Chap. 12 



CIRCULATION AND TRANSPORTATION- — BODY FLUIDS 



217 



Exchange station 

 (Blood and tissue fluid) 



Pump to 

 keep lilood 

 in motion 



(Heart) 



station 



carbon dioxide 



Renewal 



station 



hormones 



(Endocrine glands) 



Renewal station Removal station Removal station 

 Food Waste -f products Excess f heat 



\ 



\ 



/ 



Digestive tract 



Kidneys 



Skin 



Fig. 12.16. Diagram illustrating how a suitable environment within the body 

 is maintained by the circulation of the blood. (Courtesy, Woodruff and Baitsell: 

 Foundations of Biology, ed. 7. New York, The Macmillan Co., 1950.) 



well demonstrated as by experiments with isotopes made at the Carnegie In- 

 stitution of Washington and reported by Dr. G. W. Corner as follows: 



The use of substances such as heavy water and radio-active salts, differs little 

 from ordinary water and salts in their physiological activities but are easily identi- 

 fied as they travel through the body by their weight or radioactivity respectively. 

 In man, 78 per cent of the blood-plasma sodium and 105 per cent of the plasma 

 water is exchanged per minute with extravascular sodium and water. An amount 

 of water equal to a man's entire weight passes out of his blood capillaries, and is 

 replaced by an approximately equal amount, every 20 minutes. The capillary part 

 of human blood circulation, seen in the light of these facts, is a system of fine 

 tubules with permeable walls through which floods of water bearing salts and other 

 metabolic substances are pouring at every moment throughout life.* 



Control of the Heartbeat. The heartbeat is under two nervous directives: a 

 control by the neuromuscular mechanism and a control by the central nervous 

 system. The neuromuscular control is the one that may act for some time after 

 the heart of a frog or a mammal is completely separated from the body. Thus 

 the neuromuscular control can act without the central nervous control, but 

 the latter cannot act without the neuromuscular control. 



Neuromuscular Mechanism. Figure 12.19 shows the important features 

 of the mechanism. The sinuauricular and auriculoventricular nodes are net- 

 works of atypical muscle cells (Purkinje cells), just visible to the naked eye, 



" From Annual Report of the Director of the Department of Embryology. Carnegie 

 Institution of Washington, 1948-49, p. 129. 



