134 THE HUMAN VODY. 



CHANGES IN THE BLOOD. 



SECTION V. 1. Part of the oxygen of each breath 

 taken in, after reaching the air-cells, goes through their 

 walls, and through the walls of the capillaries outside of 

 them, into the blood. It joins itself to the red corpuscles ; 

 and they float on with it through the heart, and out into 

 the aorta, and finally into the capillaries of all parts of the 

 body: there it leaves the corpuscles, drawn by a more 

 powerful attraction through the capillary walls, to help 

 to nourish and build up the surrounding substance. 



2. The blood, as it discharges its load of oxygen from 

 the capillaries, takes up, at the same place, a new load 

 of carbonic-acid gas. The oxygen is the nourishment for 

 the tissues, the fuel for the fire; and the carbonic-acid 

 gas is like the ash, which must be removed, or else the fire 

 will be clogged. As the blood thus changes its load, a 

 marked change in its color takes place. As it conies from 

 the lungs, it is scarlet, and so continues through the heart 

 and arteries until it reaches the capillaries: there it turns 

 blue, and so continues through the veins and the heart 

 and the pulmonary artery until it reaches the lungs 

 again. 



In the capillaries of the larger, or systemic, circulation, 

 the blood turns blue. 



In the capillaries of the lesser, or pulmonary, circulation, 

 the blood turns scarlet. 



3, We know that it is the oxygen taken in by the lungs 

 that gives blood its scarlet color, and that it is the loss of 

 oxygen in the tissues that makes it turn blue. For if we 

 take some blue blood from a vein, and shake it up in the 

 air, it will turn scarlet; and the color will be the same 



