Oxygen and carbon dioxide are transported between the lungs and body tissues through diffusion and chemical reactions in the blood. Oxygen diffuses from the alveoli into the bloodstream and binds to hemoglobin, forming oxyhemoglobin to be carried to tissues. In tissues, oxyhemoglobin releases oxygen which then diffuses into cells. Carbon dioxide diffuses out of tissues and undergoes a series of reactions to form bicarbonate ions that are carried back to the lungs, where it diffuses out of the bloodstream and is exhaled. This continuous cycle transports oxygen to tissues for cellular respiration and removes carbon dioxide as a byproduct.