'118 PRINCIPLES OF ANIMAL BIOLOGY 



can resist the urgent demand of the respiratory center that breathing 

 be resumed. 



Mechanism of Oxygen Collection. — It has already been stated 

 that oxygen does not spread through dry surfaces. This is because 

 the movement of oxygen in entering an organism is a process of diffusion, 

 which can occur freely only when the oxygen is in solution. Aquatic 

 animals, except a few air-breathing types like whales and other swimming 

 mammals, never meet oxygen except in solution. When air comes in 

 direct contact with an animal, its oxygen cannot enter unless it is first 

 dissolved. All that is necessary is to have the surfaces moist; oxygen 

 dissolves in the film of moisture, then passes readily inward through 

 the membranes. Lungs and tracheae have no difficulty in maintaining 

 this moisture, since they possess internal cavities in which there can 

 be little evaporation. Land animals with gills, however, must either 

 live in places that are perpetually moist, such as swamps, or must prevent 

 evaporation in some way. Land-dwelling crayfishes protect their gills 

 from drying by means of chitinous flaps of the exoskeleton (page 

 90) and have the habit of burrowing in the soil until moisture is 

 reached. 



The passage of oxygen through moist membranes depends on the same 

 principle as that which causes water to flow down hill, or winds to blow 

 from areas of high atmospheric pressure to those of low pressure. Oxygen 

 goes from places of high oxygen pressure to those of lower pressure. This 

 pressure is not entirely a matter of quantity, for a small amount of oxygen 

 dissolved in a certain volume of water may exist at a greater pressure than 

 does a greater amount in the same volume of air. When oxygen enters 

 the gills of an aquatic salamander, it is because the oxygen in the water is 

 at greater pressure than is the oxygen in the gills. In a land animal with 

 liuigs, the oxygen in the air in the lungs is at higher pressure than in the 

 tissue of the lungs. In the human lungs the air in the remote alveoli, 

 being diluted with waste products there, exhibits an oxygen pressure 

 somewhat lower than the oxygen pressure of open air; and yet it is nearly 

 three times as great as the oxygen pressure in the tissues of the lungs; 

 hence the transfer to the tissue. From the cells lining the alveoli of the 

 lungs it is a very small step to the blood, for the capillaries are closely 

 applied to the alveoli. Oxygen enters the plasma, the liquid portion of 

 the blood, again in response to a pressure gradient : pressure is lower in the 

 plasma. Pressure is constantl}^ kept lower in the plasma, because the red 

 blood cells contain a protein which takes up (]uantities of oxygen in 

 chemical combination. Moreover, the blood is circulating; blood that 

 has absorbed oxygen is continually l)eing replaced by blood that has 

 little of it. So a perpetual transfer oi (jxygen to the blood is set up in the 

 lungs. 



