Q: How do the lungs work?
A: The lungs perform many functions, including making, activating and inactivating certain substances (including some hormones), helping defend the body from infection and enabling us to speak. But, the main roles the lungs play are delivering oxygen to and off-loading carbon dioxide from the blood. Having sufficient blood oxygen is crucial to adequately supply the body’s cells. Taking carbon dioxide out of the blood is critical to regulate its pH (acid-base balance) in order to ensure a healthy cell environment.
When the diaphragm moves downward and the chest wall muscles expand, the volume of the chest increases. This draws air into the lungs in much the same was as a bellows sucks air in when the handles are pulled apart. As the diaphragm and chest wall muscles relax the elasticity of the lungs pulls the chest back to its resting state, pushing the air out in much the same way as the elasticity of a blown-up balloon pushes air out as it squeezes back to its uninflated state.
Air pulled into the chest, as noted above, enters the body through the nose and/or mouth and then travels down the main breathing tube, the trachea. The trachea branches into the left and right main bronchi, and then branches further to smaller bronchi, to even smaller bronchioles and eventually to the 300 million or so alveoli, the small sacs of air, which are surrounded by small blood vessels (capillaries).
Once air is in the alveoli the oxygen in it must make its way into the blood stream. This is accomplished by the partial pressure of oxygen in the air “pushing” oxygen to diffuse across the alveolar/capillary interface to dissolve into the blood. At the same time, carbon dioxide leaves the blood and enters the alveolar space where it can be pushed out during exhalation.
The balance of the amount of blood flowing through the capillaries and the amount of air inside the alveoli must be carefully regulated throughout the lungs (called ventilation-perfusion matching). If there is not enough blood flow the under-perfused alveoli are wasted space (called dead space). If there is insufficient air for the amount of blood that perfuses an area of the lung, the blood will not be sufficiently oxygenated and hence the overall blood exiting the lungs may have too low an oxygen supply to fulfill the cell’s needs.
Oxygen does not dissolve well in water (the liquid part of blood is mostly water), so the amount of oxygen blood can carry in solution is very limited. Red blood cells (RBCs) bind oxygen, allowing the blood to carry much more oxygen in order to supply the body’s needs. In fact, about 98 percent of the oxygen in blood is bound to RBCs.
Many diseases can cause an imbalance in the ventilation-perfusion match of the lungs. Diseases that prevent normal air filling of the alveoli include those that cause excessive air flow resistance in the airways (such as asthma or emphysema), those that “clog up” the alveoli (such as congestive heart failure which can cause fluid to build up in the alveoli or pneumonia which can cause infection and inflammation within the alveoli), as well as many other conditions. Diseases that prevent sufficient blood supply to the alveoli include blockage of the pulmonary arteries (such as pulmonary emboli) and many other conditions. Some diseases compromise the lungs in other ways, such as a collapsed lung.
Many conditions can also affect the ability of oxygen to bind to RBCs. Even if the ventilation and perfusion of the lungs are normal, this may limit the amount of oxygen available to supply the body’s needs.
Without proper lung function the body’s cells may not get sufficient oxygen and/or may not have an ideal environment to function. Taking good care of your lungs is important to a long and healthy life. So do not start smoking, and if you do smoke, work hard to quit. Regular exercise and a well-balanced, healthy diet are also key to keeping your lungs functioning normally.
Jeff Hersh, Ph.D., M.D., can be reached at DrHersh@juno.com.
Dr. Jeff Hersh: Breathe easy - all about lung function
Q: How do the lungs work?