8 CIRCULATION OF THE BLOOD 



whence it passes to the left ventricle and from thence 

 the light red blood is forced into the aorta and systemic 

 circulation. 



These alterations of colour from light to dark and from 

 dark to light are indications of chemical changes. They 

 are due to the chemical combination of the red substance 

 of the blood with oxygen in the case of the light blood, 

 and with carbon dioxide in the case of the dark blood. 

 The nature and meaning of this change will be discussed 

 in a companion volume of this series dealing with respira- 

 tion. In this book we shall only consider the movement 

 of the blood from a mechanical point of view. 



Let us now again trace a particle of blood on its journey. 

 This time we can follow it through the entire circulation. 

 Leaving the left ventricle, when the walls of that cavity 

 contract, the particle is forced through the valves into 

 the great artery known as the aorta. From the aorta it 

 passes into smaller and ever smaller arteries, finally 

 reaching the systemic capillary network. After travelling 

 through that network it enters a vein. Thence it passes 

 into larger and ever larger veins, until it ultimately enters 

 the great vein known as the vena cava that opens into 

 the right auricle. It has now completed the systemic 

 circulation. As the right auricle contracts, our particle 

 of blood passes through the valves between the right 

 auricle and right ventricle into the right ventricle. From 

 there it passes into the great pulmonary artery, which 

 conducts it to the lung. In the lung the pulmonary 

 artery breaks up into branches and finally into capillaries. 

 Through these our particle travels until it reaches a 

 tributary of the pulmonary vein and finally the pulmonary 

 vein itself. The pulmonary vein empties its blood into 

 the left auricle, carrying our particle with it. From the 

 left auricle the particle of blood passes at last into the 

 left ventricle, from which it started. 



