88 INTRODUCTION TO GENERAL PHYSIOLOGY 



it becomes crimson as the oxygen is given up to the active cells, 

 which use it for combustion purposes (E., p. 201). 



A fact rather difficult to reconcile with chemical combination is 

 that oxygen is not the only gas or vapour taken up by haemo- 

 globin. Carbon monoxide, nitric oxide, carbon dioxide, and chloro- 

 form are absorbed, apparently in a similar way to oxygen, although 

 in different numerical proportions. 



The taking up of carbon dioxide by haemoglobin leads, naturally, 

 to the question of the carriage of this gas in the blood, so as to 

 remove it from the tissues, where it is being continually produced 

 by oxidation of carbon compounds. As the arterial blood reaches 

 the tissues, oxy-haemoglobin gives up part of its oxygen, since the 

 tension of oxygen in the cells is low. The tension of carbon 

 dioxide in the cells becoming higher than in the blood, owing to 

 the above-mentioned combustion process, it passes into the blood, 

 and is taken up by the haemoglobin. Experiments show that 

 carbon- dioxide drives off a part of the oxygen from the 

 haemoglobin. Or, put in another way, under a given reduced 

 tension of oxygen, less is held by the haemoglobin in the presence 

 of carbon dioxide than in its absence. The advantage of this is 

 clear, since more oxygen is set free for use. We are reminded of 

 a similar state of affairs in the case of enzymes, where one sub- 

 stance can drive another out of adsorption on the surface. 



The Lungs 



The blood returning to the heart by way of the veins contains 

 then less oxygen and more carbon dioxide than when it arrives at 

 the tissues. How does it replenish its oxygen and get rid of the 

 excess carbon dioxide? It is sent by the heart to the lungs for 

 this purpose. These organs consist essentially pf an elaborate 

 system of little bags full of air, on the walls of which there is a fine 

 network of minute blood vessels (capillaries). The blood in these 

 vessels is separated from the air by a very thin membrane, so that 

 the haemoglobin is quickly exposed to a tension of oxygen high 

 enough to saturate it, and to one of carbon dioxide low enough to 

 remove a great part of the carbon dioxide from it. But there must 

 clearly be some means of renewing the air in the lungs. This is 

 done by alternately expanding and contracting the cavity of the 

 chest, in which the lungs are contained, a process known as breath- 

 ing. There are muscles which raise the ribs from an oblique to a 

 more horizontal position, and there is a muscular partition between 

 the chest and the abdomen, the diaphragm, which at rest is in the 

 form of a dome projecting into the chest. When it contracts, the 

 top is pulled towards the abdomen, since the lower edge is fixed 



