Diffusion is important to animals because it is the process by which useful molecules enter the body cells and waste products are removed.
Digested food molecules such as amino acids and glucose move down a concentration gradient from the intestine to the blood.
Waste products such as carbon dioxide or urea travel by diffusion from body cells into the bloodstream.
Oxygen moves from high concentration (in the air sac) to a lower concentration (in the blood). Carbon dioxide moves from high concentration (in the blood) to a lower concentration (in the air sac).