648 PHYSIOLOGICAL CHEMISTRY. 



from the tissues it receives the products of catabolism, and carries 

 them to their proper organs of elimination. 



The bright red color of the arterial blood is due to oxyhaBmoglobin. 

 A large portion of this oxygen absorbed by the haemoglobin is given 

 up to the tissues as the blood passes through the capillaries, and we 

 have then the reduced hemoglobin, to which is due the dark color of 

 the venous blood. 



In almost the reverse manner, the hemoglobin takes up carbon from 

 the tissues and conveys it to the lung. It is important to note that 

 carbon dioxide, in distinction from carbon monoxide, is not attached to 

 hemoglobin in the same manner in which the oxygen is attached. It 

 has been shown that the dark color of the venous blood is not due to 

 the presence of carbon dioxide, but to a decrease of the oxygen. 



The details of the manner in which oxidation in the animal body is induced 

 and how it proceeds are not known. In some way the atmospheric oxygen, 

 which under ordinary conditions has no action on the proteins, fats, and carbo- 

 hydrates, is so changed as to become active. It is commonly believed that the 

 process is carried on by enzymes, some of which (peroxidases) have been actually 

 demonstrated, while others (oxygenases) are merely hypothetical (see page 637). 



Waste products of animal life. The changes which the food 

 suffers after having been absorbed by the animal system are ex- 

 tremely complicated, and far from being thoroughly understood. 

 Numerous products and organs are formed and nourished from and 

 by the blood ; among them muscular, nerve, and brain substance, ex- 

 cretions and secretions, 1 such as milk, saliva, bile, gastric and pan- 

 creatic juice, etc., together with bones, teeth, hair, and many others. 



Most of these substances (some secretions, such as milk and others, 

 excepted) suffer a constant oxidation in the system, and are finally 

 eliminated as waste products ; in regard to the intermediate com- 

 pounds formed in the tissues we know little, but it is highly probable 

 that the reduction of the complicated food material to the simple 

 forms of the waste products is very gradual. There are three chan- 

 nels through which the waste products are given off; they are the 

 lungs, the skin, and the kidneys. By the lungs are eliminated 

 chiefly carbon dioxide and some water, by the kidneys urine (which 

 is a weak aqueous solution of urea, uric acid, urates, phosphates, 

 chlorides, and sulphates of calcium, magnesium, sodium, potassium, 

 etc.), and by the skin are constantly eliminated carbon dioxide and 

 water, and during the process of sweating also more or less of the 

 constituents of urine. 



1 An excretion consists of material detrimental to the organisms, and removed from it by 

 certain glands. A secretion contains peculiar compounds especially elaborated by the glands 

 for the purpose of serving certain requirements of the organism or its offspring. Thus, urea 

 and sweat are excretions, while pepsin and milk are secretions. 



