The respiratory system is divided into two main components, the upper respiratory system (URS) and the lower respiratory system (LRS). The URS is composed of the nose, nasal cavity, paranasal sinuses and pharynx. The pharynx is further subdivided into the nasopharynx (superior portion), oropharynx (middle portion) and the laryngopharynx (inferior portion). The LRS is composed of the larynx, trachea, bronchial tree and lungs. The functions of the respiratory system include: (1) providing a gas exchange surface between the air and the blood, (2) moving air to and from the gas exchange surfaces, (3) participate in the regulation of blood pH, blood volume and blood pressure, (4) sound production, (5) defensive barrier against airborne pathogens, and (6) protection of gas exchange surfaces from environmental changes. Respiration refers to two integrated phases: external respiration and internal respiration. External respiration includes the processes involved in the exchange of oxygen and carbon dioxide between the blood and interstitial fluid of the body and the environment. While internal respiration consists of the utilization of oxygen and production of carbon dioxide by the cells of the body. This latter process is also called cellular respiration. It is important not to confuse breathing and respiration. The two terms are not interchangeable. Breathing or respiratory rate and depth, is controlled by the respiratory centers located in the pons and medulla of the brain stem. The respiratory centers are sensitive to the amount of oxygen in the blood, the pH of the blood and the amount of carbon dioxide in the blood. However, the respiratory centers are most sensitive to CO2; for this reason, it is the amount of CO2 that normally controls breathing. During exercise, as the demand for O2 increases, the amount of CO2 produced by cells also increases. The increase CO2 leads to an increase in the respiratory rate, which not only brings in more O2, it also eliminates the excess CO2. External respiration involves the movement of air into and out of the lungs. The process is driven by pressure differences that develop between air pressure in the environment and the pressure deep within the lungs. As the diaphragm contracts during inspiration (inhalation), the volume of the thoracic cavity increases and the pressure within the lungs falls below that of the atmosphere (negative pressure). As a result, air flows down the pressure gradient into the lungs. Once in the lungs, O2 can diffuse from the alveoli into the blood and CO2 can diffuse from the blood into the alveoli. After a brief equilibrium period, the diaphragm relaxes, returns to its original position. In doing so, the volume of the thoracic cavity decreases and the pressure within the lungs increases above atmospheric pressure (positive pressure). Air now flows down its pressure gradient out of the lungs. This process is called expiration (exhalation). As mentioned previously, the frequency of diaphragmatic contractions (rate) and degree of contraction (depth) are controlled by the respiratory centers in the brain stem. Internal respiration, sometimes called cellular respiration, is the process whereby the oxygen in the blood is utilized by the cells of the body. When blood reaches a capillary bed, the O2 in the blood will diffuse down its concentration gradient into the cells surrounding the capillary. Ultimately diffusion will carry the O2 into the mitochondria of the cell where it is utilized in the production of ATP. At the same time, CO2, a by product of internal respiration, will diffuse from the cells into the blood and be carried to the lungs for elimination.
Posted on: Fri, 30 Aug 2013 23:26:43 +0000
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