150 PRINCIPLES OF ANIMAL BIOLOGY 



It is estimated that in man the hemoglobin which reaches the capil- 

 laries out in the tissues is nearly saturated with oxj'^gcn, while the tissues 

 and the Ij^inph in which the cells are bathed are practically devoid of 

 oxygen. Expressed in the usual way the tension of oxygen in the hemo- 

 globin as it arrives in the capillaries is estimated at about 75 to 80 mm. of 

 mercury while in the tissues the tension of the oxj'gen is practically zero. 

 Hence the oxygen rapidly diffuses from the blood plasma and the hemo- 

 globin into the lymph and thence into the tissue cells where it is used in 

 metabolism. The passage of the blood through the capillaries is esti- 

 mated to require about one second ; yet in this brief period of time the 

 tension of oxygen in the hemoglobin is reduced from about 75 or 80 mm. 

 of mercury to about 37.6 mm., a loss in pressure equal or about 50 per 

 cent. The blood plasma gives up practically all of its oxygen. 



External and Internal Respiration. — From the above account it may 

 be seen that respiration in animals that use a circulating medium for the 

 transportation of oxygen to the cells resolves itself into two parts, namelj^ 

 the passage of the oxygen from the outside medium (air or water) into 

 the blood, and the passage of oxygen from the blood into the tissue cells. 

 These parts have been designated external and internal respiration, 

 respectively. ,Intej:iial respiration -faay^^je- compared to respiration in 

 "^ Amceba, Paramecium or Hj'dra. This is true respiration. External 

 respiration is merely an accessory operation, loading oxygen upon the 

 transportation system. The oxygen conveyed to the cells is consumed 

 in the oxidation of carbohydrates, fats, and proteins, a process which is 

 accompanied by the release of energy, and the production of materials 

 which cannot be used in the body. 



Excretion. — The materials which result from the oxidation of fats, 

 carbohydrates, or proteins, are wastes or excretions. They are largely 

 carbon dioxide and water. The oxidation of proteins also gives rise to 

 nitrogen compounds as well as carbon dioxide and water and certain sul- 

 phates and phosphates. The methods of elimination of these substances 

 are described seriatim. 



Carbon Dioxide Elimination. — A very small proportion of the carbon 

 dioxide produced by the metabolic activities is discharged, in the mam- 

 mals, through the skin in the secretion of the sweat glands; but in the 

 frog a large proportion (in some species as much as 74 per cent.) of the 

 carbon dioxide may be eliminated through the skin. A small quantity 

 is eliminated through the kidneys, but in mammals and most other air 

 breathing vertebrates the lungs are the most important organs for the 

 elimination of carbon dioxide. The relative efficiency of gills and skin 

 in aquatic forms with respect to carbon dioxide elimination is apparently 

 not well estabhshed. The mechanism employed in carbon dioxide elimi- 

 nation from the tissues and through llic lungs is about the same as that 

 of the oxygen intake. The operation of the mechanism depends upon 



A, 



