Respiration is a term that is variously applied to the acts of inhalation and exhalation, to the movement of gas molecules in the lungs between alveolar air and blood in the alveolar capillaries, to the exchange of dissolved gases in the tissues between the systemic capillaries and the surrounding interstitial fluid and to the process conducted within the mitochondria of cells that results in the production of ATP (and CO2) from small organic molecules by using O2. The respiratory system is a set of tubes that branch to increase in number and decrease in size, within an elastic structure that is moved by muscles. The lungs and chest wall together act like a bellows to move air into and out of the alveoli. The epithelial cells of the walls of the alveoli are part of the respiratory membrane that separates the air in the alveoli from the blood in the alveolar capillaries. The endothelial cells of the capillary walls are also part of the respiratory membrane.