Final answer:
During gas exchange, oxygen and carbon dioxide move independently down their pressure gradients. Oxygen diffuses out of capillaries and into tissue cells due to a pressure gradient, while carbon dioxide moves out of tissue cells and into capillaries. This exchange occurs in both systemic capillaries and lung capillaries.
Step-by-step explanation:
Oxygen and carbon dioxide move independently of each other; they diffuse down their own pressure gradients.
As blood leaves the lungs through the pulmonary veins, the venous Po₂ = 100 mm Hg, whereas the venous PCO₂ = 40 mm Hg.
As blood enters the systemic capillaries, the blood will lose oxygen and gain carbon dioxide because of the pressure difference of the tissues and blood. In systemic capillaries, Po₂ = 100 mm Hg, but in the tissue cells, Po₂ = 40 mm Hg.
This pressure gradient drives the diffusion of oxygen out of the capillaries and into the tissue cells.
At the same time, blood PCO₂ = 40 mm Hg and systemic tissue PCO₂ = 45 mm Hg.
The pressure gradient drives CO₂ out of tissue cells and into the capillaries.
The blood returning to the lungs through the pulmonary arteries has a venous Po₂ = 40 mm Hg and a PCO₂ = 45 mm Hg.
The blood enters the lung capillaries where the process of exchanging gases between the capillaries and alveoli begins again.