Difference Between Alveoli and Alveolar Sac

The respiratory subdivisions that make the lungs include respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. Alveoli and alveolar sac make the farthest end of the respiration pathway. They also make the sites where most of the gas exchange take place within the lungs. Both alveoli and alveolar sac are found at the end of alveolar ducts.

What is Alveoli?

Alveoli are the final ends of the respiratory pathway, connected to alveolar ducts. They are thin-walled spheres and the sites where highest percentage of gas exchange take place. There are about 300 million alveoli found in each lung of a human. The surface area resulted by alveoli for surface diffusion is about 80 m2, which is about 42 times the surface area of whole human body. The air space opening into two or more alveoli is called alveolar sac. Each adjacent alveolus (singular term of alveoli) is separated by a common wall called interalveolar septum, which is composed of connective tissue with numerous anastomosing capillaries and a network of fine elastic and reticular fibers. The alveoli wall contains mainly type 1 alveolar cells (simple squamous epithelium), which make the main sites of gas exchange. In addition, it also contains type II alveolar cells (septal cells), fibroblasts, and alveolar macrophages. Fibroblasts are responsible for the production of reticular and elastic fibers, whereas type II alveolar cells (cuboidal epithelial cells) are responsible for the secretion of alveolar fluid that containing surfactant, which keep the respiratory surface moist. The macrophages are important for defensive actions against foreign particles.

What is Alveolar Sac?

An alveolar sac is a common air space at the end of an alveolar duct. It opens into two or more alveoli in the lungs. So the alveoli are clustered around the alveolar sacs. Hence, the alveoli sac is lined by the same epithelium that form the lining of alveoli.

What is the difference between Alveoli and Alveolar Sac?

• Alveolar sacs are the common air spaces that open into two or more alveoli. (Alveoli are outpouchings of alveolar sacs)

• The amount of alveoli present in the lungs is higher than that of alveolar sacs. 


Further Reading:

  1. Difference Between Alveoli and Alveolus