Pulmonary Alveolus


Alveoli in Human Lungs

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A pulmonary alveolus refers to a tiny hollow cavity of cup shape found in the lungs where gas exchange of carbon dioxide and oxygen takes place. The plural form is known as pulmonary alveoli. The word alveolus is a Latin term meaning a little cavity. At the starting of the respiratory system, alveoli are found in the acini. They are located in the lining of alveolar ducts, sparsely in the respiratory bronchioles, and unaccountably in the blind end sacs of alveoli. As we inhale air into our lungs, the air passes through our bronchi and finally, it flows into numerous alveoli present at the end of bronchioles to provide lesser air passages. The carbon dioxide-rich air is exhaled similarly from the alveolar sacs through the passages as inhaled before.

Structure of Pulmonary Alveolus

The lungs have alveolar sacs which house the alveoli in the respiratory zone, especially in the pulmonary lobules. The pulmonary alveolus is the smallest functioning unit in the respiratory system. Extending from the lumens as scattered out pockets, they are also located in the bronchioles. It further deepens down in the alveolar ducts which are heavily lined with alveoli. Each bronchiole separates into two to eleven alveolar ducts each of which further separates into five to six alveolar sacs. These contain clusters of alveoli. Until the age of eight years, new alveoli formation takes place. The lungs and alveoli form a major part of the respiratory system. Each alveolus is wrapped by fine capillaries that cover about three-fourths of its surface area. The gas exchange surface is the alveolar membrane itself. As per lung anatomy alveoli breathes out oxygen and collects carbon dioxide from the capillaries.

Functioning of Pulmonary Alveolus

The alveoli in human lungs have majorly three types of cells. Type 1 and Type 2 are pneumocytes found in the alveolar walls. The other one is the alveolar macrophage, a large phagocytic cell that moves in between the connective tissues of the lumen. Type 1 cells are usually thin, flat, squamous which forms the structure of the alveoli. To reduce surface tension, Type 2 cells release pulmonary surfactant. It can also replace damaged type 1 cells. The Type 1 cells are majorly used in the gas exchange process from the membrane to the blood. The thin lining of the cells helps in effective diffusion faster. But type 1 cells are susceptible to toxicity and cannot replicate. 

The Type 2 cells are much smaller than the Type 1 cells are cuboidal. They mainly secrete the pulmonary surfactant with the help of special lamellar organelles. Without the coating of the phospholipids, the alveoli would have collapsed. The blood air barrier region typically houses these cells. After twenty-six weeks of gestation, usually, Type 2 cells start to develop. But secretion of the surfactant starts after 35 weeks. The alveolar macrophages present in the lung anatomy alveoli are scavengers who engulf and foreign substances such as dust, carbon, bacteria, and more. 

Diseases Associated With Pulmonary Alveolus

Atelectasis is caused by insufficient surfactant secretion. In infants, it can cause infant respiratory distress syndrome (IRDS) due to the same reason. Pulmonary alveolar proteinosis is also due to surfactant inflammation. Acute Respiratory Distress Syndrome (ARDS) is caused by alveolar damage of the lungs. Whenever an abundance of mucus is produced in the lungs it causes chronic bronchitis. All the air passages of the bronchioles and alveoli become packed with a thick layer of mucus. Sometimes a cavity is produced by alveolar destruction which is known as cavitary pneumonia that affects the gas exchange in the alveoli. Another disease where the elastin of the alveolar walls is broken down is known as emphysema. It prolongs the time of exhalation. Pulmonary Alveolar Microlithiasis is caused by small stones formed inside the alveoli. There are plenty of diseases associated with the lungs and alveoli but these were some of the serious ones.

Did You Know?

  • An adult pair of typical human lungs has about 480 million alveoli. 

  • It produces about fifty to seventy-five square meters of surface area in totality.

  • The diameter of alveoli in human lungs is about 200 to 500 micrometers.

  • The interconnected airways present in the alveolar walls are known as pores of Kohn.

FAQ (Frequently Asked Questions)

1. What is the Microanatomical Structure of Pulmonary Alveolus?

Answer: The alveoli are composed of very thin epithelial layers of flattened cells or squamous epithelium. It also has an extracellular matrix consisting of blood capillaries. The thin epithelial layer helps in the gaseous diffusion. There is a thin lining of fluid known as the surfactant and the membrane has multiple numbers layers such as the epithelial layer, basement layer, capillary endothelium membrane, and more. The air passages present in the alveoli are called pores of Kohn. Elastic and collagen fibers are present in the Alveolar septa separating the sac and alveoli. The stretching of the alveoli during inhalation of air is possible due to the elastic fibers. They again help is the exhalation of the carbon dioxide bore air by springing back. Apart from the Alveolar cells are basically of three types. Two are pneumocytes while the other one is a large phagocytic cell known as Alveolar macrophages.

2. What Do You Mean By Pulmonary Alveolus?

Answer: Pulmonary alveolus is a cavity located primarily in the respiratory system of the lungs. The respiratory tract starts and terminates at the alveolar air sacs. The total surface area provided by them is approximately a hundred-meter squares which helps in the gaseous diffusion of oxygen and carbon dioxide. Seventy percent area of the alveolar surface is covered by a mesh of capillaries. About seventy meter square of the area is produced by approximately 700 million pulmonary alveoli present in common human lungs. The diameter of an alveolus is normally 200 micrometers. The alveoli consist of a squamous epithelium layer which is surrounded by capillaries. It also consists of collagen and elastic fibers. There are also some air pores known as pores of Kohn. The three types of cells, mainly the pneumocytes and the large phagocytic scavenger cell have different functionality.