21 8 ESSENTIALS OF PHYSIOLOGY. 



The perfusing fluid most commonly used is known as Ringer's fluid, 

 and has the following composition : 



NaCl . . 0-65 per cent. 



KC1 ... 0-03 



CaCl 2 . . . 0-02 

 A trace of sodium carbonate. 



If O'l per cent, dextrose is added to this solution, it is called Locke's 

 fluid. 



By means of this or similar methods, the influence of changes in the 

 composition of the perfusing fluid upon the rate and force of the beat 

 can be readily ascertained. The muscle of the frog's heart possesses a 

 large store of energy, and will continue to beat for a long time without 

 any fresh supply of nutrient material so long as the perfusing fluid 

 contains oxygen. The force of the beat is dependent, however, not 

 only on the oxygen supply, but also on the presence or absence of 

 certain salts and on the reaction of the perfusing fluid. 



(1) If the heart is perfused with a solution free from calcium, the 

 contractions of the ventricle gradually become feebler, and the heart 

 soon stops in diastole. When calcium, but not potassium, is present, 

 the ventricle after a short time fails to relax completely during diastole, 

 and eventually may come to a standstill in a fully contracted state. 

 The presence of both calcium and potassium, as, for example, in Ringer's 

 fluid, seems to be essential for the maintenance of the normal beat. 



(2) The heart is extremely sensitive to slight changes in the 

 reaction, that is, the H ion concentration, of the perfusing fluid. It 

 beats most forcibly when the perfusing fluid is neutral ; if the fluid is t 

 made slightly acid or slightly alkaline, the beats become smaller. 



In the mammal the heart receives its blood supply through the 

 coronary arteries, along which blood is flowing during both systole and 

 diastole. The conditions which modify the supply of blood to the 

 heart by the coronary vessels can be readily studied in the heart-lung 

 preparation (p. 199). In the first place, the amount of blood flowing 

 through the coronary vessels varies directly with the blood pressure in 

 the aorta. Secondly, it is increased by the addition of adrenalin to the 

 circulating blood, this substance dilating the coronary vessels. Thirdly, 

 carbonic acid and other metabolic products of muscular activity dilate 

 the coronary vessels. By one or other of these means the blood supply 

 to the heart is increased whenever it does more work. 



Ligature of a coronary artery, or of one of its main branches, is 

 followed almost immediately by stoppage of the ventricles, and the 

 heart cannot be made to beat again. 



