Respiration is essential in the survival of all living organisms. It is a process where gas exchange occurs in all biological organisms. In the case of animals such as humans, respiration is affected by the ratio of the concentration of oxygen and carbon dioxide in the body. The human body generally requires oxygen gas because it is only through this aerobic respiration by which metabolic functions are assumed to maintain homeostasis. In humans there is a specific organ system which is solely dedicated to respiration known as the respiratory system. The human respiratory system constitutes different types of organs which performs specific functions. These include the nasal cavity, pharynx or throat, larynx or voice box, epiglottis, trachea, the bronchial tree, pleural membrane and the lungs and the air sacs known as alveoli. Other respiratory accessory organs include the oral cavity, intercostal muscles, and the diaphragm (Starr, Evers and Starr, 2008).
Generally air moves through the nostrils lest breathing becomes laborious the mouth helps as an accessory organ. The cilia act as a mechanical filter when large particles enter the nostrils while the mucus secreted by cells of the nasal lining captures most fine particles and airborne chemicals. As the air moves to the nasal cavity, it gets warmed and moistened before it flows to the pharynx. The pharynx is an airway connecting the nasal cavity and the mouth with larynx and is also connected with the esophagus. This organ also augments in the production of sound. From the pharynx the air moves to the larynx where sound is produce and works in unison with the epiglottis. That is, the contraction of the vocal chords in the larynx closes the epiglottis and during swallowing the epiglottis closes off the larynx to prevent food and other particles from entering the lungs. When the epiglottis close food and fluid enters the esophagus connects the pharynx and the stomach.
However, when the epiglottis is wide open air, the moves into the trachea or windpipe. The trachea connects the larynx with two bronchi which leads into the lungs. The bronchi are increasingly branched airways which end at the air sacs of the lungs known as alveoli where gas exchange occurs. These airways are lined with epithelial cells and mucus secreting cells which is responsible for the prevention of respiratory tract and infections. The cilia sweep the mucus toward the throat to expel pathogens.
Each lung is protected by a rib cage and a bilayer pleural membrane which separates the lungs from the body and two lungs from each other. Because of the lungs elasticity it allows and increase in surface area and thus gas exchange between the internal and external environment is further enhance. The bronchial tree inside the lungs is connected to the alveoli which allow gas exchange with blood through the pulmonary capillary (Jaryszak et al. 2000; Starr, Evers and Starr, 2008).
In the medulla oblongata, the respiratory center releases nerve impulses to the diaphragm and the intercostal muscles of the ribcage which causes the contraction of the diaphragm and the up and down movement of the rib cage.
When thoracic cavity increases in size, the air pressure in the expanded lungs decreases but this is eventually balanced by the air coming from the nostrils. When the respiratory center ceases to send stimulants to the diaphragm the normal relaxation state is assumed and exhalation occurs (Body Systems and Homeostasis, 1997).
Body Systems and Homeostasis. (1997). McGraw-Hill College Division. Retrieved from: http://www.mhhe.com/biosci/genbio/maderbiology/supp/homeo.html
Jaryszak, E. M., Baumgartner Jr., W. A., Peterson, A. J., Presson Jr., R. G., Glenny, R. W. and Wagner Jr., W. W. (2000) Selected Contribution: Measuring the Response Time of Pulmonary Capillary Recruitment to Sudden Flow Changes. Journal of Applied Physiology, 89:1233–1238.
Starr, C., Evers, C. A. and Starr, L. (2008). Biology: Concepts and Applications. Belmont CA: Thomson Brooks/Cole.