434 



LECTURE XVIII. 



the other lung is increased. 1 The carbon dioxide elimination is similarly 

 affected. 



Stimulation of the vagus leads to the opposite effect. It is not so easy 

 to decide how much these results are due to an influence upon the lungs, 

 and how much is due to the influence of the vasomotor fibres. Apparently 

 the latter is not sufficient to account for the whole phenomenon. The 

 influence of the vagus nerve has also been observed with mammals. Stim- 

 ulation of this nerve tends to make the respiratory quotient approach the 

 value 1. 



Now if we consider once more the gas-exchange in the lungs, we see that 

 two processes are taking place side by side. Oxygen diffuses from the 

 alveolar air, which is relatively rich in this gas, and saturates the venous 

 blood with this element that is so important for the whole metabolism. 

 Simultaneously, the blood laden with carbon dioxide gives up the latter 

 to the alveolar air, which contains relatively less of it, and this takes place 

 until the partial pressure of the carbon dioxide in the alveoli is equal to 

 that of the blood, i.e., until equilibrium has been established. Now begins, 

 without doubt, the activity of the epithelium of the lungs by means of 

 which oxygen from the alveolar air is secreted in the blood-vessels, and has 

 the effect of overbalancing the equilibrium between the oxygen tension of 

 the blood and that of the alveolar air, in favor of the former. The carbon 

 dioxide is eliminated with equal avidity, and given up to the alveolar air. 



Oxygen now circulates anew with the blood to the tissues whose oxygen 

 content is relatively less than that of the blood, so that oxygen is con- 

 stantly diffusing from the blood, and first of all the oxygen is lost, which 

 is merely dissolved in the blood. When this dissolved oxygen is lost, the 

 reserve supply, i.e., that combined with the oxyhemoglobin, comes into 

 play. The oxyhemoglobin now dissociates and oxygen is given up to the 

 plasma, in order to keep the oxygen tension of the blood up to a cer- 

 tain value. Now the question arises whether this internal respiration 

 takes place in accordance with the gas diffusion laws, or whether we must 

 assume that here also a secretion, i.e., an active giving up of oxygen, 



1 V. Maar: Skand. Arch. Physiol. 13, 269 (1902). 



