Blood-air barrier

Blood-air barrier

The blood-air barrier (also known as the alveolar-capillary barrier or membrane) is a thin partition where gas exchange occurs between the alveolar space and the capillaries. This barrier is permeable to molecular oxygen, carbon dioxide, and other gases.

Structure

The blood-air barrier is composed of the alveolar epithelium, the capillary endothelium, and their shared basement membrane. This barrier is extremely thin (about 0.5 micrometers at its thinnest part), allowing for efficient gas exchange.

Alveolar epithelium

The alveolar epithelium is primarily composed of two types of cells: Type I pneumocytes and Type II pneumocytes. Type I pneumocytes are thin and flat, covering most of the alveolar surface. Type II pneumocytes are more numerous but occupy a smaller portion of the alveolar surface. They are responsible for the production and secretion of pulmonary surfactant, which reduces surface tension in the alveoli.

Capillary endothelium

The capillary endothelium is a single layer of thin, flat cells that line the interior surface of blood vessels. These cells are tightly joined together, forming a barrier that controls the passage of materials in and out of the bloodstream.

Basement membrane

The basement membrane is a thin, fibrous matrix that separates the alveolar epithelium from the capillary endothelium. It provides structural support and acts as a filter, allowing certain molecules to pass through while blocking others.

Function

The primary function of the blood-air barrier is to facilitate gas exchange. Oxygen from the inhaled air diffuses across the barrier into the blood, while carbon dioxide in the blood diffuses across the barrier into the alveoli to be exhaled. This process is known as respiration.

Clinical significance

Damage to the blood-air barrier can lead to conditions such as pulmonary edema, where fluid leaks into the alveoli, and acute respiratory distress syndrome (ARDS), a severe form of lung injury.

See also

• Respiratory system
• Gas exchange
• Pulmonary circulation