Answer
The direction of diffusion of oxygen and carbon dioxide is determined by their concentration gradient. The gases diffuses from higher concentration to lower concentration.
Work Step by Step
In lungs the alveoli have high concentration of oxygen and the blood passing through the capillaries have a high concentration of carbon dioxide. Hence the oxygen diffuses into blood and the carbon dioxide diffuses from blood into the alveoli.
The tissue cells have higher concentration of carbon dioxide, whereas the blood passing through the systemic capillaries have a higher concentration of Oxygen. Hence the carbon dioxide diffuses to the blood from the tissue cells and the oxygen diffuses into the tissue cells from the blood.
