420 



Our Surroundings 



the veins. They are very numerous and exceedingly small tubes 

 and, as has already been stated, form the connecting link between 

 the arteries and the veins. They penetrate all the spaces between 

 the cells of the body, forming a network that surrounds them. 

 Blood plasma, containing food and oxygen, diffuses by osmosis 

 from the walls of the. capillaries into these spaces. Wastes from 

 the cells also gather in these spaces. The mixture thus formed 

 is called lymph and the spaces are called lymph spaces. From 

 this fluid the cells absorb their food and oxygen. 



The capillaries not only bring the food and oxygen, but they 

 also take up and remove some of the wastes that are in solution. 

 They are well adapted for their work by the exceedingly thin 



tissue of their walls. The food and 

 oxygen pass into the lymph, thence 

 into the cells by the process of os- 

 mosis, and the wastes pass out of 

 the cells into the lymph, thence into 

 the capillaries by the same process. 

 The wastes flow on in the larger 

 blood vessels by which they are 

 carried to the organs of excretion, 

 where they are eliminated. 



Course of the Circulation of 

 the Blood. Starting in the left ven- 

 tricle the blood in a complete circuit 

 takes the following course: left ven- 

 tricle, through the left semilunar 

 valve into the aorta, the great artery 

 leading from the heart, from which 

 it is distributed by arteries to all 

 parts of the body. It then passes 

 through the capillaries into the veins 

 and on to the right auricle of the 

 heart. From the right auricle it 



passes through the tricuspid valve to the right ventricle, then 

 through the pulmonary artery into the capillaries of the lungs, 

 from which it passes through the pulmonary veins to the left 



Kidneys 



CIRCULATION OF THE BLOOD 



Trace its course. 



