RESPIRATION 



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the blood, the mixed venous blood of the body — viz., that found 

 in the right ventricle of the heart — should be examined. An 

 analysis of this, from the dog, compared with arterial blood, 

 is given in the following table : » 



The mean and the extremes are shown in the above table, 

 which gives a good idea of the- variations in the amount of gases 

 met with, especially in venous blood ; and the same doubtless 

 holds good for all animals. Both nitrogen and argon are found 

 in blood. It is believed they are in simple solution and of no 

 physiological value. 



Oxygen in the Blood. — If the oxygen in blood were held in 

 simple solution, as it is in water, and the blood introduced into 

 a mercury pump, with every reduction in pressure in the pump 

 oxygen would be given off from the blood. But in effect this is 

 not so. It is not until the pressure in the pump stands at one- 

 third of an atmosphere that oxygen begins to escape, and when 

 the pressure is reduced to one-sixth of an atmosphere, the oxygen 

 is suddenly and vigorously liberated. It is evident that its union 

 with something has been suddenly broken, and this something is 

 haemoglobin. Further evidence that the oxygen is not simply 

 absorbed by blood is afforded by the experiment of shaking 

 air and blood, and oxygen and blood, together. The blood 

 absorbs more oxygen from the air than an equal bulk of water 

 would, while it absorbs no more from an atmosphere of pure 

 oxygen than it does from atmospheric air. The explanation is 

 that the oxygen and haemoglobin form a definite chemical 

 compound. 



The oxygen in blood is carried in two forms : a very small 

 quantity, amounting to 0*65 volumes per cent., is simply 

 dissolved in the plasma ; the other and larger portion is the 

 oxygen chemically combined with haemoglobin, which may be 

 conveniently taken at 20 volumes per cent. Oxyhaemoglobin 

 possesses the property of giving off its oxygen when the pressure 

 in the surrounding medium falls sufficiently low. We have 

 already seen this occur in the blood-pump. 



