The main respiratory surface in humans is the alveoli, which are small air sacs branching off from the bronchioles in the lungs. They are one cell thick and provide a moist and extremely large surface area for gas exchange to occur. Capillaries carrying deoxygenated blood from the pulmonary artery
run across the alveoli. They are also extremely thin, so the total
distance gases must diffuse across is only around 2 cells thick. An
adult male has about 300 million alveoli, each ranging in diameter from
75 to 300 µm.
Inhaled oxygen is able to diffuse into the capillaries from the alveoli, while CO
2 from the blood diffuses in the opposite direction into the alveoli. The waste CO
2 can then be exhaled out of the body. Continuous blood flow in the capillaries and constant breathing maintain a steep concentration gradien