Question

In which direction will CO2 flow if the PCO2 in the alveoli were normal and PCO2 in the venous blood entering the lungs were 30 mmHg?

a) CO2 will diffuse out of the pulmonary capillaries, and into the alveoli.

b) CO2 will diffuse out of the alveoli, and into the pulmonary capillaries.

c) No net diffusion of CO2 will occur because the partial pressures of CO2 within the tissues and systemic capillaries are at equilibrium. d) It cannot be determined without knowing the partial pressures of O2 and H2O, within the tissue and systemic capillaries as well.

Answer 1

B is the correct answer, i.e CO2 will diffuse out of the alveoli, and into the pulmonary capillaries.

Explanation:

Henry's Law

a. Gasses dissolve into liquids in proportion to their partial pressures.
b. The direction of gas movement (between dissolved and undissolved) is determined by the partial pressures in the two phases.

The gas will flow in a direction to attain equilibrium across the pressure gradient.

eg:

Examples: Suppose we have this mixture:
P x = 100
Py= 50
When this mixture comes in contact with a liquid, gas x will dissolve faster than gas y.

Suppose our mixture and liquid have been in contact for a while. Px in the air will equal Px in the liquid, that is, they reach equilibrium. If Px is lowered in the air, then some gas x will go out of solution until a new equilibrium is reached.

The normal partial pressure of carbon dioxide in the blood of the capillary is about 45 mm Hg, whereas its partial pressure in the alveoli is about 40 mm Hg, this way CO2 flows from capillaries towards alveoli.

So when there is a decrease in PCO2 in the blood of the capillary to 30 mmHg and the partial pressure in the alveoli iremains about 40 mm Hg the flow of CO2 will be from alveoli to capillaries to attain equilibrium leading to more concentration of CO2 in the blood increasing the PH.

• Option A is not correct as per the above-stated law.
• Option C is not correct as the partial pressures of CO2 within the tissues and systemic capillaries are not at equilibrium, as the normal partial pressure in the alveoli is about 40 mm Hg and given PCO2 in the venous blood entering the lungs were 30 mmHg, causing a pressure gradient.
• Option D is incorrect as the diffusion is affected by only the pressure gradient but in turn, the entire process affects the PH levels, ventilation, HCO3 levels, etc.