Structure of an Alveolus and its Blood Supply

Alveoli are tiny air sacs located at the end of the bronchioles in the lungs, where gas exchange (the exchange of oxygen and carbon dioxide) happens.
Alveoli are shaped like small, rounded bags or balloons, which allows them to expand as they fill with air during inhalation.
They are extremely small, but their huge number (approximately 600 million in each lung) creates a large surface area (about 75 square metres) for gas exchange.
Every alveolus is enveloped by tiny capillaries, which are the smallest blood vessels in the body. These capillaries carry blood rich in carbon dioxide and poor in oxygen.
The wall of the alveolus (alveolar wall) and the capillary are only one cell thick each, creating a thin surface (termed the respiratory membrane) for gas exchange.
The oxygen from air breathed in diffuses across this thin respiratory membrane from the alveolus into the blood present in the capillaries. At the same time, carbon dioxide diffuses from the blood to the alveolus, to be exhaled out.
The alveoli walls are moist, which aids the diffusion of gases.
Surfactant is a special substance produced by the alveoli, which reduces surface tension, preventing the alveoli from collapsing and sticking to each other when air leaves them during expiration.
The blood supply ensures that the gases are transported to and from the cells: oxygen is delivered to cells for respiration (energy production), and carbon dioxide, a respiration waste product, is removed.
The efficient functioning of alveoli is essential for maintaining the oxygen-carbon dioxide balance in the body, which underpins all vital biological processes.
Smoking can cause serious damage to the alveoli, leading to conditions such as emphysema where the alveoli lose their elasticity and can’t exchange gases effectively. This emphasises the importance of a healthy respiratory system.