210 BIOLOGY 



puscles, absorbs large quantities of oxygen as the blood is flow- 

 ing through the lungs and skin. The oxygenated blood then 

 passes from the lungs back to the heart and is pumped out 

 through arteries to the tissues. Here the red blood corpuscles 

 give up their oxygen, and at the same time the blood absorbs 

 carbon dioxid (CO 2 ) from the tissues. When the blood, there- 

 fore, leaves the capillaries on its journey back to the heart, it 

 has left behind its oxygen and taken in its place carbon dioxid, 

 which it gives up when it next reaches the lungs or the skin, 

 at the same time taking up oxygen. The process of respiration 

 is therefore a system of gas exchange. 



Metabolism. In the living tissues the food and oxygen are 

 chemically combined, an oxidation of the food taking place. 

 The chemical changes that occur are numerous and result in 

 the formation of new materials for the body, producing growth , 

 development of muscular activity, and all of the other phenom- 

 ena of life, and finally resulting in the appearance of waste 

 products. The waste products are chiefly three : (1) a gas, carbon 

 dioxid (CO 2 ) ; (2) a liquid, water (H 2 0) ; (3) a solid, called urea 

 (CON 2 H 4 ), which contains the nitrogen. Although the urea 

 is solid under all ordinary conditions, it is dissolved in the liq- 

 uids of the body, since it is soluble in water, and is therefore in a 

 state of solution while in the body. These three waste products 

 are not only valueless but distinctly harmful, and it is necessary 

 for the body to get rid of them. The series of chemical changes 

 which finally results in waste products is called metabolism. 



Excretions. The elimination of the waste products of me- 

 tabolism is known as excretion. The carbon dioxid gas passes 

 into the blood, and when the blood reaches the lungs the gas 

 diffuses from the blood into the air. The waste water also passes 

 into the blood and is passed off from the body through the kid- 

 neys, the lungs, and the skin. The urea finds its way into the 

 blood, and as the blood flows through the kidneys (Fig. 92), 

 they take the urea from it. They then pass it through their 

 ducts dissolved in the urine, and it goes to the bladder and 



