The process of breathing, so far as we ourselves are concerned, is so obvious that it is easy to overrate its importance. It will be seen that this process in itself is not fundamental but is merely the process leading to tissue respiration. We may define tissue respiration as the setting free of energy in a useful form from food, generally by oxidation. The phrase ’a useful form of energy ’is rather vague, but must be used for want of a better one. In the great majority of cases it is kinetic energy, I.e. work is done because parts are moved, but we may generalize by stating that any activity requires energy, whether it is to bring about secretion, excretion or a chemical change, as well as the more obvious nervous and muscular activities. The food that is used up when this process occurs is generally a simple sugar. When such a substance is oxidized, carbon dioxide and water are the principal substances produced. The process may be given in its simplest form by the equation :—

C6H1206 + 602 = 6COa + 6HaO + useful energy, but the actual process consists of a series of reactions commencing with sugar and oxygen and ending with the liberation of carbon dioxide and water.

Exhaled breath is warmer and contains more C02 and water vapour and less oxygen than inhaled air; facts which are accounted for in the above equation.

Experiment 15—To show the Presence of Carbon Dioxide in Exhaled Air

On breathing through this apparatus, air enters through one flask of lime water and leaves through the other. The lime water in the first case remains clear, while that in the other turns milky, showing the presence of carbon dioxide in the exhaled air.

To show Presence of Carbon Dioxide in Exhaled Air.

Experiment 16—To show the Amount of Oxygen absorbed by the Lungs

A certain volume of exhaled air is collected, by displacement of water in the graduated tube A and its volume read. A piece of yellow phosphorus attached to the end of a long copper wire is then inserted and pushed to the top of the tube. The phosphorus ’uses up ’the air by oxidation. When no more white smoke is seen, the phosphorus is withdrawn and the volume of the remaining ’air ’is read. The difference of the two readings gives the volume of oxygen originally in the ’breathed out ’air. If the experiment is repeated, using the same volume of ordinary air, the volume of oxygen in the ’breathed out ’air will be found to be less than that in the same volume of ordinary air.

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