How Pulmonary Alveoli Facilitate Efficient Gas Exchange
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.
FAQs on Pulmonary Alveolus: Structure, Function, and Significance
1. What is a pulmonary alveolus and what is its main role in the lungs?
A pulmonary alveolus is a tiny, balloon-shaped air sac located at the very end of the respiratory tract within the lungs. Its primary role is to serve as the main site for gas exchange. Here, oxygen from the inhaled air diffuses across a thin membrane into the bloodstream, while carbon dioxide from the blood diffuses into the alveolus to be exhaled.
2. What is the difference between the terms 'alveolus' and 'alveoli'?
The terms refer to the same structure, with the only difference being grammatical. 'Alveolus' is the singular form, referring to a single air sac. 'Alveoli' is the plural form, referring to multiple air sacs. The human lungs contain millions of alveoli, which together create a massive surface area for breathing.
3. What are the different types of cells found in a pulmonary alveolus and their functions?
The alveolar wall is primarily composed of three main cell types, each with a specific function:
- Type I Pneumocytes: These are extremely thin, flat cells that constitute about 95% of the alveolar surface. Their thin structure is vital for allowing rapid and efficient diffusion of gases.
- Type II Pneumocytes: These are cuboidal cells that secrete pulmonary surfactant, a substance that reduces surface tension inside the alveoli, preventing them from collapsing during exhalation.
- Alveolar Macrophages: Also known as 'dust cells', these are immune cells that move along the alveolar surface to engulf dust, bacteria, and other inhaled debris, keeping the lungs clean and protected from infection.
4. How does the specific structure of the alveoli facilitate efficient gas exchange?
The structure of alveoli is highly adapted for efficient gas exchange in several key ways:
- Vast Surface Area: With approximately 700 million alveoli in human lungs, the total surface area for gas exchange is enormous, roughly the size of a tennis court.
- Extremely Thin Walls: The respiratory membrane—formed by the alveolar wall and the capillary wall—is incredibly thin (about 0.5 micrometres), minimizing the distance gases need to travel.
- Rich Blood Supply: Each alveolus is wrapped in a dense network of blood capillaries, ensuring that a large volume of blood is always close to the inhaled air, maintaining a steep concentration gradient for diffusion.
- Moist Lining: The inner surface is lined with a thin layer of fluid, which allows oxygen to dissolve before diffusing across the membrane into the blood.
5. What is the importance of pulmonary surfactant in the alveoli, and what happens if it's absent?
Pulmonary surfactant is a critical lipoprotein mixture secreted by Type II pneumocytes. Its main importance is to reduce the surface tension of the fluid lining the alveoli. This prevents the air sacs from collapsing at the end of exhalation and reduces the effort needed to inflate the lungs during inhalation. If surfactant is absent or insufficient, as in Neonatal Respiratory Distress Syndrome, the alveoli collapse, making breathing extremely difficult and inefficient.
6. Why do diseases like emphysema, which damage alveolar walls, lead to severe breathing difficulties?
Emphysema causes the breakdown of the walls between adjacent alveoli. This destruction leads to severe breathing problems for two main reasons. Firstly, it drastically reduces the total surface area available for oxygen to enter the blood and for carbon dioxide to leave. Secondly, the loss of alveolar walls also means a loss of elastic fibres, which diminishes the lungs' natural elastic recoil. This makes it difficult to exhale fully, trapping stale air in the lungs and impairing ventilation.
7. How does the respiratory surface of the alveoli compare to other parts of the respiratory tract, like the bronchioles?
The key difference lies in their function and structure. Bronchioles are part of the conducting zone, designed to transport air, so they have thicker, more rigid walls containing smooth muscle. In contrast, alveoli form the respiratory zone and are solely designed for gas exchange. Their walls are exceptionally thin (a single cell layer), lack muscle, and are directly fused to capillaries to create the thinnest possible barrier for diffusion, a feature absent in the bronchioles.
