2015-05-26
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The term «respiration» means the exchange of gases (oxygen and carbon dioxide) which takes place between the living organism and the environment. One must consider that in higher organisms this exchange takes place at several different levels. An initial exchange must occur between the air in the lungs, from which the oxygen is being continually taken up and into which carbon dioxide is being continually poured, and the external air. This is the process of external respiration.
The composition of the air inside the lungs is different from that of the air which we inhale. The content of alveolar air is very constant, especially the one of carbon dioxide, the partial pressure of which is normally 40 mm of mercury. This constancy is the result of a self-regulating mechanism by which the respiratory activity is governed by the amount of carbon dioxide which has been eliminated from the organism.
The exchange of gases varies according to the size and activity of the organism. In man at rest the absorption of oxygen reaches about 0.25 litre a minute and the elimination of carbon dioxide 0.2 litre. At a time of maximum muscular activity, the consumption of oxygen and the production of carbon dioxide may both exceed 4 litres a minute.
The movement of air into the lungs is brought about by an increase in the volume of the thoracic cavity with the action of the respiratory muscles. The lungs follow this movement passively. Some of the in-spiratory muscles have a fixed point on the ribs; when the ribs are being raised the muscles increase the anteroposterior and transverse diameters of the thoracic cavity (costal respiration). Another important muscle is the diaphragm, a thin dome-shaped «sheet», which closes the lower part of the thorax and separates it from the abdomen. The diaphragm contracts and flattens; it contributes in this way to the extension of the vertical diameter of the thoracic cavity and raises the ribs (abdominal respiration). At the time of expiration, the thorax returns to its initial
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82 ♦ Learning to Understand a Medical Text
position, and air is expelled through the same tracts that had been used by fresh air during inspiration.
In an individual at rest the number of inspirations per minute is 10 to 15; the pulmonary ventilation, or the volume of air which passes through the respiratory system each minute, is about 6 litres per minute. During intense muscular activity the inspiration rate may rise to 50 and the ventilation to 150 litres or more per minute.
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1. The term «respiration» means the exchange of gases. 2. An initial exchange between the air in the lungs and the external air is called external respiration. 3. The exchange of gases varies according to the size and activity of the organism. 4. The content of alveolar air is very constant. 5. The diaphragm contracts and flattens.
