The Micromechanics of Lung Alveoli

The alveoli are microscopic, balloon-like structures in the lungs that look like little clusters of grapes at the ends of the bronchial branches in the lungs. The Alveoli are the functional units of the lungs.

Alveoli Function

Although alveoli are microscopic,they are the workhorses of the respiratory system. Alveoli function to move oxygen in and carbon dioxide out of the bloodstream. The balance between the lungs' expanding and collapsing forces determines the size of an alveolus at any specific point in time.

Lung recoil is the term used to describe the tendency of alveoli to rebound after being inflated. The primary determinant of lung recoil is surface tension, which is a force imposed by water molecules at the surface of liquids. It occurs whenever there is an air-liquid interface, as in the case of the interior lining of alveoli and acts to minimize the surface area.

The other principal determinant of recoil is the composition of the lung tissue itself, which has a high content of collagen and elastin fibers giving it large elastic properties. Even though these collapsing forces are crucial to exhale air after an inspiration, they must be counteracted by expanding forces at end-expiration to prevent alveolar collapse. One of the principal forces that prevent complete airway closure, is the extensive collagen extracellular matrix running throughout the interalveolar septa that provide "radial traction," i.e., forces pulling in all directions away from the center of each alveolus, keeping it inflated.

The other factor that prevents alveolar collapse is the presence of surfactant, a substance produced by type II pneumocytes. Since water lines the interior surface of alveoli, surface tension pulls the water collection as well as the alveoli inwards. Without a mechanism to counter this force, the alveolus would collapse. 

How do Alveoli Adapt to Their Function?

1. Large surface area - many alveoli are present in the lungs with a shape that further increases surface area.

2.Thin walls - alveolar walls are one cell thick providing gases with a short diffusion distance.

3. Moist walls - gases dissolve in the moisture helping them to pass across the gas exchange surface.

4. Permeable walls - allow gases to pass through.

5. Extensive blood supply - ensuring oxygen rich blood is taken away from the lungs and carbon dioxide rich blood is taken to the lungs.

6. A large diffusion gradient - breathing ensures that the oxygen concentration in the alveoli is higher than in the capillaries so oxygen moves from the alveoli to the blood. Carbon dioxide diffuses in the opposite direction.

How Many Alveoli Are in the Lungs?

When we inhale, air enters the lungs and travels through passageways to reach 480,000,000 alveoli. The size of a single alveolus has an approximate diameter of 200-500 microns, regardless of lung size. As a point of reference, one micron is a millionth of a meter. The diameter of a human hair is about 70 microns, so one alveolus would be close to equal the diameter of three human hairs put together. The total surface area of all alveoli in a healthy adult set of lungs is approximately 70 square meters (approximately the size of half a tennis court). 

Can Alveoli Repair Themselves?

If alveoli are damaged, they may be able to self-repair. The lung is a highly quiescent tissue, previously thought to have limited reparative capacity and a susceptibility to scarring. It is now known that the lung has a remarkable reparative capacity, when needed, and scarring or fibrosis after lung injury may occur infrequently in scenarios where this regenerative potential is disrupted or limited. 

Over time, the alveoli may be able to repair any damage. Alveolar regeneration after an acute lung injury has been observed in many mammals. Results in animal models have shown that alveolar type II cells function as resident alveolar stem cells that can proliferate and differentiate into alveolar type I cells to build new alveoli after lung injury.

Smoking Destroys the Alveoli

Cigarette smoke inhibits alveolar repair. Over time, the toxins from inhaled cigarette smoke break the thin walls of alveoli, leaving larger, less efficient air sacs. The sacs also begin to lose their bounce, making it harder to bring in the oxygen and expel carbon dioxide. Both can become partially trapped in the lungs. In a smoker, this process signals the beginning of emphysema, a form of chronic obstructive respiratory disease (COPD). However, if exposure to cigarette smoke stops soon enough, the damage can be halted. If smoking continues, there will come a point where lung damage will progress regardless of whether a person stops smoking or not. Emphysema causes severe shortness of breath.

Conditions That Impact Alveoli

In addition to smoking, there are other conditions that can affect alveoli. Some of these conditions can occur due to smoking, but some may be related to other medical conditions.

Pneumonia can cause inflammation in the alveoli. Pulmonary edema causes fluid to build up in the alveoli, potentially resulting in respiratory failure. Acute respiratory distress syndrome causes fluids to accumulate in the alveoli. Asthma causes inflammation and leads to air becoming trapped in the alveoli. COPD damages the alveoli and reduces the surface area in the lungs, making it more difficult for gas exchange to occur. Emphysema causes air to become trapped in the alveoli, making it more difficult to expel air from the lungs.Some types of lung cancer, such as bronchioloalveolar carcinoma, begin in the alveoli of the lungs.

Acute respiratory distress syndrome (ARDS) is a potentially life-threatening condition where the alveoli are damaged thereby letting fluid leak into the lungs which makes it difficult to exchange gases and oxygenate the blood. Diffuse alveolar damage (DAD) is an acute lung condition with the presence of hyaline membranes. These hyaline membranes are made up of dead cells, surfactant, and proteins. The hyaline membranes deposit along the walls of the alveoli, where gas exchange typically occurs, thereby making gas exchange difficult. DAD can be caused by infections, drugs as aspirin, radiation therapy, oxygen toxicity, heroin, cocaine, inhalational injuries, connective tissue diseases, chemotherapeutic agents, chlorine gas, nitrogen dioxide, smoke, kerosene and other forms of insult. It is the most common seen in patients with ARDS. The most common causes of ARDS are pneumonia, non-pulmonary sepsis, and aspiration.

Tips to Take Care of The Alveoli

Don't Smoke. Breathe clean air and limit your exposure to pollutants. Wash your hands regularly. Eat a healthy diet with various fruits, vegetables, grains, and protein sources. Maintain moderate body weight.

Stay hydrated. Staying well hydrated by taking in fluids throughout the day helps keep the mucosal linings in the lungs thin. This thinner lining helps the lungs function better.

Exercise Regularly. Exercise helps keep your lungs in good shape by making them work harder. Do breathing exercises. Lungs at rest and during most daily activities are only at 50 percent of their capacity. Lungs thrive on movement and activity.

Pulmonary rehabilitation can help address any muscle weakness. Watch your posture. Since the lungs are soft structures, they only take up the room that you make for them. You want to occasionally sit tall and reach overhead to make more room for your lungs. A simple technique for giving your lungs even more room is leaning back slightly in a stable chair, lifting the chest and opening the front of your body as you breathe deeply.

Laughing is a great exercise to work the abdominal muscles and increase lung capacity. It also clears out your lungs by forcing enough stale air out that allows fresh air to enter into more areas of the lung.

Get regular health checkups to know how your physical health is generally doing. Maintain a strong immune system. This can include keeping up to date with vaccinations and flu shots.