What is the Difference Between Alveoli and Alveolar Sac

The main difference between alveoli and alveolar sac is that alveoli are tiny air sacs that functions as basic respiratory units, whereas alveolar sac is a cluster of alveoli that resembles a bunch of grapes. 

The alveoli and alveolar sac are two structures that occur in the lungs. Their function is to be involved in the gas exchange in the lungs.

Key Areas Covered

1. What are Alveoli
– Definition, Structure, Function
2. What is Alveolar Sac
– Definition, Structure, Function
3. Similarities Between Alveoli and Alveolar Sac
– Outline of Common Features
4. Difference Between Alveoli and Alveolar Sac
– Comparison of Key Differences

Key Terms 

Alveolar Sac, Alveoli

What are Alveoli

Alveoli are the many tiny air sacs of the lungs, occurring at the end of the respiratory airways. Pulmonary alveoli are the alveoli in the mammalian respiratory system. Generally, the sac diameter of the alveoli is 0.2-0.5 mm. Collectively, alveoli look like a bunch of grapes. The average surface area of alveoli in a pair of human lungs is around 70 m3. Besides, the respiratory membrane of the alveoli is made up of a simple squamous epithelium.

Figure 1: Gas Exchange

Furthermore, the main function of alveoli is to facilitate the exchange of respiratory gases. Generally, deoxygenated blood reaches the lungs through the pulmonary artery from the heart’s right ventricle. This blood takes oxygen from the air inside the alveoli at the alveoli, releasing carbon dioxide into the alveoli. It forms oxygenated blood, which returns to the heart’s left atrium through pulmonary veins.  

What is Alveolar Sac

The alveolar sac is a collection of alveoli. An adult lung contains 480 million alveoli that form alveolar sacs. Alveoli are hollow, distensible cup-shaped cavities where pulmonary gas exchange occurs. In the alveoli, oxygen is exchanged for carbon dioxide at the blood-air barrier. However, alveoli are the functional tissue of the alveolar sac. Therefore, alveoli take up around 90% of the total lung volume.

Figure 2: Alveolar Sac

Moreover, the respiratory zone of the lungs begins at the respiratory bronchioles. Alveoli occur sparsely in the bronchioles. An alveolar duct connects the alveolar sacs to the bronchioles. Five or six alveolar sacs occur per alveolar duct. Also, blood capillaries surround the alveolar sac, facilitating gas exchange. The pulmonary artery and vein are the blood vessels that supply blood to the alveolar sac. Acinus refers to the primary respiratory unit of the lungs containing respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. 

Similarities Between Alveoli and Alveolar Sac

• The alveoli and alveolar sac are two structures that occur in the lungs.
• They undergo gas exchange in the lungs. 

Difference Between Alveoli and Alveolar Sac

Definition

Alveoli refer to tiny air sacs that occur at the end of the respiratory tract in the lungs. In contrast, the alveolar sac refers to sacs of many alveoli, which are the cells that exchange oxygen and carbon dioxide in the lungs.

Significance

Alveoli are tiny air sacs functioning as basic respiratory units, while an alveolar sac is a cluster of alveoli that resembles a bunch of grapes.

Structure

The alveoli contain thin, squamous, and flat epithelial lining cells, while the alveolar sac is a pocketlike structure made up of several individual alveoli. 

Function

Alveoli facilitate the gaseous exchange of carbon dioxide and oxygen between inhaled air and the bloodstream, while the alveolar sac combines several individual alveoli. 

Conclusion

In brief, the alveoli and alveolar sac are two structures that occur in the lungs. Alveoli are tiny air sacs that serve as the basic respiratory unit. They contain thin, squamous, and flat epithelial lining cells. Alveoli facilitate gaseous exchange. In comparison, the alveolar sac is a cluster of alveoli that resembles a bunch of grapes. It is made up of several individual alveoli. Its function is to combine several individual alveoli. Therefore, the main difference between alveoli and alveolar sac is their structure and function.

References:

1. Alice Ferng. (2023, July 24). Alveoli. Kenhub. 

Image Courtesy:

1. “Gas exchange in the aveolus” By domdomegg – Own work (CC-BY 4.0) via Commons Wikimedia
2. “Alveolus diagram” By   LadyofHats – Own Work (Public Domain) via Commons Wikimedia