182 Prof. Draper on the Cause of the 



which has been obtained in its passage over the air-cells of 

 the lungs, an oxidation which is indicated by its bright crim- 

 son tint. On reaching its final distribution in the tissues, it 

 effects their oxidation, producing heat ; and as it loses its 

 oxygen, and receives the metamorphosed products of the tis- 

 sues, it takes on the blue colour characteristic of venous 

 blood. 



If now we contrast the relations of arterial and venous 

 blood to the tissues, it is obvious that the former, from the 

 fact that it can oxidize them, must have an intense affinity for 

 them ; but the latter, as it is the result of that action after all 

 affinities have been satisfied, must have an attraction which is 

 correspondingly less. 



Arterial blood has therefore a high affinity for the tissues ; 

 venous blood little or none. But the change from arterial 

 to venous blood takes place in the manner I have just indi- 

 cated ; and therefore, upon the first of the foregoing general 

 rules, motion will take place, and in a direction from the arte- 

 rial to the venous side. 



By the deoxidizing action of the tissues upon the blood, that 

 liquid ought upon these principles to move from the arteries 

 into the veins, in the systemic circulation. The systemic cir- 

 culation is therefore due to the deoxidation of arterial blood. 



The Pulmonary Circulation. — In this circulation ve- 

 nous blood presents itself on the sides of the air-cells of the 

 lungs, not to carbonaceous or hydrogenous atoms, but to 

 oxygen gas, which being the more absorbable of the consti- 

 tuents of the air, is taken up and held in solution by the moist 

 walls of those cells. Absorption of that oxygen takes place, 

 and arterialization is the result. The blood from being blue 

 turns crimson. 



What now are the relations between venous and arterial 

 blood and oxygen gas ? For that gas venous blood has a high 

 affinity, as is shown by its active absorption ; but this affinity 

 is satisfied and has ceased in the case of arterial blood. 



The change from venous to arterial blood, which takes place 

 on the air-cells which are charged with oxygen gas, ought upon 

 these general principles to be accompanied by movement in 

 a direction from the venous to the arterial side. 



The pulmonary circulation is due to the oxidation of venous 

 blood, and ought to be in a direction from the venous to the 

 arterial side. These considerations therefore explain the 

 cause of the flow in opposite directions in the systemic and the 

 pulmonic circulation ; in the former the direction is from the 

 arterial to the venous side, in the latter from the venous to 

 the arterial. It arises from the opposite chemical reactions 



