76 



THE INDIVIDUAL ORGANISM 



air through the bronchial tubes, trachea, and upper air passages to fill the 

 expanded lungs. 



Relaxation of the muscles allows the lifted ribs and tightened dia- 

 phragm to return to their resting positions, and the contracting thoracic 

 walls force the excess air from the lungs. The alternation of these opposing 

 movements — inspiration when the thoracic cavity expands, expiration 

 when it contracts — serves to keep the air in the lungs continually 

 refreshed. 



External respiration. The air entering the alveoli is brought into 

 very close proximity to the blood that flows through the lungs. The 

 lining of each alveolus consists of a single layer of flat epithelial cells, 



EXTERNAL 



INTERNAL 



An alveolus of lung body tissues 



Fig. 5.2. Scheme of internal and external respiration. 



on the outer surface of which the thin-walled capillaries of the blood 

 system form a close network. Gases readily diffuse through the alveolar 

 epithelium and capillary walls and so can pass from a region of higher 

 concentration to one of lower concentration. The blood entering the 

 lungs has come from the other body tissues, where it has lost most of its 

 oxygen and taken up much carbon dioxide. The air in the alveoli, on 

 the other hand, is comparatively high in oxygen and low in carbon 

 dioxide. Consequently oxygen passes from the alveoli into the blood and 

 carbon dioxide from the blood into the alveoli. The fluid blood can take 

 into solution only a small amount of oxygen, but the hemoglobin of the 

 red blood cells absorbs oxygen molecules to form the loosely combined 

 oxyhemoglobin that gives arterial blood its red color. Hemoglobin thus 

 immensely increases the capacity of the blood for oxygen. 1 The gaseous 



1 Hemoglobin is a protein with a molecular weight of about 68,000. Its molecule 

 contains four smaller units of structure, each having a weight of about 17,000 and 

 containing globin and hemin, the latter including a single atom of ferrous iron. Each 



