HEART 411 



But the flow of blood is determined by the difference of 

 pressure between the arteries and veins, and hence the 

 arterial pressure must be maintained when the dilatation of 

 the small arteries is allowino- the increased outflow of blood. 



This must be met by an increased action of the heart to 

 maintain the pressure. The heart must be capable of great 

 variations in its action, so that at one time it may pump out 

 only a small amount of blood, at another an enormously 

 greater quantity. The heart must be able to perform very 

 varying amounts of work. 



To determine the variations in the work done, it is 

 necessary to measure the amount of blood expelled and the 

 resistance against which it is expelled. A certain amount 

 of work is done in giving velocity to the blood, but this is 

 small when compared with the work of overcoming resist- 

 ance. 



(1) The resistance in the aorta may be measured by a 

 mercury manometer (p. 447). 



(2) The determination of the amount of blood expelled 

 from the heart has proved a very difficult problem. It has 

 been attempted both by direct and by indirect methods. 



A. Direct Methods. 

 So far it has not proved possible to measure the output 

 of the heart by a direct method with the heart acting 

 normally in situ. 



(1) Cardiometer Method.— The output of blood at each 

 beat of the heart of the dog may be measured by a cardio- 

 meter, a rigid walled air-tight case, which is placed round 

 the heart and connected with a piston-recorder, so that the 

 decrease in the volume of the enclosed heart, due to the 

 blood leaving it, may be directly recorded by means of a 

 lever attached to the piston. 



(2) The Isolated Heart-Lung Preparation.— We shall presently 

 consider the way in which Starling was able to remove the 

 heart and lunofs from a dos^ and to allow blood to circulate 

 throucrh them. Fis:. 178 shows how, bv means of the side 

 tube, the blood driven from the heart may be collected and 

 its amount measured. 



