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Abstract This article, which is the last in the life sciences series and the second of two articles on the respiratory system, describes gaseous exchange in the lungs, transport of oxygen and carbon dioxide, and internal and external respiration. The article concludes with a brief consideration of two conditions that affect gas exchange and transport: carbon monoxide poisoning and chronic obstructive pulmonary disease.
Keywords
Gaseous exchange, respiration, respiratory system and disorders
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RESPIRATION HAS THREE main stages: ventilation, and external and internal respiration. Ventilation is the movement of air from the atmosphere into the lungs, as described in part one of this article. Once air has entered the alveoli, oxygen and carbon dioxide are exchanged between the alveoli and blood in the pulmonary capillaries. This process is known as external or pulmonary respiration. The gases then circulate in the blood reaching the tissues, where oxygen and carbon dioxide are exchanged between the capillaries and tissue cells. This process is known as internal or tissue respiration (Tortora and Derrickson 2009).
External respiration
The movement of oxygen from the alveoli into the blood via the alveolar capillaries, and the movement of carbon dioxide from the blood into the alveoli (Figure 1), is known as external respiration. The blood in the alveolar capillaries is oxygenated during external respiration. The movement of gases is dependent on gas concentration (Gould and Dyer 2011).
Air contains several gases, including nitrogen, oxygen, carbon dioxide, argon, neon, helium, methane and hydrogen. Each gas exerts its own pressure, referred to as the partial pressure (Jenkins and Tortora 2013). Regardless of where these gases are, collectively they make up the total pressure of the mixture, which can be calculated by adding the partial pressures of each gas, for example to calculate atmospheric pressure (Jenkins and Tortora 2013). Clinically, the partial pressure of a gas can be expressed in millimetres of mercury (mmHg), although the term kilopascal (kPa) is usually used for respiratory gases. The pascal (Pa) is an internationally recognised measure of pressure (Clancy and...