CHAPTER XXI 

 RESPIRATION 



WE have seen in the previous chapters how the essential energy changes in cells 

 are of the nature of oxidation, and we have discussed the nature of the mechanism 

 by which ordinary molecular oxygen, arriving at the cell, is rendered active in order 

 to burn up substances, not otherwise easily oxidised. 



Oxygen must, theiefore, be supplied to the cells and, in warm-blooded animals, 

 at a considerable rate. In unicellular organisms, no special mechanism is necessary, 

 but in larger organisms, it is clearly of importance that oxygen should be conveyed 

 directly to the active cells, without having to diffuse through thick layers of cells, 

 themselves consuming oxygen. At the same time, provision must be made for the 

 escape of the carbon dioxide formed in combustion. The mechanisms concerned in 

 this process are known as those of respiration. 



In the majority of organisms, oxygen is conveyed to, and carbon dioxide 

 removed from, the tissues in a state of solution of some kind in a liquid circulating 

 through a system of tubes. This liquid is the blood. In insects, there is a system of 

 ramifying tubes containing air. These are known as tracheae, and the air contained 

 in them is periodically changed by muscular movements as well as by diffusion. In 

 organisms provided with circulating blood, there is usually a means by which free 

 gaseous interchange of blood with the external medium containing oxygen, be it 

 water or air, is enabled to take place. In water animals we have gills, in land 

 animals, lungs. Arrangements are also present by which the water or air, with 

 which the interchange of gases takes place, is periodically replaced by a fresh supply. 

 This is done by the aid of muscular movements. The external surface of the 

 organism not being of a sufficiently large area for gaseous exchange, special organs 

 are developed for the purpose of affording a larger surface. The mechanisms here 

 referred to are usually known as those of external respiration, as contrasted with 

 the oxidation process in the tissues, called internal respiration. Intervening 

 between the two, we have to consider the process by which oxygen is carried in the 

 blood. The question of internal respiration is closely connected with that discussed 

 in the preceding chapter, although there are some aspects of it more appropriately 

 described here. 



THE HISTORY OF THE DISCOVERY OF OXYGEN 



That a continued supply of air is necessary to life, at all events in the higher 

 animals, was shown clearly by Robert Hook (or Hooke) (1667, p. 539) in experi- 

 ments made before the Royal Society at some of their early meetings. He showed 

 at one meeting a dog, which was kept alive, after removal of the ribs and the 

 diaphragm, by blowing air into the windpipe with bellows. The absence of 

 convulsions was noted. These made their appearance when the supply of air was 

 stopped, but were put an end to by renewing the blowing in of air. He showed 

 also that the actual mechanical movement of the lungs had nothing to do with the 

 recovery, as had been supposed, since he caused a continuous current of air to be 

 blown through, and allowed to escape by means of holes pricked in the lungs. Hook 

 himself points out that " it was not the subsiding or movelessness of the lungs that 

 was the immediate cause of death, or the stopping of the circulation of the blood 

 through the lungs, but the want of a sufficient supply of fresh air." (The italics 

 are in the original.) . 



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