The lungs exchange oxygen and carbon dioxide during breathing. Gas exchange happens in alveoli, which are tiny, balloon-shaped compositions that are located throughout the lungs. There are millions of alveoli in the lungs. Alveoli have a large surface area that increases their ability to absorb oxygen and release carbon dioxide.
They have thin walls that are one cell thick and in direct contact with capillaries, which reduces the diffusion distance. The walls are also moist, which helps gases to pass easily through them. Alveoli contain collagen and elastin, which offer firmness to the air sac structure and bounce back during exhalation. The lungs have more than 70 square meters of surface area made up of a network of alveolar sacs.
Inhaled air passes from the lungs’ conducting zone through microscopic branches called bronchioles to alveolar ducts. Each alveolar duct contains alveoli and opens into a cluster of alveolus, which has a grape-like shape. The cluster of alveoli together has a total surface area about the size of half a tennis court.
Each alveolus is surrounded by fine pulmonary capillaries. The alveolar-capillary membranes prevent liquid in the capillaries from entering the air sacs. The alveolar walls are made up of a thin layer of type I lung cells that are permeable to oxygen molecules and lipids (fats).
Oxygen diffuses into the alveoli from the blood stream, which in turn transports it to the body’s tissues and organs. Carbon dioxide diffuses out of the alveoli into the bloodstream, where it is eventually breathed out of the body. Conditions like pneumonia, emphysema, and tuberculosis affect how well the alveoli function. In some cases, these conditions cause the alveoli to fill with fluid, which makes it hard to breathe.