The branching air passageways of the mammalian lung end in clusters of tiny sacs called alveoli. The alveoli are the sites of gas exchange. Each alveolus is surrounded by capillaries. Oxygen and carbon dioxide can diffuse rapidly across the thin walls of an alveolus and its adjacent capillary. Blood entering lung capillaries from the systemic circulation is low in oxygen and high in carbon dioxide. Air inside the alveolus is high in oxygen and low in carbon dioxide. Each gas follows its concentration gradient. Oxygen entering the bloodstream binds to hemoglobin in red blood cells; carbon dioxide entering the lungs is exhaled.