DIGESTION AND RESPIRATION 529 



for the various cellular activities. To maintain it, oxygen must be 

 continuously supplied and the by-products, carbon dioxide and water, 

 must be continuously removed. In vertebrates this involves the uptake 

 of oxygen and the release of carbon dioxide in the respiratory organ, 

 the transportation of these gases by the blood and their exchange be- 

 tween the blood and cells. These processes were fully considered in 

 Chapter 5. Here we are concerned with the structure and function of 

 the vertebrate respiratory organs, in which gas exchange with the en- 

 vironment occurs. 



All respiratory surfaces, whether in a worm, a fish or a man, con- 

 sist of a moist, semipermeable, vascular membrane exposed to the 

 external environment so that gas exchange by diffusion can take place 

 between the blood and the environment. The entire body surface of 

 primitive organisms may serve as a respiratory membrane, but the 

 respiratory surface in the higher animals is generally confined to a 

 limited region and protected in various ways. This reduces the chance 

 of mechanical injury and the amount of body water lost or gained by 

 osmosis via this route, but restricting the extent of this membrane poses 

 the problem of providing adequate surface for gas exchange. Each 

 kind of vertebrate has had to solve the dilemma of how to expose these 

 delicate membranes to the environment yet protect them from it to 

 some extent. 



225. The Respiratory System of Fishes 



Superficially, there is little resemblance between the respiratory 

 system of mammals and that of most fishes. The respiratory organs of 

 fishes are gills located in the gill slits and attached to the visceral 

 arches. A fish respires (Fig. 26.6) by expanding its pharynx and taking 

 water in through the mouth. Then the mouth is closed, or in certain 

 species oral valves close, the pharynx is contracted, and water is forced 

 out through the gill slits. Water cannot go down the esophagus, for 

 this is collapsed except when swallowing. Gill rakers act as a strainer 

 to prevent food from clogging the gills. In sharks, each gill slit opens 

 independently at the body surface. In bony fishes, all of the slits empty 

 into an opercular chamber which is closed when water is taken in, but 

 opens when water is expelled. 



The gills themselves consist of numerous, thin-walled lamellae, or 

 filaments, containing a rich network of capillaries. They are protected 

 in the gill slits, they have a large surface area, the blood and external 

 environment are in close proximity, and gas exchange occurs readily 

 as water passes over them. In addition, the body gains or loses water 

 through the gills, and some nitrogenous wastes are excreted here. The 

 salt-water teleosts also excrete salts through the gills. These fishes live 

 in an environment in which the salt concentration is greater than that 

 in their bodies so they tend to lose water by osmosis. They must drink 

 large amounts of salt water and then excrete the salts by specialized 

 cells in their gills. 



A number of fishes live in water which has a low oxygen content, 



