What is the partial pressure of O2 in the venous system?
40 mmHg
160 mmHg
100 mmHg
46 mmHg
The correct answer and explanation is:
The correct answer is 40 mmHg.
In the venous system, the partial pressure of oxygen (pO2) is typically around 40 mmHg. This value is much lower than the pO2 in the arterial blood, which is usually around 95-100 mmHg. The partial pressure of oxygen in the venous blood reflects the oxygen that has been used by the tissues throughout the body during cellular respiration.
Oxygen is transported in the blood by binding to hemoglobin molecules in red blood cells. In the arteries, oxygenated blood is pumped from the lungs to the tissues, where oxygen is delivered to the cells. Once the oxygen is used by cells for metabolic processes, it is exchanged for carbon dioxide, a waste product of metabolism. The blood, now carrying carbon dioxide, returns to the heart through the veins. The pO2 in the venous blood is therefore lower because much of the oxygen has been consumed, and carbon dioxide levels are higher, creating a partial pressure of around 40 mmHg.
The significant difference between the arterial pO2 (approximately 95-100 mmHg) and the venous pO2 (around 40 mmHg) is important for understanding how oxygen diffuses from the blood into tissues and how carbon dioxide is transported back to the lungs. In the lungs, oxygen moves from the alveoli, where the pO2 is around 100 mmHg, into the blood. In contrast, carbon dioxide moves in the opposite direction—from the blood (with a higher pCO2) into the alveoli to be exhaled. This exchange occurs through diffusion based on partial pressure gradients, with oxygen diffusing from areas of high to low pO2 and carbon dioxide moving in the opposite direction.