Gas Exchange: Structure and Function of Alveoli - Essay Example

? Gas Exchange – Structure and Function of Alveoli The lungs are the organs of the respiratory system responsible for gaseous exchange. Each lung divides into lobes, which divide into lobules, which also divide to eventually lead to the final basic respiratory unit, the alveolus. Alveoli make up the pulmonary parenchyma. When air enters the lung through the bronchus, it continues to pass through dividing smaller and smaller airways called bronchioles. The smallest bronchiole at the end of the airway, the terminal bronchiole, leads into the alveolar duct, that opens into several alveoli. The bronchi and bronchioles are lined by a columnar epithelium with cilia. The epithelium produces mucus, and the cilia beat in the upwards direction, so that mucus traps any dust and foreign particles harboring pathogenic microorganisms in the inspired air. The mucus is then swept up to the trachea and throat to be swallowed, thus ensuring that only very clean air reaches the alveoli, to prevent infection. The alveolus itself is a sac-like structure filled with air, with walls made up of a single layer of epithelial cells. These epithelial cells are called pneumocytes. There are two types of pneumocytes – type I and tpe II. Type I pneumocytes are flat, thin cells that cover 90% to 95% of the alveolar surface. In between the walls of adjacent alveoli is present interstitial tissue made by elastic and connective tissue fibers, as well as pulmonary capillaries. These walls between alveolar air sacs are also called alveolar ‘septa’. The capillaries are lined by a single layer of endothelial cells. Gas exchange of oxygen and carbon dioxide occurs between the blood in the pulmonary capillary and the air in the alveolus. The gases have to diffuse through the ‘air-blood barrier’ to reach the other side. This air-blood barrier consists of the alveolar epithelium, the capillary endothelium, and the interstitial space between them. The thickness of this barrier is normally only 05. – 1 micrometers. This very small diffusion distance allows the gases to diffuse very rapidly, so that equilibrium is achieved within a second during inspiration. There are about 300 million alveoli in the adult, with the total surface area of the air-blood barrier estimated around 70 m2. This large surface area allows all of the blood in the body to be distributed within the two lungs, so that oxygenation can occur quickly. When a large amount of alveolar surface area is lost, damaged or unable to carry out gas exchange, such as occurs in diseases such as emphysema, this leads to significant shortage of oxygen in the blood. During inspiration, the fresh air entering the lungs has a high concentration of oxygen and low concentration of carbon dioxide. The blood in the pulmonary capillary, on the other hand, has a low level of oxygen but a high level of carbon dioxide. During gas exchange, carbon dioxide diffuses out of the blood, passes through the air-blood barrier, and enters the alveolar air. This carbon dioxide is then removed during expiration. Similarly, oxygen from the alveolar air diffuses across the air-blood barrier to enter the pulmonary capillary and the red blood cells in the capillary, until the concentration of oxygen in the alveolar air and blood becomes equal. When the next inspiration takes place, fresh air with a high oxygen content enters the alveolar air so that this new oxygen can now enter the blood. Breathing is a motion that performs the function of replacing the air in the alveolar sacs with fresh air, in order to maintain the diffusion gradient for oxygen and carbon dioxide between the blood and alveolar air. This ensures that gas exchange continues constantly at a regular rate. One of the lung functions that enables this breathing movement to take place is the inherent elastic recoil of the lung, which is due to the large amount of collagen fibers in the alveolar septum. During inhalation, the diaphragm and chest wall muscles contract to increase the chest cavity space and expand the lung, so that fresh air enters the lung. When the muscles lose the contraction, it is the elastic recoil of the lung that causes it to shrink and produce the force of expiration, so that air can leave the lung after gaseous exchange is complete. The inner wall of the alveoli, adjacent to the air, is lined by a layer of a lipid-like substance called surfactant, which is produced by type II pneumocytes. Surfactant lowers the surface tension at the inner wall of the alveolar sac, and prevents collapse of the alveolus during expiration. Thus, surfactant helps maintain the patency of the air walls. Surfactant production starts in the fetal stage, and is completed by 32 weeks of gestation. In the absence of surfactant, as happens in preterm babies who are born before their lungs have made sufficient surfactant, many alveoli collapse during expiration and fail to reopen at inspiration due to high surface tension, so that the surface area for gas exchange is much reduced, leading to severe hypoxia. This is ‘termed acute respiratory distress syndrome’ in preterm newborns. A third type of cell present in the lung parenchyma is the alveolar macrophage. This is a white blood cell that moves out of the blood and scouts along the alveolar wall, ingesting any dust or foreign particles, and bacteria or microorganisms that reach the alveoli. The macrophages thus protects the lung from infections. In the alveolar septa are present holes, termed ‘pres of Kohn’. These pores allow inter-alveolar communication, and are a protective feature that help maintain lung surface area in the situation where bronchioles are blocked. If a bronchiole becomes obstructed by mucus or any other substance, all the alveoli that it supplies would become unable to do any gas exchange as no fresh air would enter the alveoli. However, the pores of Kohn allow fresh air from neighbouring alveoli to enter the alveolus with the obstructed bronchiole, thus ensuring that lung surface area is not lost in this situation and gas exchange can still take place. Thus, the structure of the alveolus is highly specialized, and adapted to maximizing its ability to carry out its function of gas exchange. References Barrett K.E., Barman S.M., Boitano S., Brooks H. (2010). Chapter 35. Pulmonary Function. In K.E. Barrett, S.M. Barman, S. Boitano, H. Brooks (Eds), Ganong's Review of Medical Physiology, 23e. Retrieved November 26, 2011 from http://www.accessmedicine.com/content.aspx?aID=5245931 Levitzky M.G. (2007). Chapter 1. Function & Structure of the Respiratory System. In M.G. Levitzky (Ed), Pulmonary Physiology, 7e. Retrieved November 26, 2011 from http://www.accessmedicine.com/content.aspx?aID=2773000. Mescher A.L. (2010). Chapter 17. The Respiratory System. In A.L. Mescher (Ed), Junqueira's Basic Histology: Text & Atlas, 12e. Retrieved November 26, 2011 from http://www.accessmedicine.com/content.aspx?aID=6182422. Wilson F.J., Kestenbaum M.G., Gibney J.A., Matta S. (1999). Chapter 19. Respiratory System. In F.J. Wilson, M.G. Kestenbaum, J.A. Gibney, S. Matta (Eds), Histology Image Review. Retrieved November 26, 2011 from http://www.accessmedicine.com/content.aspx?aID=403312. Read More