444 THE RESPIRATION 



Much important information is being collected concerning these values 

 in various abnormal conditions. When the percentage of hemoglobin is 

 increased or. decreased the oxygen consumption by the tissues remains 

 within the normal limits unless the percentage of hemoglobin is reduced 

 below 30. Thus Luiidsgaard 67 found in a polycythemic patient with 33.4 

 volumes per cent oxygen capacity (corresponding to 181 per cent 

 hemoglobin) a venous 2 content of 28 vols. per cent, giving a differ- 

 ence of 5.4 vols. per cent or 16.1 per cent unsaturation ; and in an anemic 

 patient with only 6.7 vols. per cent oxygen capacity (36 per cent hemo- 

 globin) he found the venous oxygen to be 1.5, a difference of 5.2 vols. 

 per cent or 77.6 per cent unsaturation. This result is significant, since 

 it shows that the tissues are efficiently oxygenated whether or no the 

 arterial blood carries a great reserve or no reserve at all of oxygen. 

 When the arterial oxygen falls below the minimum (which is about 30 per 

 .cent hemoglobin), the bloodflow must become increased in order to sup- 

 ply the tissues with the normal amount of oxygen. The observations on 

 the mass movement of the blood in the hands and the viscera referred 

 to elsewhere in this volume (page 208) are of interest in this connection. 



Cyanosis. Before considering the fundamental causes for cyanosis it 

 is important to note that a condition not unlike it may be due to marked 

 polycythemia (erythrosis or false cyanosis). In this condition the oxy- 

 gen unsaturation of the venous blood is normal. Cyanosis itself is prob- 

 ably always due to an excessive degree of oxygen unsaturation of the 

 blood, although sometimes, as in poisoning by certain drugs, it is caused 

 by conversion of oxyhemoglobin into methemoglobin. The unsaturation 

 may be due to excessive reduction of the blood in the tissues, in which 

 case the arterial blood remains normal, or it may depend on inadequate 

 aeration of the blood in the lungs when both venous and arterial blood 

 will give high unsaturation values. 



Excessive reduction in the tissues may result either from increased 

 activity, as in vigorous muscular exercise, or from a sluggish circula- 

 tion, so that, although the rate of reduction itself is not altered, the blood 

 loses too much of its oxygen. This sluggish state of the capillary circu- 

 lation may be due to venous obstruction, to independent dilatation of 

 the capillaries (see page 252), or to a slow circulation "time of the blood 

 as a whole. Faulty aeration of the blood in the lungs may be the result 

 of interference with the ventilation of the alveoli from pneumonia, 

 bronchitis, edema, etc., or it may be due to disturbances in the lesser 

 .circulation, as in valvular or congenital heart disease. 



Lundsgaard 67 has furnished some interesting figures to show the de- 

 gree to which the unsaturation of the venous blood must occur to cause 

 cyanosis. The practical yalue which such information has already af- 



