CHAPTER XXV 

 RESPIRATION AND EXCRETION 



RESPIRATION 



Cell breathing. — Cell breathing, the exchange of carbon 

 dioxide for oxygen, or the process of oxidizing the living cell, 

 is the essential act of respiration. 



The amoeba takes its oxygen from the water which sur- 

 rounds it, and discharges its carbon dioxide into the water. 

 Cells of the body take their oxygen from and discharge car- 

 bon dioxide into the lymph which surrounds them. From 

 the lymph carbon dioxide passes into the blood. The blood 

 carries it to the lungs. The respiratory or breathing appa- 

 ratus is a mechanism for supplying the blood with its needed 

 oxygen and for removing from the blood its load of carbon 

 dioxide. The apparatus may be considered to consist of 

 the lungs and the air passages leading into them. 



Position and structure of the lungs. — ^The lungs (figs. 175 

 and 159 R. L., L. L.) fill the greater part of the thoracic 

 cavity. This cavity is lined by the pleural membrane which 

 folds neatly back over all the organs in the cavity. The lungs 

 are therefore suspended within this sac of pleural membrane. 

 The portion of the membrane covering the lungs is separated 

 from the portion lining the thoracic cavity by a liquid which 

 reduces friction between the two walls. The lungs are shaped 

 to fit around the heart which also lies in this cavity. The 

 thoracic cavity is completely divided into two parts by a 

 partition of connective tissue, within which lie the trachea, 

 the oesophagus and large blood-vessels. One lung lies on 



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