136 THE BIOLOGY OF MARINE ANIMALS 



CATEGORIES OF RESPIRATORY EXCHANGE 



Respiration in animals embraces three processes, namely gaseous exchange 

 with the external medium at the surface of the body or in internal respira- 

 tory passages, transport of gases through the body and exchange of gases 

 between the internal medium and the tissue cells. In all animals these 

 several respiratory processes take place in aqueous media. In aquatic 

 animals, of course, the surface of the body is bathed with water. But even 

 in amphibious and terrestrial species the external bounding surface of the 

 respiratory structures — gills, lungs, tracheae — is kept moist by fluid exuda- 

 tion or secretion. The passage of gas across the external surface is almost 

 invariably accomplished by diffusion. The velocity of diffusion depends 

 upon the power of the liquid to dissolve the gas, and is directly propor- 

 tional to the absorption coefficient of the gas in the liquid. The direction 

 of diffusion is from the side of the film where pressure of the gas is high to 

 the side where it is low, and the rate of diffusion is dependent on the con- 

 centration gradient across the film. The rate of diffusion, or amount of 

 substance crossing a certain area in known time, is given by Fick's law, 

 which states: 



dw , 

 dQ = - DA -7- df 

 ^ dx 



Here dQ represents amount of substance diffusing in time dt across a 

 plane of area A at right angles to the direction of diffusion, when the 

 concentration gradient is du/dx. D is the diffusion coefficient, and the 

 negative sign appears because diffusion takes place from a region of higher 

 to one of lower concentration. 



Diffusion constants of oxygen, determined for a number of substances 

 of biological importance, are as follows — 



The diffusion constant (10 4 x diffusion coefficient) is defined as the number 

 of cubic centimetres of 2 (0 C C, 760 mm Hg) penetrating per min through l/u 

 thickness and 1 cm 2 when the pressure difference equals one atmosphere. 

 Diffusion through tissues, whether inert or living material, is obviously 

 slower than through water, other things being equal. Differences exist in 

 the diffusion rates of various gases, C0 2 diffusing through tissues 20-30 

 times faster than 2 , owing to the higher solubility of the former. The 

 values for the diffusion constant change with shift of temperature, there 

 being a 1 % increase or decrease in the diffusion constant for each degree 

 the temperature is higher or lower than 20°C. 



