492 



EESPIRATION 



ingly small quantity of blood until an equilibrium has resulted, which in this 

 case requires a much shorter time than by any of the procedures mentioned pre- 

 viously. The apparatus itself consists of a tonometer and a tubular receptacle 

 for the analysis of the gas bubble. The latter is first played upon by a small jet 

 of blood led in by a narrow cannula, its size being then measured by drawing 

 it into a graduate. The absorption of carbon dioxid and oxygen is carried out 

 in the usual manner by using potash and pyrogallic acid. 



Another method frequently employed for the determination of the tension of 

 the gases in the venous blood of the lungs requires the use of a pulmonary catheter, l 

 which consists of two tubes, one being situated within the 

 other. The outer tube is somewhat shorter than the inner, 

 and is closed by a rubber balloon which after the insertion 

 of the ^catheter in the bronchus, is inflated until it com- 

 pletely blocks the respiratory passage. Samples of air 

 are then withdrawn through the inner tube at intervals, 

 until the diffusion of the gases between the alveoli and the 

 blood has continued long enough to establish an equili- 

 brium. Haldane and Smith 2 have estimated the oxygen 

 tension in the arterial blood in the following manner: 

 The subject is permitted to breathe known quantities of 

 carbon monoxid until the hemoglobin has combined with 

 as much of this gas as it will acquire. The percentage 

 amount 'of this gas in the hemoglobin is then ascertained 

 in a sample of blood taken either from the finger or from 

 the lobule of the ear. Eventually, when the absorption 

 of carbon monoxid has ceased, its tension in the aerated 

 blood of the lungs will be the same as that in the inspired 

 air. The latter value, as well as the extent to which the 

 hemoglobin has been saturated with carbon monoxid, 

 being known, the tension of the oxygen in the blood leaving 

 the lungs is also known. 



While the values obtained with these methods 

 show considerable fluctuations, it may safely be 

 concluded that the tension of the gases in the 

 arterial blood closely coincides with that of the 

 corresponding gases in the alveolar air. To be 

 exact, the carbon dioxid of the alveoli is always 

 under a slightly lower pressure than that of the 

 blood, while the oxygen is under a slightly higher 

 pressure. In the latter case, the difference amounts 

 to 1-4 per cent, of an atmosphere; moreover, it has 

 been shown by Krogh to persist even if the com- 

 position of the alveolar air is altered artificially. 

 That is to say, while any change in the tension of 

 the constituents of the alveolar air is immediately followed by a cor- 

 responding alteration in the tension of the gases in the blood, the 

 oxygen pressure is always greater in the alveoli than in the blood, 

 whereas the carbon dioxid tension is higher in the blood than in the 

 alveoli. 



Much greater differences have been ascertained in the venous 



1 Loewy and Schrotter, Zeitschr. fur exp. Pathol. und Therapie, i, 1905, 197. 



2 Jour, of Physiol., xxii, 1897, 231. 



FIG. 254. DIA- 

 GRAM ILLUSTRATING THE 

 DIFFUSION OF THE 

 GASES BETWEEN THE 

 TIDAL Am AND THE 

 BLOOD. 



T, trachea; TA, 

 tidal air; B, bronchi; 

 J, inf undibulum ; C, 

 capillaries; O, oxygen 

 atoms; CO2, molecules 

 of carbon dioxid. 



