The lungs are composed of millions of tiny, balloon-like air sacs called alveoli. These structures are the fundamental units of respiration, where the exchange of gases between the lungs and the blood occurs. When you breathe in, air travels down a series of branching tubes and finally enters the alveoli. It is here that oxygen from the air passes into the bloodstream, while carbon dioxide, a waste product, moves from the blood into the alveoli to be exhaled.
Anatomy and Location of Alveoli
The alveoli are situated at the very end of the respiratory tract, capping the smallest airways known as bronchioles. These structures do not appear in isolation; instead, they are organized into clusters called alveolar sacs, which resemble a bunch of grapes.
A typical pair of human lungs contains approximately 480 million of these microscopic sacs. This vast number creates an immense surface area for gas exchange, estimated to be between 50 and 75 square meters. Each individual alveolus is a cup-shaped cavity with extremely thin walls, measuring only about 200 micrometers in diameter.
The Process of Gas Exchange
This process occurs across a specialized surface known as the respiratory membrane, which is formed by the thin walls of the alveoli and the equally thin walls of the capillaries. This shared membrane can be as thin as 0.2 micrometers in some places, allowing for rapid and efficient gas movement.
Gas exchange is driven by diffusion, where molecules move from an area of higher concentration to one of lower concentration. When you inhale, the concentration of oxygen is high in the alveoli and low in the blood arriving at the lungs. This pressure difference causes oxygen molecules to move from the alveoli, across the respiratory membrane, and into the capillaries, where they bind to hemoglobin in red blood cells.
Simultaneously, the blood that reaches the lungs is rich in carbon dioxide, a metabolic waste product. The concentration of carbon dioxide is therefore higher in the capillaries than it is in the inhaled air within the alveoli. This gradient drives carbon dioxide to diffuse from the blood, across the same membrane, and into the alveolar space.
Specialized Alveolar Cells and Surfactant
The majority of the alveolar surface, about 95%, is composed of Type I pneumocytes. These are extremely thin, flat cells that form the primary structure of the alveolar wall. Their primary role is to provide a physical barrier while facilitating gas exchange.
Interspersed among the Type I cells are Type II pneumocytes. Though less numerous, these cells have an important function: they produce and secrete a substance called pulmonary surfactant. Surfactant is a complex mixture of lipids and proteins that coats the inner surface of the alveoli.
This coating works to reduce surface tension within the fluid lining the air sacs. Without surfactant, the natural tendency of the moist alveolar surfaces to stick together would cause the tiny sacs to collapse, especially during exhalation. Surfactant counteracts this force, keeping the alveoli open and ready for the next inhalation.
Conditions Affecting Alveolar Function
In a condition like emphysema, the walls between adjacent alveoli break down. This destruction of the alveolar walls leads to the formation of larger, inefficient air sacs and a reduction in the total surface area available for gas exchange, making it difficult to get enough oxygen.
Pneumonia is another condition that directly impacts the alveoli. This infection causes inflammation that leads to the alveoli filling with fluid, such as pus or water. This fluid creates a barrier that prevents oxygen from diffusing from the inhaled air into the bloodstream.
Newborn respiratory distress syndrome occurs when an infant’s lungs are not developed enough to produce adequate amounts of surfactant. Without this substance, the alveoli collapse with each breath. This requires immense effort to reinflate and severely hinders the baby’s ability to breathe independently.