Question

Which of the following explains why the PO₂ of the blood leaving pulmonary capillaries is 104 mm Hg, but the blood leaving the lungs to enter the left atrium is 95 mm Hg?

O Some oxygen diffuses out of the blood as it travels through the pulmonary veins.
O Some carbon dioxide in the alveoli is mixed with oxygenated blood from alveolar circulation.
O Some carbon dioxide in the alveoli is mixed with oxygenated blood from alveolar circulation.
O Some oxygen is incorporated into bicarbonate ion, removing it from solution and reducing PO2.
O Deoxygenated blood from bronchial circulation is mixed with blood from oxygenated alveolar circulation.

Answer 1

The blood leaving the pulmonary capillaries has a Po2 of 104 mm Hg due to oxygen binding to hemoglobin. The Po2 decreases to 95 mm Hg as it reaches the left atrium because of the mixing of oxygenated blood with deoxygenated blood from the bronchial circulation. The correct option 5 is that deoxygenated blood from bronchial circulation is mixed with oxygenated alveolar circulation.

Step-by-step explanation:

The partial pressure of oxygen (Po2) of the blood leaving the pulmonary capillaries is 104 mm Hg due to the gas exchange process occurring in the lungs.

As oxygen diffuses from the alveoli into the capillaries, it binds to hemoglobin in red blood cells, raising the Po2. However, the blood leaving the lungs to enter the left atrium has a slightly lower Po2 of 95 mm Hg. This is primarily because some oxygen is used to convert carbon dioxide into bicarbonate ions in red blood cells, and additionally, deoxygenated blood from the bronchial circulation mixes with the oxygenated blood from the alveolar circulation.

The correct option explaining the difference in Po2 is: 'Deoxygenated blood from bronchial circulation is mixed with blood from oxygenated alveolar circulation.' This bronchial circulation is part of the systemic circulation that provides oxygen to the lung tissue itself; the blood from this system enters the pulmonary veins and mixes with the alveolar oxygenated blood, which slightly lowers the overall Po2.