In Brief 



There is a dual circulation in plants : one part carries liquids up from the 

 roots, the other part carries food down from the leaves. 



The blood of human beings and other vertebrates consists of a colorless 

 fluid, the plasma, in which numerous red and white corpuscles float. The 

 blood, circulating in a closed system of vessels, transports oxygen, carbon 

 dioxide, food and wastes. 



The colorless lymph fills spaces between tissue masses and between cells. 

 This constitutes an internal fluid medium from which the cells of the body 

 obtain their food and oxygen and into which they discharge carbon dioxide 

 and other wastes. 



When blood vessels are injured, the interaction of special substances leads 

 to the formation of a clot; the clear liquid left by the clotting and the 

 separation of the corpuscles is the serum. 



The white blood corpuscles resemble the ameba. They wander in the 

 body fluids and engulf foreign particles or organisms that enter the body, 

 or particles of cells that have been destroyed. 



The blood is propelled through the vessels by the rhythmical contraction 

 of the heart. 



In warm-blooded animals there is a double circulation; the left ventricle 

 supplies the systemic circulation, and the right ventricle supplies the pul- 

 monary, or lung, circulation. 



The circulating blood distributes heat to the body extremities and equal- 

 izes the temperature of the whole body. 



The stability of the body fluids, or homeostasis, is maintained by imme- 

 diate and automatic compensatory responses to chemical deviations and to 

 changes in concentration and temperature. 



EXPLORATIONS AND PROJECTS 



1 To observe the beating of the heart, anesthetize a rat or guinea-pig, open 

 the ventral side, exposing the abdominal viscera, as well as the heart, lungs, and 

 vessels of the thorax, but without cutting any of them. Note the rhythmic pul- 

 sation of the arteries, which carry blood from the heart. Observe a gradual filling 

 of the auricles during the resting period. Note whether the heart begins a beat 

 at one end or contracts all at once. Describe the heartbeat. 



2 To study the structure of the heart and of the adjoining vessels, use a 

 "haslet" (lungs with heart attached, as removed from animal) from a butcher 

 shop. 



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