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BIOLOGY FOR BEGINNERS 



The more oxygen the tissues need, the more rapidly the lungs have 

 to operate to supply the blood with it, to be carried to the tissues. 

 Air Changes in Breathing. Air contains only about 20 per cent 

 of oxygen. Of this, only about a quarter is absorbed in the lungs 

 by the haemoglobin of the blood. In the circulation, the haemo- 

 globin can give out only about one-half the oxygen it contains, so, 



FIG. 126. Diagram to show the changes in the sternum, diaphragm, and 

 abdominal wall in respiration. A, inspiration; B, expiration; Tr, trachea; 

 St, sternum; D, diaphragm; Ab, abdominal wall. The shaded part is to indi- 

 cate the stationary air. From Martin- Fitz. 



unless we breath deeply and keep our breathing apparatus in 

 healthy working order, the tissues may receive too little oxygen. 

 Since oxidation (union of oxygen with tissue) is the only source 

 of life energy, this matter is of very great importance. 



Expired air loses about one-fourth of its oxygen, but receives 

 100 times as much carbon dioxide as it had when taken in, also a 



