128 A MANUAL OF VETERINARY PHYSIOLOGY 



been found, which on section prevent the further accumulation 

 of gas. 



In the researches of Haldane and Priestly previously alluded 

 to the alveolar air was found to contain a remarkably constant 

 percentage of carbon dioxide. This constancy is maintained by 

 the increase in alveolar ventilation, which results from the 

 increased production of carbon dioxide. The respiratory centre, 

 these observers say, is exquisitely sensitive to the slightest 

 increase in the alveolar C0 2 pressure, a rise of 0*2 per cent, of 

 an atmosphere being sufficient to double the amount of alveolar 

 ventilation during rest. It is an astonishing fact that the lungs 

 should be able to arrange for their effective ventilation. 



Internal or Tissue Respiration. — This is infinitely more obscure 

 than pulmonary respiration — in fact, the means by which the 

 tissues utilise the oxygen given to them is quite unknown. 



We have previously seen that in the passage of blood through 

 the capillaries of the tissues it loses in about one second 40 per 

 cent, of its oxygen. This loss occurs in consequence of the dis- 

 sociation of the oxyhemoglobin in the presence of tissues whose 

 oxygen tension is practically nil. The blood, when it left the pul- 

 monary capillaries, was nearly saturated with oxygen — perhaps 

 19 c.c. of oxygen for every 100 c.c. of blood, equal to a partial 

 pressure of 100 mm. of mercury. In the tissue, the oxygen being 

 built up into living substance as fast as it is supplied, its partial 

 pressure is nil. Under these circumstances, there is no difficulty 

 in accounting for the transfer of oxygen from the blood to the 

 tissues on purely physical and chemical grounds, the dissociation 

 of haemoglobin being in all probability assisted by the presence 

 of carbon dioxide in the tissues, for, as previously mentioned, it 

 is known that an increased tension of carbon dioxide in blood 

 favours the giving up of oxygen to the tissues. 



In consequence of the above changes, it is evident there is a 

 reduction in oxygen tensions from the blood to the tissues; 

 being highest in the blood-cell, it is less in the plasma, still less 

 in the vessel wall, lower in the lymph, and at its lowest in the 

 tissue elements. In fact, these various tissues deal with the 

 haemoglobin in order to obtain its oxygen, exactly as we deal 

 with it outside the body in order to obtain its gases — viz., 

 expose it to reduced pressure, as, for instance, in a mercurial 

 pump. The blood, as we have mentioned earlier, does not give 

 up all its oxygen in the tissues, probably on account of the 

 rapidity of its passage. At no period, excepting under the con- 

 dition of asphyxia, is the whole of the haemoglobin reduced. 

 There may be more oxygen returned to the lungs unused than 

 has been abstracted by the tissues. 



The whole of the carbon dioxide in blood is not got rid of in 



