The lung alveoli are microscopic, balloon-like air sacs located at the end of the respiratory tree within the lungs. These numerous structures are the primary sites where the body brings in oxygen and releases carbon dioxide. Hundreds of millions of these tiny sacs provide an immense surface area, comparable to a tennis court, facilitating the continuous exchange of gases essential for life.
Anatomy of the Alveoli
Each alveolus resembles a tiny balloon or grape with extremely thin walls. Type I pneumocytes, flattened cells, primarily maintain the structural integrity of these delicate sacs and form most of the alveolar surface. These cells are specialized for gas diffusion.
Interspersed among Type I cells are Type II pneumocytes, cuboidal cells that produce pulmonary surfactant. This specialized lipoprotein complex lines the inner surface of the alveoli, reducing the surface tension within the fluid lining. Without surfactant, the strong cohesive forces of water molecules would cause the tiny alveolar sacs to collapse during exhalation, making re-inflation difficult.
A dense network of capillaries encases each alveolus, bringing deoxygenated blood into close proximity with the inhaled air. The walls of these capillaries are remarkably thin, often just one cell thick, mirroring the delicate structure of the alveolar walls. This close arrangement of the alveolar membrane and capillary wall creates an efficient barrier for gas transfer.
The Gas Exchange Process
The exchange of gases within the alveoli occurs through a passive process known as diffusion, driven by differences in partial pressure. When air is inhaled, oxygen fills the alveoli, creating a higher concentration of oxygen there compared to the deoxygenated blood arriving in the surrounding capillaries. This pressure gradient causes oxygen molecules to move from the alveoli, across their thin walls, and into the bloodstream.
Simultaneously, the deoxygenated blood reaching the capillaries carries a high concentration of carbon dioxide, a waste product from the body’s metabolic processes. The partial pressure of carbon dioxide in the blood is greater than that in the alveoli. Consequently, carbon dioxide molecules diffuse from the blood, across the capillary walls, and into the alveolar air.
This bidirectional movement of gases occurs across the “respiratory membrane,” a composite structure formed by the alveolar epithelium, the capillary endothelium, and their fused basement membranes. This membrane is exceptionally thin, measuring 0.2 to 0.6 micrometers, which minimizes the distance gases must travel. The efficient diffusion across this membrane ensures that oxygen is delivered to the blood for circulation throughout the body, while carbon dioxide is exhaled.
Conditions Impacting Alveolar Health
Various conditions can compromise the delicate structure and function of the alveoli, impairing the body’s ability to perform gas exchange. Emphysema, a chronic obstructive pulmonary disease (COPD), causes progressive destruction of the alveolar walls. This damage leads to the merging of smaller air sacs into larger, irregularly shaped airspaces, which reduces the total surface area available for gas exchange, making it difficult to get enough oxygen.
Pneumonia, a lung infection, results in the alveoli becoming inflamed and filled with fluid, pus, and cellular debris. This accumulation obstructs the passage of oxygen and carbon dioxide across the alveolar-capillary membrane. The presence of fluid creates a diffusion barrier, hindering the transfer of gases and leading to reduced oxygen levels in the blood.
Acute Respiratory Distress Syndrome (ARDS) is a severe inflammatory response in the lungs, often triggered by serious illness or injury elsewhere in the body. In ARDS, the inflammation causes the capillary walls surrounding the alveoli to become leaky, allowing fluid and proteins to flood into the alveolar spaces. This fluid accumulation impedes gas exchange, leading to difficulty in oxygenating the blood and requiring intensive medical support.