What are Alveoli?
Alveoli are tiny, sac-like structures found in the
lungs where gas exchange occurs. They are the terminal ends of the respiratory tree and are critical for the process of
respiration. Each lung contains millions of alveoli, providing a large surface area for the exchange of oxygen and carbon dioxide.
Structure of Alveoli
The alveoli are composed of a thin epithelial layer and are surrounded by a network of
capillaries. The epithelial lining consists of two main types of cells:
Type I alveolar cells (pneumocytes): These are thin, flat cells that cover about 95% of the alveolar surface. They facilitate the exchange of gases between the alveoli and the blood.
Type II alveolar cells (pneumocytes): These cells are more cuboidal in shape and are responsible for the secretion of
surfactant, a substance that reduces surface tension within the alveoli, preventing their collapse.
Function of Alveoli
The primary function of alveoli is to enable the exchange of gases between the air and the blood. Oxygen from the inhaled air diffuses through the thin walls of the alveoli into the blood in the capillaries, while carbon dioxide from the blood diffuses into the alveoli to be exhaled. This process is facilitated by the large surface area and the thin barrier between the alveoli and the capillaries.Histological Staining of Alveoli
To study the alveoli under a microscope, histological staining techniques are used. Common stains include
Hematoxylin and Eosin (H&E), which highlight the cellular structures. Hematoxylin stains the nuclei of cells blue or purple, while eosin stains the cytoplasm and extracellular matrix pink. Special stains like
Periodic Acid-Schiff (PAS) can be used to identify specific components like glycogen and mucopolysaccharides.
Pathological Changes in Alveoli
Various diseases can affect the structure and function of the alveoli. For instance: Emphysema: This condition involves the destruction of the alveolar walls, leading to enlarged air spaces and reduced surface area for gas exchange.
Pulmonary Fibrosis: This disease is characterized by the thickening and scarring of the alveolar walls, which impairs gas exchange.
Pneumonia: Infection can cause the alveoli to fill with fluid or pus, hindering their ability to exchange gases.
Clinical Importance of Alveoli
Understanding the histology of alveoli is crucial for diagnosing and treating respiratory diseases. Techniques like
bronchoscopy and
lung biopsy allow for the examination of alveolar tissue, aiding in the identification of pathological changes. Additionally, treatments such as
surfactant therapy are directly related to the functional role of alveolar cells.
