14 



NATURE 



[January 5, 1922 



surrounding it. We can thus vary at pleasure the 

 resistance which has to be overcome by the left 

 ventricle, and, by maintaining a normal pressure 

 in the beginning of the aorta, ensure a proper 

 supply of oxygenated blood through the coronary 

 arteries to .the muscular tissue of the ventricles. It 

 is this fact vv^hich makes it possible for the warm- 

 blooded heart to continue to beat for eight to 

 twelve hours after removal from the body. On the 

 other side of the artificial resistance the blood is led 

 through a spiral immersed in warm water to keep 

 the blood at body temperature, and then passes into 

 a reservoir from which a wide rubber tube leads to 

 a glass tube placed in the big vein opening into 

 the right auricle. By means of a screw clip on this 

 tube the inflow of blood may be regulated to any 

 desired extent, and can be kept constant while other 

 conditions are varied. Thus in this preparation 

 the three chief factors, temperature, the inflow of 

 blood, and the resistance to the outflow of blood, 

 can be varied separately and at the will of the 

 operator. Any of the heart cavities or any part of 

 the circuit can be connected to manometers so as to 

 record the pressure of the fluid, and by means of 

 a side tube placed just beyond the artificial resist- 

 ance we can allow the blood to flow off into a 

 graduated cylinder, and thus measure the time taken 

 by the left ventricle to expel 50 or 100 c.c. of blood, 

 thus measuring the average output of the organ. 



An Exferiment Described. 



A typical experiment may be divided into six 

 stages. Records of one experiment show that in the 

 first stage the heart was beating at a normal rate 

 (72 per minute), the blood pressure varied from 

 100 mm. Hg, and the output of the heart was 

 240 c.c. of blood per minute. In the second stage 

 the resistance to the flow of blood through the 

 tubes was increased to such an extent that the pres- 

 sure rose to 160-180 mm. Hg. The heart con- 

 tinued to beat, and for a time put out just as much 

 blood as it did at the lower pressure. In the third 

 stage the artificial resistance was suddenly reduced 

 to zero, the arterial pressure fell to about 20 mm. 

 Hg, but the heart beat regularly and the outflow 

 of blood was unaltered because the inflow of blood 

 had not been altered. In the fourth stage the inflow 

 of blood was raised suddenly to 600 c.c. per minute. 

 The heart became bigger, but the regularity of its 

 contractions remained unaltered, and it drove for- 

 ward all the blood that it received. 



The same thing happened in the fifth stage, in 

 which the artificial resistance was raised simultane- 

 ously with the venous inflow. The reason for these 

 phenomena is that within certain limits the heart 

 isolated fyom the body can respond to all the de- 

 mands made upon it; it can overcome a higher 

 resistance, and it can pump out more fluid. In the 

 sixth stage the inflow of blood was further increased 

 to 1200 c.c. per minute, and the artificial resistance 

 was increased until the blood pressure rose to 

 200 mm. Hg. This was too much for the heart, 

 which began to beat irregularly and dilate widely. 

 It would have failed altogether if the pressure sur- 

 NO. 2723, VOL. 109] 



rounding the thin rubber tube had not then been 

 released to allow the artificial pressure to drop to 

 a level at which the left ventricle could empty itself. 

 If during this experiment the amount of oxygen 

 taken up by the blood had been measured, and also 

 the amount of carbonic acid given off by this fluid 

 in passing through the lungs, an increase in both 

 these amounts would have been found during the 

 stage at which greater demands were being made on 

 the heart. That is to say, the greater the work 

 done by the heart, the greater the chemical changes 

 to supply energy. A motor-car may be running 

 steadily with an even beat of its engines along a 

 level road ; when it comes to a hill it will slow up 

 and finally stop unless the chauffeur increases the 

 chemical changes and the energy of each explosion 

 within the cylinder by opening the throttle and 

 letting in more mixture of petrol and air. In the 

 case of the heart there is no chauffeur, but there is 

 some automatic regulation by which the heart in- 

 creases its chemical changes, and therefore the 

 energy of each beat, in exact proportion to the work 

 which is demanded of it. It is the nature of this 

 automatic regulation which concerns us now. 



The Nature of ike Automatic Regulation of the 

 Heart. 



By a careful observation of the changes in the 

 heart in the experiment described above we may 

 arrive at some clue to the nature of the pressure, 

 but more accurate methods are necessary if we are 

 to be certain of the correctness of our guess. We 

 must, under these varying conditions, measure : 

 (i) the pressure in the heart cavities produced at 

 each contraction; (2) the volume of the heart cavi- 

 ties — i.e. the length of the muscle fibres of their 

 walls. The first we measured in the experiment 

 described by connecting the interior of each cavity 

 in turn with a quickly acting manometer, the excur- 

 sions of which are registered by an optical method 

 so as to avoid the instrumental vibrations of a lever. 

 The curve of pressure obtained under two condi- 

 tions — i.e. low and high artificial resistance — could 

 then be plotted. It must be remembered that the 

 heart was sending on in each case all the blood that 

 it received, though the work necessary under the 

 high pressure was two or three times as great as 

 that necessary to send on the blood at the low 

 pressure. To measure the volume of the heart the 

 ventricles are enclosed in an instrument known as 

 a cardiometer. This communicates with a piston 

 recorder so that the change of the volume of the 

 ventricles at each beat can be registered on a moving 

 surface. 



The question we have to decide is : How does the 

 heart know when it is relaxed that at the next con- 

 traction it will have to exert more force than it did 

 previously, when the arterial resistance to be over- 

 come was lower? If we measure the pressure in 

 the ventricles in the manner just described we find 

 that during the period of relaxation of the ven- 

 tricles the pressure in its cavities is approximately 

 zero, whether the artificial pressure which it has to 

 overcome at its next beat is 50 or 150 mm. Hg. It 



