What is partial pressure gradient?
A partial pressure gradient is the difference in the concentration of a gas in a mixture of gases, in which the gas is at a higher pressure in one location and a lower pressure in another location. A gas will diffuse from a higher pressure to a lower pressure down the gradient.
This is how oxygen and carbon dioxide diffuse into and out of our bodies.
Gas exchange occurs in the alveoli (air sacs) in our lungs, which contain capillaries. The partial pressure of oxygen is greater in the external environment than in the capillaries, so oxygen diffuses into the capillaries. The partial pressure of carbon dioxide is higher inside the capillaries than in the external environment, so carbon dioxide diffuses out of the capillaries.