THE PURIFICATION OF THE BLOOD, ETC. 33 



into, and distends the lungs, they receive a tide 

 of blood which fills all the capillary branches 

 of the pulmonary artery spread over the walls 

 of the thin air vesicles, and the air and the 

 blood are thus brought into contact, and its 

 purification is effected. At each expiration, the 

 expansion of the lungs is greatly diminished, 

 and a tide of blood, now arterial, and received 

 by venous capillaries from the capillaries of 

 the pulmonary artery, is propelled along the 

 pulmonary veins to the left side of the heart. 

 The quantity of air received into the lung! 

 during ordinary inspiration is perhaps little 

 more than a pint each time, but it may be 

 increased naturally and without effort to two 

 pints and a half. It must be remembered, 

 however, that the lungs are never, in their 

 natural state, exhausted of air, nor can any 

 effort of expiration utterly exhaust them ; when, 

 however, this is effected as far as possible, and 

 followed by as forcible an inspiration as possible, 

 a quantity of air, varying from five to seven 

 pints, will be received. Upwards of nine pints 

 have been so received, but this is beyond the 

 average. About two ounces (by weight) of 

 blood are received by the heart at each dilatation 

 of the auricles respectively, and the same quan- 

 tity is expelled from it at each contraction of 

 the ventricles ; consequently, as the heart dilates 

 and contracts four times to one respiration, or 

 seventy-two times on the average in a minute, 

 it sends every minute 144. ounces to the lungs, 

 which in the same space receive about eighteen 



