154 The Vertebrate Organ Systems 



muscular activity. Instead of reoxidizing the lactic acid so produced 

 back into glycogen, the lactic acid is eliminated as a waste product. 

 The parasite thus acquires no oxygen debt. It should be realized that 

 anaerobiosis is a comparative condition, varying in degree from forms 

 such as man -who can acquire only a limited oxygen debt to those para- 

 sites which can live an entire life span in the absence of free oxygen. 



External and Internal Respiration. — The term respiration may 

 be applied to any process involving not only the mechanical intake and 

 transport of air, but also the utilization of the oxygen in the cells. It 

 is actually better to distinguish between two phases of respiration : ex- 

 ternal and internal. External respiration includes the movement of 

 oxygen into the circulatory system and the transport by that system 

 of the oxygen to the cells. It also includes the loss of the waste ma- 

 terial, carbon dioxide, to the atmosphere. Internal respiration refers to 

 the actual utilization of the oxygen within the cells. In mammals, ex- 

 ternal respiration consists of the inspiration of the air into the lungs, the 

 movement of the oxygen into the blood, and the transport of this oxygen 

 to the cells. In the blood the oxygen is carried by the red blood cells 

 which possess the respiratory pigment, hemoglobin. This hemoglobin 

 combines with oxygen to form oxyhemoglobin. When oxyhemoglobin 

 reaches the capillaries, it gives up the oxygen and again is simply hemo- 

 globin. The carbon dioxide produced by the oxidation of foods is re- 

 turned by the blood to the lungs for release. 



Internal respiration is carried on within the cells by a complex 

 series of enzymes. This phase of respiration produces the energy for 

 all the life processes. 



Mechanics of Respiration. — Among fish, water is forced over the 

 vascular gills and here gaseous exchange occurs. With the advent of 

 terrestrial life and lungs, other mechanisms developed to increase the 

 efficiency of this exchange. The frog, which has no special chest muscles 

 for respiration, literally swallows its air by the action of the muscles 

 of the throat. Most of the reptiles have advanced greatly over this 

 simple method : they have developed ribs and muscles for aiding 

 respiratory exchange. 



It is among the mammals that the greatest ef^ciency in the me- 

 chanism for facilitating this gaseous exchange is seen. Here the rapid 

 movement of air is based upon differences in pressures which are brought 

 about by the interaction of the intercostal muscles and the muscular 

 diaphragm. 



