Question

The partial pressure of $CO_2$ in the venous blood is
A. Greater than the tissue spaces
B. Lesser than the tissue spaces
C. Greater than the arterial blood
D. Less than in alveoli

Answer 1

Partial pressure of carbon dioxide is the measure of $CO_2$ in the venous or arterial blood. In normal conditions the range of $PCO_2$ is 35-45 mmHg. It increases during exercise.

Complete Answer:
- The partial pressure of carbon dioxide is used to determine the alveolar ventilation in the lungs.
- During cellular respiration, we breathe in oxygen and breathe out carbon dioxide. Carbon dioxide is continuously produced. The partial pressure of the carbon dioxide in the tissue is higher which causes it to diffuse out of the tissue.
- When the blood reaches the heart after oxygenation, the partial pressure of oxygen is around 40 mmHg. And that of carbon dioxide is 45 mmHg.

- The partial pressure of carbon dioxide is
(I) High in pulmonary capillaries.
(II) Low in the alveoli.
So the carbon dioxide diffuses across the membrane from blood into the alveoli.

Thus the correct answer is (B). The partial pressure of $CO_2$ in the venous blood is lesser than the tissue spaces.

Note: Significance of partial pressure- The internal and external respiration is driven by the partial pressure difference. The diffusion of gases from the capillaries to the tissues depends on the partial pressure of carbon dioxide and oxygen. The amount of $CO_2$ and $O_2$ that diffuses across the membrane is similar.