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The reduction of pressure of oxygen is caused by the addition of water vapour (humidification) to the entire mixture of gases, thus reducing the pressure of the other gases. Therefore, oxygen partial pressure within the upper airway is noted inspired PO 2 (PiO 2). Once air is warmed and humidified in the nose and upper respiratory tract, the pressure of oxygen decreases while concentration of H 2O increases, thus altering effective PO 2 in this gas mixture. Īlveolar partial pressure of oxygen (PAO 2) This changes are normally local, consequently, short-term temporal (time scale of minutes, hours, days and weeks) variations in BP in a same location usually range around 5-15 mmHg. At sea level BP is about 760 mmHg, although can be affected not only by altitude: latitude, humidity, temperature and even the season of the year may also affect BP. Geographical altitude is an important factor affecting BP, because as altitude increases, the amount of gas molecules in the air decreases, so the air becomes less dense than at sea level. Thus, the partial pressure of oxygen (PO 2) depends mainly on the atmosphere’s barometric pressure (BP) and its fractional concentration. ĭalton’s law establishes that within a combination of any given gases, the total pressure is the same as the sum of the partial pressures of each individual gas present in that mixture. The composition of gases within the troposphere is constant at approximately the following ratio: 78.08% nitrogen, 20.95% oxygen, 0.93% argon and finally less than 0.038% for carbon dioxide and other gases. The amount of dissolved oxygen within the tissues and the cells depends on several factors including: barometric pressure (BP), climatological conditions (temperature, relative humidity, latitude, altitude), as well as physiological, pathological, and physical-chemical processes within the organism itself. To maintain homeostasis, the amount of oxygen within the tissues should respond to a gradient of pressure that pushes oxygen by diffusion throughout the membranes into the tissues. During aerobic respiration the presence of oxygen in addition to pyruvate, produces adenosine triphosphate (ATP), thus yielding energy to the entire organism. The human body is a highly aerobic organism that consumes oxygen according to its metabolic demand. This review also included the most relevant measuring methods of PO 2 as well as a practical overview of its reference values in several tissues. Our objective is to offer to the reader a summarized and practical appraisal of the mechanisms related to the oxygen’s supply within the human body, including a facilitated description of the gradient of pressure from the atmosphere to the cells. Several factors such as anaesthesia, hypoxia, and stress affect the regulation of the atmospheric, alveolar, arterial, capillary and tissue partial pressure of oxygen (PO 2).
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Understanding the role of the main factors affecting oxygen availability, such as the gradient of pressure of oxygen during normal conditions, and during hypoxia is an important point. Along metazoan evolution, an exquisite control developed because although oxygen is required as the final acceptor of electron respiratory chain, an excessive level could be potentially harmful. The human body is a highly aerobic organism, in which it is necessary to match oxygen supply at tissue levels to the metabolic demands.