Alveoli are composed of epithelial layers and extracellular matrix enclosed in capillaries while alveolar sacs are the distal ends of alveolar ducts .The alveoli sacs are formed by a group or cluster of alveoli, and it is there where they communicate while the alveoli are made up of collagen and elastic fibers . The alveoli process the gases that the blood caries to the lungs expelling the carbon dioxide and letting the blood transport the oxygen to the cells in the body while it is in the alveolar sacs where this process takes place.

Difference between Alveoli, Alveolar Duct, and Alveolar Sac

The respiratory system consists of various structures that work together to facilitate the exchange of oxygen and carbon dioxide between the lungs and the blood. Three important structures involved in this process are the alveoli, alveolar ducts, and alveolar sacs. While these structures are closely related and function together, they have distinct characteristics and roles in the respiratory system.

Alveoli:
- Alveoli are tiny air sacs located at the terminal ends of the respiratory bronchioles.
- They are responsible for the primary gas exchange between the respiratory system and the blood.
- Alveoli are surrounded by a network of capillaries, allowing for the exchange of oxygen and carbon dioxide.
- The walls of alveoli are thin and composed of a single layer of epithelial cells, making them highly efficient in gas exchange.
- They have a large surface area, providing ample space for gas exchange to occur.
- Alveoli are numerous, with estimates suggesting that there are around 300 million in each lung.
- The alveolar walls contain specialized cells called type I and type II pneumocytes. Type I pneumocytes are responsible for gas exchange, while type II pneumocytes secrete a substance called surfactant that helps prevent alveolar collapse.

Alveolar Ducts:
- Alveolar ducts are airway passages that connect the respiratory bronchioles to the alveolar sacs.
- They are lined with simple squamous epithelium, similar to the alveoli.
- Alveolar ducts are wider and longer than the respiratory bronchioles, allowing for the passage of air.
- They have a branching structure, with numerous alveolar sacs branching off from them.

Alveolar Sacs:
- Alveolar sacs are clusters of multiple alveoli that are interconnected.
- They are located at the end of the alveolar ducts and serve as reservoirs for air.
- Alveolar sacs have a shape resembling a grape cluster.
- The walls of alveolar sacs are thin and consist of a network of capillaries, similar to individual alveoli.
- The clustering of alveoli in alveolar sacs provides a larger surface area for gas exchange.

Summary:
In summary, alveoli, alveolar ducts, and alveolar sacs are all essential components of the respiratory system. Alveoli are the primary sites of gas exchange, while alveolar ducts connect the respiratory bronchioles to the alveolar sacs. Alveolar sacs, on the other hand, serve as reservoirs for air and contain clusters of interconnected alveoli. Understanding the distinct characteristics and functions of these structures helps to appreciate the complexity and efficiency of the respiratory system.