Alveoli are tiny air sacs deep inside the lungs where oxygen enters the blood and carbon dioxide leaves it. They are the main site of gas exchange, the process that connects every breath you take with the body’s need for oxygen and removal of carbon dioxide.
Each lung contains millions of alveoli. Although individual alveoli are microscopic, together they provide a very large surface area for exchanging gases. Their structure is highly specialized: they have extremely thin walls, are surrounded by tiny blood vessels called capillaries, and are kept open by substances that help prevent them from collapsing.
Where are the alveoli?
Air travels through the respiratory system before reaching the alveoli. When you inhale, air passes through the nose or mouth, down the trachea (windpipe), and into progressively smaller airways called bronchi and bronchioles. At the ends of the smallest bronchioles are clusters of alveoli.
An alveolus is essentially a small pocket of air surrounded by a network of capillaries. The air inside the alveolus and the blood inside those capillaries are separated by an extremely thin barrier. This short distance allows oxygen and carbon dioxide to move between the air and blood efficiently.
How do alveoli exchange gases?
Gas exchange occurs mainly through diffusion, in which molecules move from an area where their concentration or partial pressure is higher to an area where it is lower.
When you inhale, the air reaching the alveoli contains a relatively high amount of oxygen compared with the blood arriving from the body. Oxygen therefore moves across the alveolar wall and into the surrounding capillaries. Once in the blood, much of the oxygen binds to hemoglobin, a protein inside red blood cells that transports oxygen throughout the body.
Carbon dioxide moves in the opposite direction. Blood returning to the lungs contains more carbon dioxide than the air inside the alveoli. Carbon dioxide therefore diffuses from the blood into the alveolar air. When you exhale, that carbon dioxide is carried out of the lungs.
This exchange happens continuously as blood flows through the lung’s capillaries.
Why are alveoli so effective at gas exchange?
The design of the alveoli makes rapid gas exchange possible.
Their walls are extremely thin. The respiratory surface consists largely of a very thin layer of cells, allowing gases to cross a short distance between alveolar air and blood.
They have a large surface area. Millions of alveoli collectively create an extensive interface between air and blood. A larger surface area provides more opportunity for oxygen and carbon dioxide to be exchanged at the same time.
They are surrounded by capillaries. A dense capillary network keeps blood in close contact with the alveoli. Continuous blood flow carries absorbed oxygen away and brings carbon-dioxide-rich blood back to the lungs.
They maintain a useful pressure gradient. Breathing continually replaces the air in the alveoli, while circulation continually replaces the blood around them. This helps maintain the differences in oxygen and carbon dioxide levels that drive diffusion.
What keeps the alveoli from collapsing?
The alveoli are not simply empty bubbles. Their inner surfaces contain a substance called pulmonary surfactant, a mixture of lipids and proteins.
Surfactant reduces surface tension at the air-liquid interface lining the alveoli. Without enough surfactant, the walls of small alveoli are more likely to collapse, making the lungs harder to expand and increasing the effort required to breathe.
The lungs also contain connective tissue and elastic fibers that help the alveolar structures expand and recoil during breathing.
What happens to the alveoli when you breathe?
During inhalation, the diaphragm and other respiratory muscles expand the chest, causing the lungs to expand. Air flows through the airways and reaches the alveoli.
As the alveoli fill with fresh air, oxygen diffuses into the blood while carbon dioxide diffuses into the alveoli. During exhalation, the lungs recoil and air containing carbon dioxide leaves the body.
Not every molecule of air in the lungs is replaced with every breath. Some air remains in the lungs after exhalation, which helps keep gas exchange relatively continuous rather than stopping between breaths.
What is the blood-air barrier?
The blood-air barrier, sometimes called the alveolar-capillary barrier, is the extremely thin interface across which respiratory gases travel.
It includes the alveolar lining, very thin supporting tissue, and the lining of the capillaries. Its job is a delicate balancing act: it must be thin enough to allow gases to cross quickly while still providing a functional barrier between air and blood.
Oxygen crosses this barrier in one direction, and carbon dioxide crosses it in the other. Once gases have crossed, the circulatory system takes over the task of transporting them.
What can interfere with alveolar function?
Alveoli work best when they are open, well ventilated, and closely matched with blood flow. Problems that damage the alveolar walls, fill the alveoli with fluid or other material, or interfere with airflow can reduce gas exchange.
For example, in emphysema, damage to the walls between alveoli reduces the available surface area for gas exchange and can make it harder to move air out of the lungs. In pneumonia, inflammation and fluid or other material in the air spaces can interfere with the movement of oxygen into the blood. Pulmonary edema, in which fluid accumulates in the lung tissue and air spaces, can also make oxygen transfer more difficult.
Smoking and exposure to other harmful inhaled substances can damage lung tissue over time. Some infections, inflammatory conditions, and other lung diseases can affect the alveoli or the surrounding capillary network as well.
The result is not always simply “less air.” A person may have air reaching part of the lung without enough blood flow there, or blood passing through areas that are poorly ventilated. Efficient breathing depends on both ventilation and circulation being appropriately matched.
Why do alveoli matter to the whole body?
The alveoli are where the respiratory and circulatory systems meet. The respiratory system brings oxygen to the alveolar air spaces and removes carbon dioxide; the cardiovascular system carries oxygenated blood away from the lungs and returns carbon-dioxide-rich blood for disposal.
The oxygen entering the blood is ultimately used by cells to support cellular respiration, the set of chemical reactions that produces usable energy from nutrients. Carbon dioxide is a waste product of cellular metabolism and must be transported back to the lungs so it can be exhaled.
So although alveoli are tiny structures, their function is fundamental to maintaining the body’s oxygen supply and helping regulate carbon dioxide levels in the blood.
