Question

List the partial pressure of CO₂ at each stage from the atmosphere, to the lungs, to the blood leaving the pulmonary capillaries, to the blood entering the systemic capillaries, to the blood leaving the systemic capillaries, and finally back up to the blood entering the pulmonary capillaries. Describe at each stage why the partial pressure increases, decreases, or remains the same.

Answer 1

CO2 or any other gases flow according to the pressure gradient from high to low.

The partial pressure of CO2 in the atmopshere is low due to the low percentage of CO2 in air it is about 0.3 mm Hg, but the PCO2 of the alveolar is high around 47 mm Hg, this pressure difference will drive CO2 out of lungs to the outside.

When it comes it to the alveoli of the lungs, PCO2 in alveoli is 40 mm Hg versus 45 mm Hg in blood returning from the capillary, because of which CO2 will move from blood to the alveoli.

When it comes to the systemic circulation the partial pressure of CO2 is lower in blood, because cellular respiration continously produces CO2, in tissues PCO2 will be around 45 mm Hg, while the PCO2 in blood will be around 40 mm Hg, because of the pressure difference CO2 diffuse out of tissue, cross the intestitial fluids and enter the blood.

By the time blood returns to the heart, the partial pressure of carbon dioxide has returned to about 45 mm Hg.