2.^0 Tlir INTI.RNAL r.NVIRONMINT OF Mir BODY Part III 



blood passes from the general circulation into the sinus venosus, thence 

 into the auricle, and on into the muscular ventricle which forces it forward 

 via the bulbus arteriosus into the ventral aorta, and then into four pairs of 

 afferent branchial (or gill) arteries. Branches from each of these enter the 

 gill filaments where they divide still further into capillaries (Fig. 13.6). 

 These are the scene of the exchange of gases between the water and blood. 

 The blood comes to the gills with its oxygen low and its carbon dioxide 

 high; it leaves the gills by the efferent branchial arteries with these qualities 

 reversed. It flows over the body, entering the great dorsal aorta first, then 

 goes through many branches distributing oxygen and receiving carbon 

 dioxide. Finally it reaches the heart and again takes the direct route to the 

 gills. 



It is important to note that the mouth (pharynx) of fishes is a single road 



B 



Fig. 13.5. Respiratory organs of vertebrates. Left, Larva of spotted salamander 

 (Amblystoma) with blood gills. /, gills; 2, fin; 3, balancers; 4, legs. Right, Blood 

 gills of salamander larvae showing responses to differences in the amounts of dis- 

 solved oxygen in the water: A, after living in water poor in oxygen; B, control 

 animal, after an equal time in water rich in oxygen. (After Drastich. Courtesy, 

 Krogh: The Comparative Physiology of Respiratory Mechanisms. Philadelphia, 

 University of Pennsylvania Press, 1941.) 



for breathing and swallowing. This is true from frogs to man except that the 

 air route from the nose crosses the food route from the mouth (Fig. 13.8). 

 The crossing is awkward. Crumbs go down the windpipe when it is not quickly 

 covered. This happens often enough to give everybody experiences in that 

 variety of choking. 



Lungs. More difficulties are involved in absorbing oxygen from air than 

 from water. Since living organisms are largely composed of water their thin 

 membranes are soon dried and useless if exposed to air. Except for this, air 

 breathing has great advantages, because air is richer in oxygen than water, 

 holding about 20 times more. This is a boon for greater activity and a higher 

 rate of metabolism, expressed especially by birds and mammals. 



Lungs are the tools by which air-breathers have so successfully tapped the 

 oxygen supply. They have progressed toward greater efficiency by increase of 

 area, by greater diffusion of gases, and by efficient ventilation of the cavity of 

 the sac. The increased diffusion area has come with the enlargement of the 



