THE CHEMISTRY OF RESPIRATION 



1175 



ALVEOLAR CO 2 TENSIONS. 



We can thus speak of an average composition of alveolar air which, 

 in spite of the constant ventilation, differs from the external air in 

 containing an excess of carbon dioxide and a relative lack of oxygen. 

 Lavoisier, who was the first to study the chemical changes in respira- 

 tion accurately, regarded the lungs as the seat of the formation of 

 carbon dioxide and the consumption of oxygen. This view was 

 generally accepted until it was shown by Magnus, in Heidenhain's 

 laboratory, that the blood passing to the lungs contained more carbonic 

 acid gas and less oxygen than that passing away from the lungs. The 

 effects of this discovery were to transfer the chief seat of oxidation 

 to the tissues of the body, and to show that the blood acts simply as 

 a carrier of the oxygen from the lungs to the tissues, and of the carbon 

 dioxide from the tissues to the lungs. We thus learnt to distinguish 

 between external and internal respiratory processes. A consideration 

 of the chemical mechanisms involved in the process of external respira- 

 tion includes therefore an investigation of the manner in which gases 

 are held by the blood and of the factors which are responsible for the 

 transfer of oxygen and carbon dioxide from blood to alveolar air, and 

 from alveolar air to blood. 



If blood be exposed to a Torricellian vacuum at the ordinary 

 temperature, the whole of its contained gases is given off. For the 

 purpose of extracting the blood gases a great variety of pumps have 

 been devised. In every case a glass vessel is evacuated by means of 

 the mercury pump, and is then put into connection with a reservoir 

 containing blood which has been defibrinated, or has been prevented 

 from clotting by the addition of oxalate or citrate. In all these pumps 

 the main difficulty arises in the exclusion of atmospheric air, and it 

 is therefore important to dispense so far as possible with taps. One 

 of the best modifications of the Topler mercury pump is that employed 

 by Barcroft (Fig. 491), which differs little from the pump devised by 

 Bohr. 



The construction of the pump is shown in the diagram. The actual pump 

 consists of the parts A, B, c, D. The bulb B is prolonged below by a wide tube 

 dipping into the mercury in the Woulf bottle A. The upper part of the bottle 

 is filled with water and connected by two taps at w with the water-supply 

 and with a sink. The water being turned on, mercury is forced up into B ; as it 



