178 



ANIMAL BIOLOGY 



blood inducing similar changes in the capillaries that penetrate the 

 tissues. Since the total volume of blood in the body is practically 

 constant, an extra supply to one part necessitates a slightly reduced 

 supply to another. So it happens, for instance, that after a hearty 

 meal more blood is sent to points where digestion is going on, 

 leaving less for other organs — the reduced supply to the brain 

 probably resulting in the proverbial drowsiness at such times. 



Moreover, of course, the blood itself undergoes various changes 

 during its course through the body as it receives or delivers food 

 substances, secretions, and excretions. And furthermore, for in- 





>X< 



V^ 



Oxygen 

 (Lungs) 



Removal Station 

 Carbon Dioxide 



Renewal Station 

 — Hormones 

 (Endocrine Glands) 



Renewal Station 

 Food 



Exchange Station 

 (Blood and Tissue Fluid) 



Removal Station 

 Waste Products 



f 



Pump to Keep 

 Blood in Motion 

 (Heart) 



Removal Station 

 Excess Heat 



s 



s 



/ 



(Digestive Tract) (Kidneys) (Skin) 



Fig. 126. — Diagram illustrating how a suitable environment is maintained 



by the flow of blood. (From Martin.) 



stance, its supply of red blood cells is replenished from the bone 

 marrow and stabilized, in part, by a vascular organ, the spleen, 

 which is situated in the abdomen behind the stomach. (Figs. 106- 

 108, 114.) 



The elaborate mechanism in homothermal animals (Birds and 

 Mammals) that maintains a practically constant body tempera- 

 ture is largely dependent upon heat distribution, heat loss, and 

 heat conservation by the blood vascular system. At rest, a gentle 



