168 THE CIRCULATION OP THE BLOOD 



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 (0.01 

 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 Avith 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 

 convenient method for changing the hydrogen-ion concentration of such 

 fluids is by altering their tension of carbon dioxide (see page 354). In 

 a heart-lung preparation,* such alteration in carbon-dioxide tension can 

 very readily be brought about by altering the percentage of this gas in 

 the air with which the lungs are ventilated. To measure the efficiency 

 of the heartbeat in such an experiment, it is convenient to enclose the 

 organ in a cardioplethysmograph, the tracing of which will tell us the 

 degree to which the heart is contracted or relaxed, as well as the output 

 of blood per minute. By increasing the tension of carbon dioxide, it 

 has been found in such experiments that the dilatation of the ventricle 

 is encouraged, so that the heart with each beat discharges a larger quan- 

 tity of blood (Fig. 40). When defibrinated blood is used the optimum 



*A heart-lung preparation is one in which both heart and lungs are perfused outside the body, 

 the vessels being suitably connected to maintain a continuous circulation. 



