88 



GENERAL CONCEPTS 



from the gas to the liquid. This escaping tendency, known as the tension 

 ot the gas, is expressed numerically in terms oi the partial pressure of 

 the gas with which it would be in equilibrium. Notice that the gas 

 tension is a measure of the tendency of the dissolved gas to diffuse out 

 from the solution, and is not a measure of the qiumtity of gas present. 

 The actual quantity of gas in solution is a property of both the gas 

 and the liquid, and may vary considerably from one liquid to another. 

 Water and blood in equilibrium with air would each have an oxygen 

 tension of 152 mm. Hg, but the water would contain only 0.2 ml. of 

 oxygen per 100 ml. and blood (because of the presence of hemoglobin) 

 would contain 20 ml. of oxygen per 100 ml. A solution of pure hemo- 

 globin containing the same amount of hemoglobin as blood (15 gm. per 

 100 ml.) would also contain 20 ml. of oxygen per 100 ml. and have an 

 oxygen tension of 152 mm. Hg. 



The Respiratory Surfaces. The protozoa and the simpler inverte- 

 brates—sponges, coelenterates and flatworms— obtain oxygen from and 

 give off carbon dioxide to the surrounding water. This process is termed 

 direct respiration, since the body cells exchange oxygen and carbon 

 dioxide directly with the surrounding environment. The cells of the 

 larger, more complex animals cannot exchange gases directly with the 

 environment and some form of indirect respiration occurs: the cells 

 exchange gases with the body fluids (internal respiration) and the body 

 fluids exchange gases with the external environment via a specialized 



'f Aine.ba- 



Insect.-Body wall Tissue Cells 



'Spiracle 



sm 



1 Waiter 



Epitheliuin. 



0, (pa 



CO, ^^°- 



Trachea 

 "L-uTLg of vertebrate 



Tracheolc 



%,.v«™. ™s..^\v^5^ 



EndollieliuTTi -^^^ 

 Fish:' Gill f ila-ment 



Figure 5.3. Respiration in an ameba, in the tracheal system of an insect, in the gill 

 of a fish, and in the lung of a higher vertebrate. 



