Functions and Disorders of the Alveoli

How They Work and What Can Affect Them

Lungs
decade3d/istock

Alveoli are an important part of the respiratory system whose function it is to exchange oxygen and carbon dioxide molecules to and from the bloodstream. These tiny, balloon-shaped air sacs sit at the very end of the respiratory tree and are arranged in clusters throughout the lungs.

There are millions of alveoli in the human body with a surface area of roughly 70 square meters. If they were flattened and stretched from end-to-end, they could cover an entire tennis court.

Mapping the Flow of Air to and From the Alveoli

Alveoli are the endpoint of the respiratory system which starts when we inhale air into the mouth or nose. The oxygen-rich air travels down the trachea and then branch to one of the two lungs via the right or left bronchus. From there, the air is directed through smaller and smaller passages, called bronchioles, past the alveolar duct, until it finally enters an individual alveolus.

Each alveolus is lined by a fluid layer known as surfactant which maintains the surface tension and shape of air sac. The alveolus it itself is surrounded by a network of capillaries which transport oxygen to the bloodstream and carbon dioxide away from the bloodstream.

It is at this junction that oxygen molecules diffuse through a single cell in an alveolus and a single cell in a capillary to enter the bloodstream. At the same time, carbon dioxide molecules, a byproduct of cellular respiration, are diffused back into alveolus where they are expelled out of the body through the nose or mouth.

During inhalation, capillaries expand as the negative pressure in the chest is created by contraction of the diaphragm. During exhalation, the alveoli recoil (spring back) as the diaphragm relaxes.

Structure of the Alveoli

Alveoli are tiny balloon shaped structures and are the smallest passageway in the respiratory system.

The alveoli are only one cell thick, allowing the relatively easy passage of oxygen and carbon dioxide between the alveoli and the capillary. One cubic millimeter contains 170 alveoli and the area of alveoli is on average 70 square meters. The number of alveoli differs between people and larger lungs have more alveoli.

Medical Conditions Involving the Alveoli

There are a number of medical conditions that can directly affect the alveoli (which we refer to as alveolar lung diseases). These diseases can cause the alveoli can become inflamed and scarred or cause them to fill with water, pus, or blood.

Among the conditions involving the alveoli:

  • Emphysema is a condition in which the inflammation in the lungs causes the dilation and destruction of alveoli. In addition to the loss of alveoli, the cellular walls of air sacs begin to harden and lose their elasticity. This makes it difficult to expel air from the lungs (a condition called air trapping). This explains why exhaling rather than inhaling is usually more difficult in people with emphysema.
  • Pneumonia is an infection that inflames the alveoli in one or both lungs and can result in the air sacs filling with pus.
  • Tuberculosis is an infectious bacterial disease characterized by the growth of nodules in the tissues of the lungs. The disease primarily infects the alveoli as bacteria are inhaled.
  • Bronchioloalveolar carcinoma (BAC) is a form of lung cancer now considered a subtype of lung adenocarcinoma that begins in the alveoli.
  • Acute respiratory distress syndrome (ARDS) is a life-threatening lung condition that prevents oxygen from getting to the lungs as fluids begin to accumulate in the alveoli. ARDS is common in critically ill patients.
  • Respiratory distress syndrome (RDS) is seen in premature babies whose bodies have not yet produced enough surfactant to line the alveoli. Without surfactant to hold the alveoli open, the surface area of the lungs is decreased, making respiration all the more difficult.
  • Pulmonary edema is a condition caused by excess fluid in the lungs which collect in the alveoli and can lead to respiratory failure.

The Impact of Cigarettes on the Alveoli

As a single risk factor for lung disease, tobacco smoke is known to affect the respiratory tract at every level. This includes the alveoli.

Alveoli are made up of collagen and elastin which provide the air sacs their elasticity. In the same way that cigarettes damage the collagen and elastin in your skin (leading to accelerated wrinkling), they can undermine the production of these substances in your alveoli, as well. As a result, the elastic recoil of the alveoli is diminished as the cell walls begin to thicken and harden due to the mounting damage.

Cigarette smoke also affects how the alveoli work, causing damage right down to the molecular level. It disrupts our body's ability to repair itself as it might following an infection or trauma. As such, the alveolar damage is allowed to progress unhindered as the lungs are persistently exposed to toxic fumes.

Bottom Line on the Alveoli

The alveoli provide one of the most important functions our bodies perform. They're the gateway through which oxygen enters our bloodstream and the primary way in which some of the waste products of metabolism (carbon dioxide) exit the body. 

Diseases which affect the alveoli can result in reduced oxygen being delivered to the tissues of our body, and consequently, may result in damage (due to hypoxia) to every major organ.

Sources:

Hsia, C., Hyde, D., and E. Weibel. Lung Structure and the Intrinsic Challenges of Gas Exchange. Comprehensive Physiology. 2016. 6(2):827-895.

Kasper, Dennis L.., Anthony S. Fauci, and Stephen L.. Hauser. Harrison's Principles of Internal Medicine. New York: Mc Graw Hill education, 2015. Print.

Continue Reading