What is the Difference Between Bronchioles and Alveoli

The main difference between bronchioles and alveoli is that bronchioles are the small branches of the bronchial airways, connecting the respiratory airways to alveoli, whereas alveoli are the hollow, cup-shaped cavities at the end of the respiratory airways. Furthermore, bronchioles include lobular bronchioles, terminal bronchioles, and respiratory bronchioles while alveoli occur in the acini, the basic units of respiration. 

In brief, bronchioles and alveoli are the two components which collectively make up the respiratory zone of the lungs. Generally, they serve as the gas exchange surfaces, which are also surrounded by a network of capillaries. 

Key Areas Covered 

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

Key Terms 

Alveoli, Bronchioles, Gas Exchange, Respiratory Zone, Simple Cuboidal Epithelium 

What are Bronchioles 

Bronchioles are the tiny branches of the respiratory airway, leading to alveoli. Typically, each lung consists of around 30,000 bronchioles. However, they do not contain cartilage rings, encircling the tube. Basically, bronchioles are composed of smooth muscles, elastic connective tissue, and simple cuboidal epithelium with goblet cells. Moreover, there are three parts of bronchioles in the respiratory airways. They are lobular bronchioles, terminal bronchioles, and respiratory bronchioles. 

Figure 1: Bronchioles and Alveoli

Furthermore, lobular bronchioles or preterminal bronchioles branch off from the tertiary bronchi to form terminal bronchioles after passing into a pulmonary lobule. Terminal bronchioles are composed of simple cuboidal epithelium, but it lacks goblet cells. Besides, they develop the respiratory bronchioles, capable of gas exchange, which are made up of non-ciliated cells and lead to alveoli. Concerning the functions, the two major functions of the bronchioles are to conduct air to alveoli through both lobular and terminal bronchioles and to undergo gas exchange.  

What are Alveoli 

Alveoli are the many tiny air sacs of the lungs, occurring at the end of the respiratory airways. Pulmonary alveoli refer to the alveoli in the mammalian respiratory system. Generally, the diameter of the sac of alveoli is 0.2-0.5 mm. Collectively, alveoli look like a bunch of grapes. The total 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 2: Secondary Pulmonary Lobule

Moreover, 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 right ventricle of the heart. At the alveoli, this blood takes oxygen from the air inside the alveoli while releasing carbon dioxide into the alveoli. This forms oxygenated blood, which returns to the left atrium of the heart through pulmonary veins.  

Similarities Between Bronchioles and Alveoli  

• Bronchioles and alveoli are the two components of the respiratory zone of the lungs.  
• Moreover, simple cuboidal epithelium makes up of both these structures.
• Both structures are highly vascularized.  
• Furthermore, their function is to take part in the gas exchange in which oxygen from the air passes into the blood in the capillaries while carbon dioxide from the blood passes to the air inside the lungs. 

Difference Between Bronchioles and Alveoli 

Definition 

Bronchioles refer to the tiny branches of air tubes within the lungs as a continuation of the bronchus while alveoli refer to the any of the tiny air sacs of the lungs, allowing for rapid gaseous exchange. 

Significance 

Moreover, bronchioles are the small branches of the bronchial airways, connecting the respiratory airways to alveoli, while alveoli are the hollow, cup-shaped cavities at the end of the respiratory airways.  

Anatomy 

Bronchioles include lobular bronchioles, terminal bronchioles, and respiratory bronchioles while alveoli occur in the acini, the basic units of respiration. 

Conclusion 

In short, bronchioles are tiny tubules of the lungs, connecting respiratory airways to the alveoli. Furthermore, it is a component of the respiratory zone of the lungs and consists of a simple ciliated cuboidal epithelium. Alveoli, on the other hand, are the tiny air sacs at the end of the bronchioles. They are the other component of the respiratory zone as well. All these respiratory zones are highly vascularized. Thus, the main difference between bronchioles and alveoli is their anatomy. 

References:

1. Cianciolo, Rachel, et al. “Air Conduction: Terminal Bronchioles, Respiratory Bronchioles and Alveolar Ducts.” Veterinary Histology, The Ohio State University, 22 Aug. 2017, Available Here.

Image Courtesy:

1. “Illu bronchi lungs” (Public Domain) via Commons Wikimedia  
2. “Secondary-pulmonary-lobule-illustration” By Frank Gaillard – also Case courtesy of A.Prof Frank Gaillard (CC BY-SA 4.0) via Commons Wikimedia