428 LECTURE XVIII. 



blood becomes greater, and there is a more lively gas-exchange between 

 the tissues and the blood. 



We shall now attempt to trace the gas-exchange of the blood with the 

 alveolar air on the one hand, and with the tissues on the other. In the 

 lungs, two processes are continually taking place side by side. Blood 

 laden with carbon dioxide constantly reaches these organs, there to dis- 

 charge this gas and take up a fresh supply of oxygen. The dark-colored 

 venous blood is hereby changed into bright-red arterial blood, and this 

 reenters, through the veins of the lungs, the general circulation. In order 

 to understand the entire gas-exchange in the lungs, we must remember 

 that the blood-vessels of the lungs (the region where the pulmonary arteries 

 end, and the veins begin) represents an infinitely-fine, capillary network, 

 spun round the alveoli. In this way an enormous amount of surface is 

 exposed, which enables us to comprehend how, in spite of the relatively 

 quick passage of the blood through the lungs, a complete gas-exchange 

 takes place. The size of the respiratory surface has been variously esti- 

 mated. Aeby l found that the lung surface in an adult with quiet breath- 

 ing, amounted to 80 square meters. N. Zuntz, 2 assuming the alveolar 

 diameter of 0.2 millimeter, and the air volume of the lungs to be 3000 

 cubic centimeters, estimated the alveolar surface to be 90 square 

 meters. 



If we compare, first of all, the expired and inspired air, we shall find that 

 the former is poor in oxygen and rich in carbon dioxide, in comparison 

 with the latter. The outer air does not reach the alveoli in an unchanged 

 condition. It is first saturated with water vapor, and warmed to the 

 temperature of the body. It originally contains, on an average, 20.96 

 per cent oxygen, 78 per cent nitrogen, 1 per cent argon, and 0.04 per cent 

 carbon dioxide by volume. We cannot, however, apply these values 

 directly to the gas-exchange in the alveoli. For the absorption of oxygen, 

 on the one hand, and the giving up of carbon dioxide, on the other, it is 

 the composition of the alveolar air which alone comes into consideration. 

 The latter is poorer in oxygen, and richer in carbon dioxide, than the 

 expired air, which contains 16.4 per cent oxygen and 4.1 per cent carbon 

 dioxide by volume. This is because only a part of the inspired air reaches 

 the alveoli. A part of it remains unused in the air-passages, where it is 

 mixed there with the alveolar air and expired. The carbon dioxide and 

 oxygen content of the alveolar air at the moment it leaves the alveoli 

 may be computed, if we know the volume of a single inspiration, and the 

 size of the air-passages which contain the unchanged inspired air (nose, 

 pharynx, trachse, and bronchi). Such computations, it is true, are not 

 accurate, partly because this latter value is not known closely enough, and 



1 Der Bronchialbaum der Saugetiere und der Menschen, p. 90, Leipzig, 1880. 

 a In Hermann's Handbuch der Physiologic, 4, 90 (1887). 



