Breathing is important because our body needs the oxygen in the air you breathe to create the energy that keeps us alive. The Respiratory system transfers the oxygen from the Lungs into the bloodstream to travel throughout your body. This system also carries the "used" air, which is mostly carbon dioxide, back to your Lungs so that you can breathe it out. All this is done by millions of "working parts" of the Lungs, called the “alveoli”. These are small sacs like tiny folded balloons, where the air exchange takes place.

Each lung is composed of numerous air filled sacs called alveoli. They are the final branching of the Respiratory tree. They act as the primary gas exchange units of the lung, which means that it is on the inner surface of the alveoli that oxygen is absorbed and enters the bloodstream and carbon dioxide is released. The gas-Blood barrier between the alveolar space and the pulmonary Capillaries is extremely thin. This enables rapid gaseous exchange. To reach the Blood, oxygen (O2) must diffuse through the alveolar epithelium, a thin interstitial space, and the capillary endothelium; carbon dioxide (CO2) follows the reverse course to reach the alveoli.

The alveolar wall is very thin and made up of two types of epithelial cells. Type I cells which are elongated, they make the most of the alveoli and are mainly concern with the gaseous exchange. Type II cells are more compact and are responsible for producing the ‘surfactant’, a protein and fat based chemical which lines the alveoli and serves to reduce surface tension contributing to alveolar stability. The walls are so thin that the oxygen in the air can pass through them to enter your bloodstream and travel to cells in all parts of your body.

The main function of the alveoli is to keep the correct amount of oxygen and carbon dioxide in the Blood. We breathe in air by the process of respiration. The air then moves all the way down to the sac-like endings of the airways called the alveoli. In the alveoli, the oxygen from the air moves into the Blood stream that surrounds each tiny alveolus. This vital oxygen is now available for the body's needs. Carbon dioxide, which is a waste product from body tissues, is carried in the Blood stream to the Lungs. In the Lungs, the carbon dioxide is moved across the alveoli into the airways and then breathed out. The second main function of the alveoli is the secretion of the surfactant. As mentioned above, the Type 2 cells secrete the surfactant. This surfactant has an important role at the time of birth. It is due to this that the Lungs expand. Without it the Lungs collapse. During the lifetime, it is the alveoli that secrete the surfactant and prevent the Lungs from collapsing.

The alveoli have radii of about 0.1 mm and wall thickness of about 0.2 µm. The gas exchange is driven by passive process of diffusion, which does not require energy for transport. Red Blood cells transit the alveolar Capillaries in about 3/4 of a second. The Lungs contain about 300 million alveoli, each wrapped in a fine mesh of Capillaries. Normal alveolar partial pressures for O2 and CO2 are 105 mmHg and 40mmHg respectively.

Interstitial Lung disease. Acute Respiratory distress syndrome. Emphysema. Lung collapse or Atelectasis. Primary Alveolar Hypoventilation.