The existence of land bridges was used to dispute continental drift
The circulatory system circulates blood around the body. The blood passes through the lungs, where it loses carbon dioxide and gains oxygen. All cells need oxygen to function and carbon dioxide is a form of waste produced in the cells. The digestive system breaks down food and absorbs.
Since diffusion moves materials from an area of higher concentration to the lower, it is described as moving solutes "down the concentration gradient." The end result of diffusion is an equal concentration, or equilibrium, of molecules on both sides of the membrane. At equilibrium, movement of molecules does not stop.
