MOVEMENT OF BLOOD IN CAPILLARIES. 337 



the other side of the network, the blood which has become venous 

 whilst traversing it. But if the blood which enters the capillaries have 

 no such affinity, no such motor powjer can be developed. On the other 

 hand, in the capillaries of the lungs, the opposite affinities prevail. The 

 venous blood and the air in the pulmonary cells have a mutual attrac- 

 tion, which is satisfied by the exchange of oxygen and carbonic acid 

 that takes place through the walls of the capillaries ; and when the 

 blood has become arterialized, it no longer has any attraction for the 

 air. Upon the very same principle, therefore, the venous blood will 

 drive the arterial before it, in the pulmonary capillaries, whilst respira- 

 tion is properly going on ; but if the supply of oxygen be interrupted, 

 so that the blood is no longer aerated, no change in the affinities takes 

 place whilst it traverses the capillary network ; the blood, continuing 

 venous, still retains its need of a change and its attraction for the walls 

 of the capillaries ; and its egress into the pulmonary veins is thus 

 resisted, rather than aided, by the force generated in the lungs. 



599. The change in the condition of the blood, in regard to the 

 relative proportions of its oxygen and carbonic acid, is the only one to 

 which the Pulmonary circulation is subservient ; but in the Systemic 

 circulation, the changes are of a much more complex nature, every 

 distinct organ attracting to itself the peculiar substances which it 

 requires as the materials of its own nutrition, and the nature of the 

 affinities thus generated being consequently different in each case. But 

 the same law holds good in all instances. Thus the blood conveyed to 

 the liver by the portal vein, contains the materials at the expense of 

 which the bile-secreting cells are developed ; consequently the tissue of 

 the liver which is principally made up of these cells, possesses a certain 

 degree of affinity or attraction for blood containing these materials ; and 

 this is diminished, so soon as they have been drawn from it into the cells 

 around. Consequently the blood of the portal vein will drive before it, 

 into the hepatic vein, the blood which has traversed the capillaries of 

 the portal system, and which has given up, in doing so, the elements of 

 bile to the solid tissues of the liver. The same principle holds good in 

 every other case. 



600. We are now prepared, therefore, to understand the general 

 principle, that the rapidity of the circulation of a part will depend in 

 great measure upon the activity of the functional changes taking place 

 in it, the heart's action, and the state of the general circulation, 

 remaining the same. When, by the heightened vitality, or the unusual 

 exercise, of a part, the changes which the blood naturally undergoes in 

 it are increased in amount, the affinities which draw the arterial blood 

 into the capillaries are stronger, and more speedily satisfied, and the 

 venous blood is therefore driven out with increased energy. Thus a 

 larger quantity of blood will pass through the capillaries of the part in 

 a given time, without any enlargement of their calibre, and even though 

 it be somewhat diminished ; but the size of the arteries by which it is 

 supplied soon undergoes an increase, which adapts it to supply the 

 increased demand. Any circumstance, then, which increases the func- 



*11 



tional energy of a part, or stimulates it to increased nutrition^ wilJ 

 occasion an increase in the supply of blood, altogether irrespectively 



