Question

The partial pressure of oxygen in the alveoli of the lungs is
A. Less than that in the blood
B. Less than that of carbon dioxide
C. Equal to that in the blood
D. More than that in the blood

Answer 1

Hint: The sites of gaseous exchange take place in the alveoli of the lungs. In cellular respiration level, the blood is rich in carbon dioxide.

Complete answer: The partial pressure of oxygen in the alveoli of the lungs is higher than that in the blood, as higher oxygen partial pressure in alveoli helps in the diffusion of oxygen into the blood. The concentration gradient that is maintained between the alveoli and the blood helps in the passive diffusion of oxygen. It moves from the region of higher concentration to the region of lower concentration. This difference in concentration causes the regular uptake of oxygen by the lungs and the continuous diffusion of carbon dioxide out of the capillaries into the alveoli.
The partial pressure of oxygen at sea level (160 mmHg in the atmosphere, 21 percent of the standard atmospheric pressure of 760 mmHg) in arterial blood is between 75 mmHg and 100 mmHg. Oxygen tension in venous blood at sea level is between 30 mmHg and 40 mmHg. Carbon dioxide is a by-product of food metabolism and in high amounts has toxic effects including dyspnea, acidosis and altered consciousness. The partial pressure of carbon dioxide at sea level in arterial blood is between 35 mmHg and 45 mmHg. The partial pressure of a gas in the blood is significant because it is directly related to ventilation and oxygenation.
Hence, the correct option is D.

Note: The partial pressure of oxygen in alveolar air is 104 mm Hg, however the partial pressure of the oxygenated pulmonary venous blood is about 100 mm Hg. When aeration is adequate, oxygen passes in the alveoli at an enormous rate, and the partial pressure of oxygen in the alveoli remains high.