in the pulmonary capillaries to facilitate gas exchange. When alveolar ventilation is insufficient, the partial pressure of oxygen in alveoli drops, signaling the body to constrict the arterioles that serve poorly ventilated alveoli, redirecting blood to better-ventilated areas. Conversely, the arterioles supporting well-ventilated alveoli will dilate.