Answer:
Diffusion
Step-by-step explanation:
Diffusion is the process by which molecules of substances move from regions of higher concentration to regions of lower concentration until equilibrium concentration is attained. This movement of molecules is because a concentration gradient exists between the two regions.
Blood cells in the capillaries is low in oxygen because the cells have used up the oxygen in cellular respiration. However, in the lungs, the oxygen concentration is high due to inspiration of oxygen from the external environment. Therefore, a concentration gradient exists between the lungs and blood cells in the capillaries. Oxygen from the lungs diffuses into the blood cells in the capillaries, and these blood cells are then returned to other parts of the body by the circulatory system. This is a continuous process and is known as gaseous exchange.