168 THE CIRCULATION OF THE BLOOD 



substance will lead to systole or contraction, whereas union of sodium 

 or potassiumjadll-lead to relaxation or diastole. 



Observations on Mammalian Heart 



Investigation of the efficiency of various saline solutions on the iso- 

 lated mammalian heart has shown that the proportion of the above salts 

 must be somewhat different from that used for the cold-blooded heart. 

 As might be expected, the most efficient proportions are those present 

 in the blood serum of the particular animal whose heart is being per- 

 fused. Basing his proportions upon the results of analyses of the inor- 

 ganic constituents of mammalian blood serum, Locke found that an 

 inorganic solution of the following composition is most efficient: so- 

 dium chloride, 0.9 per cent; calcium chloride, 0.024 per cent; potassium 

 chloride, 0.042 per cent; and sodium bicarbonate, 0.01 to 0.03 per cent. 

 When " Locke's solution," as it is called, is perfused, with oxygen in it, 

 under pressure through the isolated mammalian heart at body tempera- 

 ture, efficient beating can be maintained for many hours. More recently 

 a solution known as Tyrode 's is commonly used. It contains a small amount 

 of magnesium and of phosphates. Although undoubtedly superior for 

 some perfused preparations, such as the intestine, it does not seem to be 

 in any way superior to Locke's for the perfusion of the heart. The bicar- 

 bonates and phosphates in these solutions endow them with a hydrogen-ion 

 concentration near that of the blood (slightly on the alkaline side of 

 neutrality), and at the same time they act as buffer substances. 



As already pointed out, the organic constituents of such perfusion 

 fluids do not appear to be relatively of nearly so much importance as 

 the inorganic. Nevertheless it appears that a small percentage (0.1 

 per cent) of glucose does materially improve the nutritive qualities of 

 the solution, and it has moreover been shown that after a while the con- 

 centration of glucose in the perfusion fluid distinctly decreases. This 

 does not of itself necessarily mean that the glucose is .actually utilized 

 by the heart muscle: it might be stored away in it as glycogen. That 

 some consumption of carbohydrate does however occur in the heart has 

 been demonstrated by measuring the intake of oxygen and the output 

 of carbon dioxide through the lungs of an isolated heart-lung prepara- 

 tion perfused outside the body with defibrinated blood. By experiments of 

 this type the attempt has been made to show that the heart of diabetic 

 animals loses the power of burning glucose as compared with the. hearts 

 of normal animals. While the experiments are very suggestive, the 

 results do not as yet justify us in claiming that in the latter disease the 

 power of burning glucose in the tissues has been materially depressed. 



The concentration of hydrogen ions in the perfusion fluid has an im- 

 portant influence on cardiac efficiency. We also know that the most 



