204 PHYSIOLOGY FOB DENTAL STUDENTS. 



of the salts of the blood; c and c 1 , the effect of carbon dioxide 

 pressures on the oxygen content of the haemoglobin in a saline 

 solution ; and d is the dissociation curve of normal blood with a 

 carbon dioxide tension of 40 mm. 



The effect of carbon dioxide is of special interest. It is seen 

 that the greater the concentration of carbon dioxide, the more 

 readily is the oxygen dissociated from the oxyhaemoglobm. Thus, 

 at an oxygen pressure of 20 mm. of mercury, the amount of 

 oxyhaemoglobin formed is 67.5 per cent of the total haemoglobin 

 at a carbon dioxide pressure of 5 mm., whereas at a pressure of 

 40 mm. of carbon dioxide the amount of oxyhaamoglobin is only 

 29.5 per cent. Inasmuch as the amount of carbon dioxide is con- 

 stantly changing in arterial and venous blood, the presence of 

 this gas would seem to be an important factor in the control of 

 the oxidation or the dissociation of the haemoglobin compounds. 

 At any rate, it would help to account for the ease with which 

 oxygen is broken from the oxyhasmoglobin molecule in the capil- 

 laries which are imbedded in the tissues where the carbon dioxide 

 is formed, and its pressure is correspondingly high. 



The Mechanism of the Respiratory Exchange. The oxygen 

 in the alveoli or air passages of the lungs comprises about 14 to 

 15 per cent of the total air, and exerts on the cells of the respira- 

 tory epithelium a pressure of about 100 mm. mercury, more or 

 less. Venous blood when it reaches the lungs contains about 50 

 per cent less oxygen than does arterial blood, and can take up 

 from 6 to 8 c. c. of oxygen for every one hundred c. c. of blood. 

 Haemoglobin solutions are almost completely saturated with oxy- 

 gen at pressures of oxygen much less than 100 mm. of mercury. 

 There are, therefore, very favorable conditions in the lungs for 

 haemoglobin to take up oxygen from the air. It must be under- 

 stood, however, that the haemoglobin does not obtain oxygen di- 

 rectly from the air. The haemoglobin is held in the blood corpus- 

 cles which are floating in the blood plasma. Between the plasma 

 and the air in the lungs lie two thin membranes, the capillary wall 

 and the wall lining the air sac of the lung. The oxygen must first 

 be dissolved by the fluid in the lung epithelium; from this the 

 cells of the capillary walls take oxygen, and the plasma in turn 



