Alveolus

An alveolus (plural: alveoli) is a word used in anatomy for hollow cavities, which are empty areas within a body.
Although there are many different types of alveoli in the body, the word alveoli is usually used to talk about small air sacs in the lungs of mammals. These are also called "pulmonary alveoli". These alveoli are located at the ends of the air passageways in the lungs. They have very thin (one cell thick), wet walls and are surrounded with a network of small blood vessels, or capillaries. This allows gases to diffuse, or move across, the surface of the alveolus.
When a mammal breathes in, the concentration of oxygen is higher in the alveolus than in the red blood cells. Therefore, oxygen leaves the alveolus and enters the red blood cells. When a mammal breathes out, the opposite happens. The concentration of carbon dioxide is lower in the alveolus than in the red blood cells, so carbon dioxide leaves the red blood cells, enters the alveolus, and is exhaled.
In an average human lung, there are 480 million alveoli. Their total surface area is around 75 square meters.
Inside the alveoli is where a gas exchange happens. Gas exchange is how different gases (usually two) move in opposite directions across a specialised respiratory surface. Gases are constantly required and produced as a by-product of cellular and metabolic reactions so an efficient system to exchange them is extremely important. It is linked with respiration in animals, and both respiration and photosynthesis in plants.