Compare the partial pressures of oxygen and carbon dioxide in alveolar air versus atmospheric air.

Step 1: Understand the composition of atmospheric air. Atmospheric air contains approximately 21% oxygen (O_2) and a very small percentage of carbon dioxide (CO_2), around 0.04%. At sea level, the partial pressure of O_2 (pO_2) in atmospheric air is about 159 mmHg, and the partial pressure of CO_2 (pCO_2) is about 0.3 mmHg. Step 2: Understand the composition of alveolar air. Alveolar air is the air present in the alveoli of the lungs. When atmospheric air is inhaled, it mixes with the residual air already present in the lungs. More importantly, continuous gas exchange occurs in the alveoli: Oxygen (O_2) constantly diffuses from the alveoli into the blood. This process removes O_2 from the alveolar air. Carbon dioxide (CO_2) constantly diffuses from the blood into the alveoli. This process adds CO_2 to the alveolar air. Step 3: Compare partial pressures. Due to the continuous gas exchange: The pO_2 in alveolar air will be lower* than in atmospheric air because O_2 is constantly being absorbed into the blood. (Typical alveolar pO_2 is about 104 mmHg). The pCO_2 in alveolar air will be higher* than in atmospheric air because CO_2 is constantly being released from the blood into the alveoli. (Typical alveolar pCO_2 is about 40 mmHg). Step 4: Evaluate the given options. (A) pO_2 lesser, pCO_2 higher: This matches our analysis. (B) pO_2 higher, pCO_2 lesser: Incorrect. (C) pO_2 higher, pCO_2 higher: Incorrect for pO_2. (D) pO_2 lesser, pCO_2 lesser: Incorrect for pCO_2. The final answer is A