Body tissues 



13 



BLOOD AND LYMPH 



Blood 



Blood is a circulating tissue that is peculiar to the higher animal 

 world. Unicellular animals and plants absorb their food from the 

 surrounding mediimi and excrete waste products into the same 

 medium. MulticelliUar organisms are not able to function in this 

 simplified manner since most of the cells are buried beyond the limits 

 of active or passive diffusion from the surrounding medium. In such 

 animals the transport of food to, and waste from, the cells is accom- 

 plished by the flow of body fluids such as blood. Although blood is 

 a living tissue, the rate of metabolism is rather small compared with 

 that of liver and other metabolically active components of the body. 



The principal functions of blood are (1) to carry nutrients from 

 the digestive tract to the body tissues, (2) to transport waste products 

 of metabolism to the excretory sites, (3) to transfer oxygen and carbon 

 dioxide between the lungs and tissues, and (4) to distribute hormones, 

 immune substances, etc., to the various parts of the body. In addition 

 to these transport and distribution functions, blood also plays impor- 

 tant roles in pH and osmotic pressure maintenance and in the dis- 

 tribution and radiation of heat. 



When blood is subjected to low-speed centrifugation, slightly more 

 than 40 volume per cent separates as solids or corpuscles. Ihe remain- 

 ing GO per cent consists of a straw-colored fluitl called plasma. The 

 corpuscular fraction consists almost wholly of red blood cells, the 

 erythrocytes. The other solid constituents, the white blood cells 



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