OXYGEN TENSION AND PARTIAL PRESSURE. 811 



and indeed even with as low a pressure as 40-30 mm., still 90-80 per cent 

 of the entire quantity of oxygen taken up chemically at 150 mm. is com- 

 bined with the hemoglobin. 



From these and other observations it follows that the oxygen partial 

 pressure may sink to one-half of that existing in the atmospheric air 

 without markedly influencing the oxygen content of the blood. This 

 also coincides with the experience of FRANKEL and GEPPERT l on the 

 action of low air pressures upon the oxygen content of the blood of dogs. 

 With an air pressure of 410 mm. Hg they found that the oxygen content 

 of arterial blood was normal. With an air pressure of 378-365 mm. 

 it was slightly diminished, and only on reducing the pressure to 300 

 mm. was a mentionable decrease observed. A. LOEWY 2 found that 

 the lowest oxygen pressure of the alveolar air wherein the exchange 

 of material can go on normally both qualitatively and quantitatively, 

 is equal to 30 mm. Hg. 



In regard to the above-mentioned action of low air pressure it must 

 be remarked that the alveolar oxygen tension is regulated by changes 

 in the respiration, so that with great diminution in the quantity of oxygen 

 of the inspired air, the alveolar air contains the same quantity of oxygen 

 as with a higher oxygen partial pressure of the inspired air (LOEWY). 

 For example, LOEWY found the same quantity of oxygen, namely, 6.1 

 per cent, in the alveolar air with 16 and w T ith 10.5 per cent oxygen in 

 the inspired air, because the respiration in the latter case was 8.5 liters 

 per minute against only 4.9 liters in the first case. 



It may be concluded from the large quantity of oxygen or oxyhaBmo- 

 globin in the arterial blood that the tension of the oxygen in the arterial 

 blood must be relatively higher. This is substantiated by the earlier 

 observations of BERT and HUFNER, as well as by the determinations 

 of HERTER, FREDERICQ and others, 3 using aerotonometric methods, 

 which will be mentioned below in connection with the carbon dioxide 

 tension. HERTER found the oxygen tension in the arterial blood of 

 dogs to be equal, on an average, to a pressure of 78.7 mm. Hg, and FRE- 

 DERICQ, by a better method, found that it was equal to 91-99 mm. Hg. 



The oxygen tension of the venous blood of dogs has been found by 

 aerotonometric means to be equal to 20.6-27.7 mm. (STRASSBURG, FAL- 

 LOISE), and by means of the lung-catheter (see below) equal to 25.5-27 

 mm. (WOLFBERG, NUSSBAUM). For human venous blood LOEWY and 



1 Ueber die Wirkungen der verdiinnten Luft auf den Organismus., Berlin, 1883. 



2 A. Loewy, Untersuch. iiber die Respiration und Zirculation, etc., Berlin, 1895; 

 also Centralbl. f. Physiol., 13, 449, and Arch. f. (Anat, u.) Physiol., 1900. 



3 Bert, La pression barometrique, Paris, 1878; Herter, Zeitschr. f. physiol. Chem., 

 3; Hiifner, 1. c.; Fr&lericq, Centralbl. f. Physiol., 7, and Travaux der laborat. de 1'inst. 

 de physiol. de Liege, 5, 1896. 



