The average pressure of the atmosphere at sea level is approximately 760 mmHg. At sea level the total atmospheric pressure is 760 mmHg so the partial pressure of O2 would be approximately 152 mmHg. The partial pressure of CO2 would then be about 0.304 mmHg at sea level. Gas transfer at the alveoli follows the rule of simple diffusion. Diffusion is movement of molecules along a concentration gradient from an area of high concentration to an area of lower concentration.
Diffusion is the result of collisions between molecules.
In areas of higher concentration there are more collisions. The net effect of this greater number of collisions is a movement toward an area of lower concentration. In a normal, non-stressed state, the respiratory system transports oxygen from the lungs to the cells of the body where it is used in the process of cellular respiration. Under normal conditions this transport of oxygen is sufficient for the needs of cellular respiration. Cellular respiration converts the energy in chemical bonds into energy that can be used to power body processes.