CHAPTER X 

 Blood Circulation and Breathing. 



ALTHOUGH it does not fall within the scope of this book to deal 

 in detail with the physiology of the circulation, yet the connection 

 between breathing and circulation is so specially intimate that a 

 chapter must be devoted to this subject. Physiology is most em- 

 phatically not a subject which can be divided off into water-tight 

 compartments. 



We have seen that it is with the composition of the arterial 

 blood that breathing is essentially correlated; but it has also been 

 shown in successive chapters that the amount and composition of 

 the blood returning from the tissues to the lungs play a most es- 

 sential part in determining the composition of the arterial blood, 

 and are thus intimately correlated with breathing. If, moreover, 

 the blood supply to the brain and other tissues is insufficient, or 

 the blood is abnormal in composition, the breathing is affected in 

 various ways. On the other hand circulation is intimately de- 

 pendent on breathing. If the breathing is hindered the circulation 

 is quickly affected; and, as Yandell Henderson was the first to 

 show, excessive breathing brings about failure of the circulation. 

 Thus we cannot at all fully understand how the breathing is regu- 

 lated and what part it is playing unless we understand the dis- 

 tribution of the circulating blood and the means by which its 

 composition in the tissue capillaries is regulated. 



It seems evident that the most urgent and immediate need for 

 an adequate blood supply to any part of the body arises from the 

 necessity for a continuous supply of fresh oxygen. If the supply 

 of oxygen to the arterial blood is cut off in a warm-blooded animal 

 by placing it in nitrogen or hydrogen, loss of consciousness oc- 

 curs as soon as the store of oxygen in the lungs and venous blood 

 is washed out. In man eight or ten breaths suffice for this during 

 rest, and still fewer breaths during exertion. In very small ani- 

 mals, with their rapid breathing and circulation, two or three 

 seconds are sufficient; and a few seconds afterwards the heart is 

 paralyzed also. The important effects of even a slight diminution 

 in the pressure of oxygen in the arterial blood have been made 

 clear in preceding chapters. 



