Function and Disorders of the Alveoli

Minute Structures of the Lung Vital to Respiration

Alveoli in the lungs.
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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.

What Are Alveoli?
Verywell / JR Bee 

Structure

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 (CO2) between the alveoli and blood vessels called capillaries.

One cubic millimeter of lung tissue contains around 170 alveoli. While the total number can vary from one person to the next, there are literally millions within the human lungs spanning a surface area of roughly 70 square meters.

Cells of the Alveoli

The alveoli are made up of two different types of cell that have different functions:

  • Type I pneumocytes are the cells that are responsible for the exchange of oxygen and carbon dioxide.
  • Type II pneumocytes perform two important functions. They are responsible for repairing damage to the alveolar lining and also secrete surfactant.

There are also many immune cells known as alveolar macrophages in the alveoli. Macrophages are essentially the "garbage trucks" of the immune system, and phagocytize or "eat" debris they come across. They are responsible for cleaning up any particles that are not caught by the cilia or mucus in the upper respiratory tract, as well as dead cells and bacteria.

Function

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 into 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.

Alveoli are lined by a fluid layer known as a surfactant which maintains the shape and surface tension of the air sac. By maintaining surface tension, there is more surface area through which oxygen and CO2 molecules can pass.

It is at this junction that oxygen molecules diffuse through a single cell in an alveolus and then a single cell in a capillary to enter the bloodstream. At the same time, CO2 molecules, a byproduct of cellular respiration, are diffused back into alveolus where they are expelled out of the body through the nose or mouth.

Diffusion of oxygen from the alveoli to capillaries occurs because the concentration of oxygen is lower in the capillaries. Similarly, carbon dioxide diffuses from the capillaries to the alveoli where the concentration of carbon dioxide is lower.

During inhalation, alveoli 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.

Medical Conditions Affecting Alveoli

There are a number of medical conditions that can directly affect the alveoli (that 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. In addition to damage caused by inflammation or infection within the alveoli, proper function depends on the body maintaining a balance between overinflation and underinflation of the alveoli.

  • Overdistention: The presence of a healthy connective tissue support system is need to prevent the alveoli from overdistending. An example of injury that may result in overdistention is mechanical ventilation (breathing via a respirator).
  • Surfactant dysfunction: Surfactant prevents the alveoli from collapsing completely between breaths. To understand why this is important you can imagine how it is easier to blow up a balloon that has been partially inflated relative to blowing up a completely collapsed balloon. Medical conditions such as COPD, respiratory distress syndrome in infants, asthma, interstitial fibrosis, as well as some genetic conditions can cause surfactant dysfuction leading to collapse of alveoli.

Among the conditions involving the alveoli:

Emphysema

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 that remain begin to harden and lose their elasticity. This makes it difficult to expel air from the lungs (a condition called air trapping).

Air trapping explains why exhaling rather than inhaling is usually more difficult in people with emphysema. This inability to expel air leads to further dilatation of the alveoli and the increased loss of function.

Pneumonia

Pneumonia is an infection that inflames the alveoli in one or both lungs and can result in the air sacs filling with pus.

Tuberculosis

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, causing the formation of pus in the air sacs.

Bronchioloalveolar Carcinoma (BAC)

Bronchioloalveolar carcinoma (BAC) is a form of lung cancer that is now instead considered a subtype of lung adenocarcinoma. These cancers begin in the alveoli and are often found diffusely in one or both lungs. Unlike many cancers that spread through the lymphatic system and/or bloodstream to distant regions of the body, BAC spreads primarily through the airways (aerogenous metastases) to other regions of the lungs.

Acute Respiratory Distress Syndrome (ARDS)

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)

Respiratory distress syndrome (RDS) is seen in premature babies whose bodies have not yet produced enough surfactant to line the alveoli, and therefore, less surface area is available for the exchange of oxygen and carbon dioxide.

Pulmonary Edema

Pulmonary edema is a condition caused by excess fluid in the lungs which collects in the alveoli and can lead to respiratory failure.

Alveolar Proteinosis

Pulmonary alveolar proteinosis is a rare disease in which proteins accumulate in the alveoli. It is most often an autoimmune condition, occurring in adults aged 20 to 50, but may occur as a congenital (from birth) condition as well.

Cigarettes and 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 sacs their elasticity. Smoking damages both of these, causing the sacs to harden and thicken. Smoking also actively dilates blood vessels, impeding the exchange of oxygen and CO2.

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.

A Word From Verywell

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.

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Article Sources

  1. Knudsen, L., and M. Ochs. The Micromechanics of Lung Alveoli: Structure and Function of Surfactant and Tissue Components. Histochemistry and Cell Biology. 2018. 150(6):661-676. doi:10.1007/s00418-018-1747-9

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