330 



RESPIRATION AND EXCRETION 



The Organs of Respiration in Man. — We have alluded to 

 the fact that the lungs are the organs which give oxygen to the 

 blood and take from it carbon dioxide. The course of the air 

 passing to the lungs in man is much the same as in the 

 frog. Air passes through the nose, and into the windpipe. This 

 cartilaginous tube, the top of which may easily be felt as the 

 Adam's apple of the throat, divides into two bronchi. The 

 bronchi within the lungs break up into a great number of smaller 

 tubes, the bronchial tubes, which divide somewhat like the small 



branches of a tree. The 

 bronchial tubes, indeed all 

 the air passages, are lined 

 with ciliated cells. The 

 cilia of these cells are con- 

 stantly in motion, beating 

 with a quick stroke toward 

 the outer end of the tube, 

 that is, toward the mouth. 

 Hence any foreign material 

 will be raised from the 

 throat first by the action 

 of the cilia and then by 

 coughing or " clearing the 

 throat." The bronchi end 

 in very minute air sacs, 

 little pouches having elastic walls, into which air is taken when 

 we inspire, or take a deep breath. In the walls of these pouches 

 are numerous capillaries, the ends of arteries which pass from the 

 heart into the lung. It is through the very thin walls of the air sacs 

 that an interchange of gases takes place which results in the blood 

 giving up part of its load of carbon dioxide, and taking up oxygen in 

 its place. This exchange appears to be aided by the presence 

 of an enzyme in the lung tissues. This is another example of 

 the various kinds of work done by the enzymes of the body. 



Changes in the Blood within the Lungs. — Blood, after leaving 

 the lungs, is much brighter red than just before entering them. 

 The change in color is due to a taking up of oxygen by the hoBmo- 



Air passages in the human lungs, a, larynx ; 

 6, trachea (or windpipe) ; c, d, bronchi ; 

 e, bronchial tubes ; /, cluster of air cells. 



