Skeletal muscle can contract very rapidly, but cannot stay contracted very long. This type of muscle also tires easily with the buildup of lactic acid. If all cells of an organism are in direct contact with the external environment, each cell can directly take in nutrients and get rid of wastes. But, such an organism can only be two cells thick and the cells must be relatively non-specialized. Blood carries oxygen from the lungs to the cells, and carries the CO 2 from the cells back to the lungs.
Air then rushes in.
To breather out, the ribcage contracts, and the diaphragm arches up into the chest cavity.