90 GENERAL CONCEPTS 



water constantly over the respiratory surface. Air-breathing animals may 

 obtain sufficient oxygen by diffusion alone. The earthworm, for example, 

 obtains enough oxygen by diffusion from the air in its burrow and need 

 not stir up that air. The marine worms which live in burrows or tubes, 

 in contrast, undulate their bodies to provide a current of water through 

 the burrow. An even more dramatic example of this is provided by the 

 shore crab, which can live in air or water. This animal has a set of gills 

 located in a gill chamber between the upper shell and the attachment 

 of the legs. A paddle-shaped part (the scaphognathite) of one leg moves 

 back and forth in the gill chamber to keep a current of water flowing 

 over the gills. If the scaphognathites are paralyzed, the crab will soon 

 die if placed in sea water, but will live indefinitely in air, for the rate of 

 diffusion from air is rapid enough to supply all the oxygen the animal 

 needs. 



The ability of blood to carry oxygen and carbon dioxide depends 

 to a large extent on the presence of a heme-protein pigment, such as 

 hemoglobin. If blood were water, it could carry only about 0.2 ml. of 

 oxygen and 0.3 ml. of carbon dioxide in each 100 ml. Whole blood, 

 because of the properties of hemoglobin, can carry some 20 ml. of oxygen 

 and 30 to 60 ml. of carbon dioxide per 100 ml. Hemoglobin is found 

 in all of the major groups of animals above the flatworms; certain groups 

 —molluscs and Crustacea, for example— have other heme pigments such 

 as hemocyanin. In the respiratory organ, the lung or gill, the heme 

 pigment unites with oxygen. For example, hemoglobin unites with 

 oxygen to form oxyhemoglobin: 



Hb + Oo ^ HbOa 



The reaction is reversible and hemoglobin releases the oxygen when 

 it reaches a region where the oxygen tension is low. The combination of 

 oxygen with hemoglobin and the release of oxygen from oxyhemoglobin 

 are controlled by the amount of oxygen present and by the amount of 

 carbon dioxide present. Carbon dioxide reacts with water to form car- 

 bonic acid, H0CO3, hence an increase in the concentration of carbon 

 dioxide results in an increased acidity of the blood. The oxygen-carrying 

 capacity of hemoglobin decreases as blood becomes more acid; thus the 

 combination of hemoglobin with oxygen is controlled indirectly by 

 the amount of carbon dioxide present. This results in an extremely 

 efficient transport system: In the capillaries of the tissues, carbon dioxide 

 concentration is high and a large amount of oxygen is released from 

 hemoglobin by the combined action of low oxygen tension and high 

 carbon dioxide tension. In the capillaries of the lung or gill, carbon 

 dioxide tension is lower and a large amount of oxygen is taken up by 

 hemoglobin by the combined action of high oxygen tension and low 

 carbon dioxide tension. 



Hemoglobin plays an important role in the transport of carbon 

 dioxide and in the maintenance of a constant blood pH; its functions 

 in these and in the transport of oxygen are intimately interrelated. Some 

 carbon dioxide is carried in a loose chemical union with hemoglobin, 



