We’ve explained why air enters the lungs, what components flow in/out and why, but we still haven’t discussed internal and external respiration!
Regard the image at right. Find the sad face. Here the blood is CO2 rich (45 mm Hg) and low in oxygen (40 mm Hg). This sad blood is being delivered to the lungs, where the alveolar PO2 is 104 mm Hg and the PCO2 is 40 mm Hg. See? You probably know right away that fresh oxygen will rush across the respiratory membrane into the blood and refresh its supply to 104 mm Hg. But the gradient of PCO2 is only 5 mm Hg. It would seem then that carbon dioxide would have a lesser tendency to evacuate the blood, but it turns out that carbon dioxide is about 20 times more soluble in blood and alveolar fluid than oxygen (Marieb 748). Even though its gradient is smaller, its solubility encourages its movement through the membrane into the alveoli.
The fresh, red blood is pumped by the heart to tissues throughout the body. Traveling down the systemic circuit, from aorta to arteries to arterioles to capillaries, oxygen is “used” and carbon dioxide is “created.” Internal respiration at the tissues creates a new equilibrium of partial pressures of oxygen and carbon dioxide, so that again the venous (return) blood is in the same sad condition we started in.
In summary, arterial blood (red) has a higher PO2 and a lower PCO2 than venous blood, and this system is perpetuated at both ends by external and internal respiration.
Pretty simple huh? Not really…