All organisms need to exchange substances such as food, waste, gases and heat with their surroundings. These substances must diffuse between the organism and the surroundings. The rate at which a substance can diffuse is given by Fick's law: So rate of exchange of substances depends on the organism's surface area that's in contact with the surroundings. Requirements for materials depends on the volume of the organism, So the ability to meet the requirements depends …showed first 75 words of 1681 total…

…showed last 75 words of 1681 total…larger and faster expiration, used in exercise These movements are transmitted to the lungs via the pleural sac surrounding each lung. The outer membrane is attached to the thorax and the inner membrane is attached to the lungs. Between the membranes is the pleural fluid, which is incompressible, so if the thorax moves, the lungs move too. The alveoli are elastic and collapse if not held stretched by the thorax (as happens in stab wounds).