The human lung has about 80 million alveoli. Give two features of the alveoli that allow large amounts of oxygen to enter the blood.

Alveoli forms the respiratory surface in human body. Each alveolus is sac like structure bounded by a single layer of epithelial cells. It is bound on the outside by a network of capillaries. The pulmonary artery from the heart containing deoxygenated blood enters the lungs and then becomes oxygenated after passing in the capillaries through the lungs.