Respiration and Metabolism 211 



in Table 34.-'="' The value for the diffusion constant* increases with rising 

 temperature, about 1 per cent per degree, taking the rate at 20° C. as unity. 

 In spite ot its larger molecular size, carbon dioxide diffuses through tissues 20 

 to 30 times faster than oxygen does, owing to the higher solubility of COo . 

 Members of the more primitive phyla (Protozoa, Rotatoria, Porifera, Coel- 

 enterata, Platyhelminthes, Nemertea, and some Annelida), as well as eggs and 

 embryos of all groups, can procure their oxygen entirely by diffusion without 

 recourse to special respiratory mechanisms or circulatory systems. The rela- 

 tionship between adequate oxygen pressure and oxygen consumption for small 

 spherical organisms is summarized by the equation: 



Co = 



6D 



in which Co is the required oxygen pressure, A the oxygen consumption of the 

 tissue in cc. oxygen per gm. per minute, r the radius of the sphere, and D the 

 diffusion coefhcient of oxygen through the tissues.' ^"^ In organisms with fairly 

 high metabolic activities diffusion will suffice if the tissue is no more than I 

 mm. in diameter.-""' However, in animals with tissues more than 1 mm. distant 

 from an oxygen supply some supplementary transporting system is necessary. 

 With an adequate oxygen source rapid utilization establishes a high gradient, 

 and diffusion over short distances ma\ be very rapid. Bronk''- has shown by 

 the microelectrode technique that oxygen diffusion in brain tissues is suffi- 

 ciently rapid to demonstrate fluctuations in oxygen tension concordant with 

 the respiratory rhythm, when the partial pressure is measured at a distance of 

 some 25 /i, from an arteriolar oxygen supply. 



TYPES OF RESPIRATORY MECHANISMS 



The striking adaptabilit\' of animals tor the procurement of oxygen is exem- 

 plified by the many modifications which have evolved to alleviate limitations 

 imposed by simple diffusion. Organisms have met similar respiratory problems 

 in different ways. 1 he elaboration of respiratory mechanisms has developed 

 along with the evolution of efficient vascular ("convection") systems. Krogh's 

 admirable survey-'" of respiratory adaptations should be consulted in this 

 connection, as well as the extensive review of respiratory systems by Guieysse- 

 Pellissier.'"^ Four main types of respiratory mechanisms are considered here: 

 (1) integument, (2) gills, (3) lungs, and (4) tracheae. 



Integument. Cutaneous respiration plays a significant role in the respiratory 

 economy of many invertebrates, and it is safe to say that it occurs to some 

 extent in all animals. A circulating mechanism is required in aquatic animals, 

 either cilia or movement of the organ as a whole, to move the water over the 

 respiratory surfaces. In air-breathing forms a moist integument is essential for 

 significant gas exchange through the skin, and mucous glands are generally 

 found in cutaneous air breathers, as the terrestrial isopods, molluscs, and 

 amphibians. Cutaneous respiration is more common than generally recognized 



* The diffusion constant (10' times the diffusion coefficient) is defined as the number 

 of cc. of oxygen (reduced to 0° C, 760 mm. Hg) passing through a distance of 1 n over 

 an area of I sq. cm. with a pressure difference of 1 atmosphere. 



