RESPIRATION AND EXCRETION 77 



interchange between the blood and the air in the lungs constitutes external 

 respiration, as a result of which the blood that leaves the lungs is rich 

 in oxygen and low in carbon dioxide. 



Internal respiration. When the blood from the lungs reaches the 

 other body tissues, it again enters capillary vessels that permit a gaseous 

 exchange. The tissues are poor in oxygen and have a high carbon dioxide 

 concentration. Oxygen therefore diffuses from the capillaries into the 

 fluid bathing the tissues and thence into the cells, while carbon dioxide 

 diffuses in the reverse direction. This interchange constitutes internal 

 respiration, in which O2 is delivered to the ultimate consumer, the tissues, 

 and CO 2 is taken from its source in the body. Since most of the oxygen of 

 the bright red oxyhemoglobin is released, the venous blood assumes the 

 dark red color of hemoglobin. 



EXCRETION AND THE EXCRETORY SYSTEM 



We have seen that the materials taken into the body through the diges- 

 tive and respiratory systems are used for growth, repair, and the libera- 

 tion of energy. These processes, in turn, produce waste substances that 

 become poisonous if allowed to accumulate. 



As was pointed out in the beginning of this chapter, the wastes result- 

 ing from cell metabolism (chiefly oxidation) are water, carbon dioxide, 

 nitrogenous salts (urea), and inorganic salts. One of these, carbon dioxide, 

 is a gas; this substance is eliminated principally through the lungs, to- 

 gether with water in the form of water vapor. The other products of cell 

 metabolism are nongaseous and require a different means of elimination; 

 it is with this group of wastes that the organs of excretion are mainly 

 concerned. In addition to the metabolic wastes, there are other waste 

 products of the body, such as feces and dead skin, hair, and nails. Feces 

 consist of the indigestible or undigested portions of the food eaten, 

 together with bile pigments and other products of digestive glands, and 

 bacteria and other organisms that multiply in the intestines. This mate- 

 rial is eliminated by the action of the large intestine. Dead portions of the 

 skin and its appendages are mechanically shed from the body surface. 



The portions of the body that function as excretory organs are not all 

 included in the excretory system proper. They are tabulated below, 

 together with the products that they eliminate. 



iron atom is able to unite loosely with an oxygen molecule (O2), so that in changing to 

 oxyhemoglobin each hemoglobin molecule takes on four oxygen molecules. The reac- 

 tion is freely reversible: Hemoglobin + 40 2 <=^ oxyhemoglobin. Since it has been 

 calculated that a single red corpuscle contains about 240 million hemoglobin molecules, 

 its oxygen capacity is approximately 960 million oxygen molecules. 



