THE RELEASE OF ENERGY — RESPIRATION 



235 



B 



Blood to 

 the heart 

 with 



Bronchiole 



Blood from 

 the heart 

 with C02+ 



onchioie 

 or wall) 



bronchiole 

 t nnuscles) 



Alveolus 



Capillaries 

 around 

 alveolus 



Fig. 13.10. A, The human lungs, each enclosed in a double walled sac and 

 attached to the body only by the bronchial tubes and trachea. The two lines 

 around the lungs represent their outer membrane and the lining of the thoracic 

 cavity (pleura). B, Diagram of five alveoli with their blood supply. C, Lung 

 tissue consists of an enormous number of bronchioles leading to microscopic air 

 sacs with their alveoli closely surrounded by capillaries. Two air sacs are shown 

 as cut in section and greatly magnified. Air is separated from the blood only by 

 the extremely thin walls of the blood capillaries and of the alveoli of the lungs. 

 It is estimated that there are 750,000,000 alveoli in the human lungs. 



humidifier, and filter. With a "cold in the head," when the mucous cells are 

 inflamed, they greatly overdo the humidifying. Worse yet, the lining swells to 

 such an extent that for the time being it stops up the nasal passages entirely. 

 Minute particles of anything of any description that may be in the air are 

 caught against the moist walls of the nasal passages. That is the reason that 

 we smell so many things. The nose is the most democratic and hospitable of 

 our body structures. 



It is easy to see why the nasal cavities and pharynx become infected and 

 how they infect adjoining cavities in the head (Fig. 13.12). Several hollow, 

 mucous-membrane lined cavities open out of the nasal ones, the frontal sinus 

 on each side above the eye, and a maxillary sinus on each side of the upper 

 jaw. The Eustachian tubes leading to the right and left middle ears open into 

 the nasopharynx just above the soft palate. The nearby tonsils and adenoids, 



