4 CIRCULATION OF THE BLOOD 



from the arteries a bright red colour. This colour 

 changes while in the capillaries and the blood that leaves 

 the capillaries and enters the veins is of a dark red colour. 

 The presence of bright red blood in a vessel is therefore 

 an indication that the blood is on its way from the heart 

 and that the vessel is an artery. The presence of dark 

 red blood in a vessel, on the other hand, is an indication 

 that the blood is on its way to the heart and that the 

 vessel is a vein. This fact gives rise to a simple rule for 

 stopping the bleeding of wounds. If the bleeding is 

 bright red it can be stopped by applying pressure above 

 the wound, i.e. on the heart side. If the bleeding is 

 dark red it can be stopped by applying pressure on the 

 side of the wound away from the heart. 



There is one organ in which the relation that we have 

 described between bright red blood and dark red blood 

 is reversed. That organ is the lung. The blood which 

 goes from the heart to the lung is dark red, and the 

 blood which comes from the lung to the heart is light 

 red. We shall speak of the movement of the blood in 

 the lung later. 



Let us recapitulate. If we were to follow any particle 

 of blood on its journey from the heart, we should find 

 that the arteries through which it passes become smaller 

 and smaller until the capillaries are reached. Then after 

 passing through the capillary network, our particle of 

 blood enters a vein from which it passes into larger and 

 larger veins, until it finally reaches the heart again. 



The heart is really a hollow muscle which acts like a 

 pump. By contracting, as we have seen, it forces blood 

 from its cavities into the arteries. The heart contains 

 several cavities, but the cavity to which our particle of 

 blood has returned is not, in fact, the same as that from 

 which it started. To reach the cavity from which it 

 started, the particle has to make yet another journey. 



