Pulmonary alveolus

A detailed overview of the pulmonary alveolus, its structure, function, and significance in the respiratory system.

Pulmonary Alveolus

The pulmonary alveolus (plural: alveoli) is a tiny, balloon-shaped structure and is the basic unit of gas exchange in the lungs. Alveoli are located at the ends of the respiratory tree and are arranged in clusters throughout the lungs. They are crucial for the process of respiration, allowing for the exchange of oxygen and carbon dioxide between the air and the bloodstream.

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Diagram of a pulmonary alveolus showing the capillary network.

Structure

Alveoli are microscopic in size, with an average diameter of about 200 to 300 micrometers. Each alveolus is surrounded by a network of capillaries, which are the smallest blood vessels in the body. The walls of the alveoli are extremely thin, consisting of a single layer of epithelial cells known as type I and type II alveolar cells.

Type I Alveolar Cells

Type I alveolar cells, also known as squamous alveolar cells, cover about 95% of the alveolar surface area. These cells are thin and flat, facilitating the rapid diffusion of gases.

Type II Alveolar Cells

Type II alveolar cells, also known as great alveolar cells, are cuboidal in shape and are responsible for the production of surfactant, a substance that reduces surface tension within the alveoli and prevents their collapse during exhalation.

Function

The primary function of the alveoli is to facilitate the exchange of gases. Oxygen from inhaled air diffuses through the walls of the alveoli and enters the blood in the surrounding capillaries. Simultaneously, carbon dioxide from the blood diffuses into the alveoli to be exhaled.

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Illustration of gas exchange in the alveoli.

Gas Exchange

Gas exchange in the alveoli occurs by diffusion, a process driven by the concentration gradients of oxygen and carbon dioxide. The high surface area and thin walls of the alveoli optimize this process, allowing for efficient gas exchange.

Role of Surfactant

Surfactant plays a critical role in maintaining alveolar stability. By reducing surface tension, surfactant prevents the alveoli from collapsing and ensures that they remain open and functional throughout the respiratory cycle.

Clinical Significance

The health and function of the alveoli are vital for effective respiration. Conditions such as pneumonia, emphysema, and acute respiratory distress syndrome (ARDS) can impair alveolar function and lead to respiratory distress.

Pneumonia

Pneumonia is an infection that inflames the air sacs in one or both lungs, which may fill with fluid or pus, leading to impaired gas exchange.

Emphysema

Emphysema is a chronic condition characterized by the destruction of alveolar walls, resulting in reduced surface area for gas exchange and difficulty breathing.

Acute Respiratory Distress Syndrome

ARDS is a severe condition characterized by widespread inflammation in the lungs, leading to fluid accumulation in the alveoli and impaired oxygenation.

Related Pages

• Respiratory system
• Lung
• Gas exchange
• Surfactant
• Capillary