134 HUMAN PHYSIOLOGY. 



Condition of the Gases in the Blood. 



Oxygen is absorbed from the lungs into the arterial blood by the coloring- 

 matter, hemoglobin, with which it exists in a state of loose combination, and 

 is disengaged in its passage through the capillaries. 



Carbonic acid, arising in the tissues, is absorbed into the blood in conse- 

 quence of its alkalinity, where it exists in a state of simple solution and also 

 in a state of feeble combination with the carbonates, soda and potassa, 

 forming the bicarbonates. It is liberated as the blood flows through the 

 capillaries of the lungs. 



Nitrogen is simply held in solution in the plasma. 



Exchange of Gases in the Air-cells. From the difference in ten- 

 sion of the oxygen in the air-cells (27.44 mm. of Hg) and of the oxygen in 

 the venous blood (22 mm. Hg), and from the difference of the carbonic acid 

 tension in the venous blood (41 mm. Hg) and in the air-cells (27 mm. Hg), 

 it might be concluded that the passage of the gases is due solely to pressure. 

 The absorption of oxygen, however, does not follow absolutely the law of 

 pressure; that chemic processes are involved is shown by the union of oxygen 

 with the hemoglobin of the blood corpuscles. The exhalation of CO 2 is also 

 partly a chemic process, as it has been shown that the quantity excreted is 

 greatly increased when oxygen is simultaneously absorbed. Oxygen not 

 only favors the exhalation of loosely combined CO 2 , but favors the expulsion 

 of that which can be excreted only by the addition of acids to the blood. 



Changes in the Blood during Respiration. 



As the blood passes through the lungs it is changed in color, from the 

 bluish-red of venous blood to the scarlet-red of arterial blood. 

 It gains oxygen and loses carbonic acid. 

 Its coagulability is increased. Its temperature is diminished. 



Asphyxia. If the supply of oxygen to the lungs be diminished and the 

 carbonic acid retained in the blood, the normal respiratory movements 

 cease and the condition of asphyxia ensues, which soon terminates in death. 



The phenomena of asphyxia are violent spasmodic action of the respiratory 

 muscles attended by convulsions of the muscles of the extremities, engorge- 

 ment of the venous system, lividity of the skin, abolition of sensibility and 

 reflex action, and death. 



The cause of death is a paralysis of the heart from overdistention by blood. 

 The passage of the blood through the capillaries is prevented by contraction 

 of the smaller arteries, from irritation of the vasomotor center. The heart 

 is enfeebled by a want of oxygen and inhibited in its action by the inhibitory 

 centers. 



