236 



THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 



A. In the lung. Oxygen (Oj) diffuses 

 from air in the lung, and combines 

 with hemoglobin (Hb) in the red 

 blood cells to make oxyhemoglobin 

 (HbOJ 



B. In other parts of the body 

 Oxygen seporotes from the loosely 

 bound oxyhemoglobin and diffuses 

 into the tissue fluid and cells. 



Fig. 13.11. A, the exchange between air and blood in the lungs (external 

 respiration); and B, between the blood and cells of the body (internal respiration). 

 The rate and direction of the diffusion of a gas is determined by its pressure. 

 When oxygen is abundant in the lungs its pressure is high and it diffuses into the 

 blood. Carbon dioxide diffuses out of tissues, e.g., muscle, to blood, and then to 

 lungs because it passes from higher to lower pressures. Carbon dioxide is carried 

 by the blood in two ways; in loose combination with hemoglobin (CO^Hb); and 

 combined with water as carbonic acid {CO-, -f- H^O). Most of the carbonic 

 acid is converted into bicarbonates through neutralization by sodium or potassium 

 ions released when oxyhemoglobin is changed to hemoglobin. The process of con- 

 verting carbonic acid back into carbon dioxide to diffuse out in the lung capillaries 

 is speeded up by a special enzyme, carbonic anhydrase. 



often large in children, are masses of lymphatic tissue in which bacteria some- 

 times accumulate. 



Whether air enters through the nose or mouth it must pass through the 

 pharynx in order to reach the open trachea with its lifted cover, the epiglottis 

 (Fig. 13.12). In mammals, the pharynx is merely a place where the paths of 

 air and food cross in an awkward fashion. It is a place where indecision is 

 quickly punished. Either air enters it and goes straightway into an open 

 trachea, or food enters it, passes a closed trachea, and goes into the esophagus. 

 If both enter simultaneously, neither arrives and choking follows. A choke is 

 a forced expiration, an attempt to dislodge the crumbs that have "gone down 

 the wrong way." In ordinary breathing air passes quietly into the open trachea 

 through the larynx and enters the bronchial tubes. 



