BOOK II 



CHAPTER IX 

 THE BLOOD 



ALL the cells of a multicellular organism are engaged in building-up 

 (anabolic) and breaking-down (katabolic) processes. In the unicellular 

 these processes are performed by a direct interchange between the 

 single cell and the surrounding medium, but in he higher organiza- 

 tions special fluid tissues the blood and the lymph have been 

 evolved, which circulate in order to supply the requisite conditions 

 for these metabolic processes. The functions of the blood may be 

 summarized as follows: 



1. To act as the medium of absorption and exchange between 

 the alimentary canal, the lungs, and the tissues, and to supply the 

 sources of energy (food, oxygen) to the tissues for metabolic 

 purposes. 



2. To supply to the tissues a medium consisting of water, colloids 

 and electrolytes (inorganic salts) of a concentration and osmotic 

 pressure kept constant within narrow limits, in which they are able 

 to carry out their vital processes. 



3. To convey from one organ to another the internal secretions 

 and the hormones (chemical messengers), which regulate the activity 

 of the organs. 



4. To convey the products of katabolism from the tissues to the 

 organs of excretion, the lungs and kidneys. 



5. To endow the body with a mechanism protective against harmful 

 micro-organisms. 



6. In the higher animals to distribute and help to regulate the 

 heat of the body. 



7. By its power of clotting to seal up wounds and prevent serious 

 loss of blood or tissue lymph. 



The blood is a thick, viscous liquid, with a saltish taste and a 

 peculiar faint odour. When viewed under the microscope it is seen 

 to consist of a transparent liquid, the blood-plasma, in which float a 

 number of formed bodies, the red and pale corpuscles of the blood. 

 According to some authorities, there is a third solid component the 

 blood-platelets. 



75 



