Partial pressure of oxygen in alveoli:
**Core Concept**
The partial pressure of oxygen in alveoli is a critical component of the respiratory process. It is the amount of oxygen available for gas exchange between the alveoli and the bloodstream, and it is influenced by factors such as alveolar ventilation, oxygen concentration in the inhaled air, and the efficiency of gas exchange.
**Why the Correct Answer is Right**
The partial pressure of oxygen in the alveoli is typically around 100 mmHg, which is approximately equal to the partial pressure of oxygen in the inhaled air. This is because the alveoli are in equilibrium with the air in the lungs, and the rate of diffusion of oxygen from the alveoli into the bloodstream is equal to the rate of diffusion from the bloodstream into the alveoli. The alveolar-capillary membrane is highly permeable to oxygen, and the concentration gradient between the alveoli and the bloodstream is relatively small, allowing for efficient gas exchange.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it implies a lower partial pressure of oxygen in the alveoli than is actually present. A partial pressure of oxygen below 100 mmHg would indicate a problem with gas exchange or ventilation.
**Option B:** This option is incorrect because it implies a higher partial pressure of oxygen in the alveoli than is actually present. A partial pressure of oxygen above 100 mmHg would indicate a problem with ventilation or oxygen concentration in the inhaled air.
**Option C:** This option is incorrect because it implies a variable partial pressure of oxygen in the alveoli, which is not the case. The partial pressure of oxygen in the alveoli is relatively stable and is influenced by factors such as alveolar ventilation and oxygen concentration in the inhaled air.
**Clinical Pearl / High-Yield Fact**
The alveolar-arterial oxygen gradient (A-a gradient) is a useful measure of the efficiency of gas exchange in the lungs. An increased A-a gradient can indicate a problem with gas exchange or ventilation, and is a useful clinical tool for diagnosing conditions such as chronic obstructive pulmonary disease (COPD).
**Correct Answer: D. 100 mmHg**