EXCHANGE BETWEEN BLOOD AND ALVEOLAR AIR 



341 



It is likewise to be assumed that the absorption of oxygen into the blood is the 

 result of a difference in oxygen tension between alveolar air and venous blood. 



The method of determining the tension of a gas in a liquid has been given 

 above (page 335). For the measurement of gas tension in the blood, Pfliiger 

 let the blood flow in a fine jet directly from the open vessel through a tube 

 charged with a mixture of gases of known composition, and afterwards analyzed 

 the gas. By this method the blood is but a short time in exchange with the 

 mixture of gases, and on this account a complete equalization of tension differ- 

 ences is not insured. 



For the purpose of obtaining pure alveolar air for analysis, Pfliiger con- 

 structed a special instrument, the lung catheter (Fig. 133). This consists of two 

 tubes, one inclosed within the 

 other. The outer tube, made 

 of hard rubber, communi- 

 cates with a soft rubber bulb 

 (a), the thin-walled end of 

 which can be inflated by 

 means of the air pump (&) 

 after it is introduced into 

 the bronchus, so as to close 

 hermetically the bronchial 

 opening. The inner tube (d), 

 an ordinary elastic catheter, 

 places the confined lung space 

 in connection with a suitable FIG. 133. The lung catheter, Ludwig's construction, 



tube (c) filled with mercury. 



After the air has been confined for the desired length of time, it can be drawn 

 into this tube c by allowing the mercury to run out. 



From determinations carried out by this method, chiefly in Pfliiger's 

 laboratory, the tension of carbon dioxide in the arterial blood has been found 

 to be 2.8 atmospheres, and in the venous blood 3.8 to 5.4 atmospheres; that 

 of oxygen in the arterial blood at most fifteen per cent of one atmosphere. 

 Since the partial pressure of carbon dioxide in the alveolar air proved to be 

 less, and that of oxygen greater, than the tensions of these gases in the arterial 

 blood, evidence was found for the conception that the respiratory exchange 

 takes place by a simple equalization of tensions. 



Bohr has entered the lists decidedly opposed to this view. By a special 

 method he determined the tension of the gases in flowing blood, and analyzed 

 the expired air at the same time. He found that the tension of carbon dioxide 

 in the arterial blood may be lower than the partial pressure of carbon diox- 

 ide in the air which passes the bifurcation of the trachea ; also that the tension 

 of oxygen in the arterial blood may be greater than the partial pressure of 

 oxygen in the same air. Other factors than the tension differences therefore 

 must be concerned in the respiratory exchange. Bohr lays special stress upon 

 the activity of the alveolar wall, which is said to secrete carbon dioxide and 

 actively absorb oxygen. 



In general it may be assumed that the total amount of carbon dioxide given 

 off in the lungs comes to the lesser circulation from the veins of the greater 

 circulation. However, the opinion was long ago expressed by Lavoisier in his 



