CHAPTER VI 

 CIRCULATION AND RESPIRATION 



A circulatory system consists of a fluid, blood or lymph, driven 

 by the contractions of one or more hearts to all parts of the body, 

 usually through tubular blood vessels. If blood vessels are 

 absent or poorly developed, as in insects, the blood shortly 

 after leaving the heart flows through the tissues unconfined by 

 vessels. The heart is a highly specialized region of a blood vessel. 

 In the frog there is a single heart for moving the blood and a 

 number of lymph hearts which maintain a flow of lymph from the 

 tissues to the veins of the blood system. Lymph hearts are 

 absent in mammals. The circulatory system serves to transport 

 to the tissues nutritive substances, absorbed from the alimentary 

 canal, and oxygen, absorbed from air or water by the respiratory 

 organs. The blood collects from the tissue products of metab- 

 olism such as carbon dioxide, water, and various other substances, 

 which are eventually eliminated. In warm-blooded animals 

 the blood is important in controlling and regulating body temper- 

 ature. The importance of the circulatory system in connection 

 with these functions is not the same for all animals. Thus in 

 insects, air containing oxygen is carried to blood-bathed tissues 

 and carbon dioxide from them, by special tubes that function as 

 respiratory organs (p. 426), the blood forming only a short link 

 in the final phase of oxygen transport and the initial phase of 

 carbon dioxide transport. Blood is a fluid tissue composed of a 

 liquid plasma in which cells or cell-like bodies called corpuscles 

 are suspended. Lymph has a somewhat similar composition. 



Blood Vessels of Vertebrates. — Arteries are vessels that carry 

 blood away from the heart. Veins carry blood toward the heart. 

 The small arteries or arterioles terminate in still smaller vessels 

 known as capillaries, which soon reunite to form veins. The 

 arterial and venous vessels are connected centrally by the heart 

 and peripherally by the capillaries, the whole providing a cir- 

 cular pathway through which the blood moves. Sometimes a 



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