The lungs are the primary location where gas exchange occurs: oxygen is delivered to the rest of the body and carbon dioxide waste is removed from the blood. In the lungs, this process of bringing in oxygen and eliminating waste is known as respiration.
Your lungs consist of many small, balloon-shaped air sacs called alveoli. They are clustered together in groups that resemble bunches of grapes. Each individual alveolus contains a network of tiny blood vessels, called capillaries. The walls of the alveoli are thin, which allows gas to pass quickly between the capillaries and the lungs’ spongy tissue.
Air entering the lungs passes down a tube called the trachea (windpipe). From there it moves through large passageways called bronchi. The bronchi then divide into smaller and smaller passageways called bronchioles, the last of which lead to individual air sacs, or alveoli. During inhalation, oxygen and carbon dioxide enter the alveoli from the capillaries of the lungs. During exhalation, the alveoli empty out of the lungs, returning the carbon dioxide to the pulmonary capillaries for reabsorption into the lungs.
Alveolar epithelial cells have long cytoplasmic extensions, which reduce the thickness of the wall between blood and air. This allows gases to move easily across the wall, which in turn increases the surface area of the alveoli. This increased surface area is important because the solubility of oxygen in the blood is low, requiring more alveolar surface area to ensure rapid gas exchange. The walls of the alveoli are also made of collagen and elastin, which helps them to expand when you inhale, then bounce back after exhalation. The lungs also secrete a fluid called surfactant, which lowers the surface tension of water and prevents the lungs from collapsing. Diseases that damage the lungs can negatively impact the amount of surfactant your lungs produce, causing them to work less efficiently.