148 INTRODUCTION TO GENERAL PHYSIOLOGY 



dilate without dilatation of the arterioles or with constriction of 

 these latter. 



The Regulation of the Heart Beat. For the sake of simplicity 

 of description of the mechanism of adjustment by changes in the 

 blood vessels, we have supposed that the heart has continued to 

 expel the same amount of blood in equal times. But the heart has 

 its own powers of regulating its output of blood, and thus of main- 

 taining a good arterial pressure even when the peripheral vessels 

 are dilated, and of moderating it if there is much increase in peri- 

 pheral resistance by vaso-constriction. 



At one time this adjustment was thought to be of a very complex 

 nature, but the work of Starling and his co-workers has resulted in 

 reducing it almost completely to a comparatively simple " /aw of 

 the heart." It will be clear that the degree to which the ventricles 

 are filled by the time at which they contract depends on the amount 

 of blood which has flowed in during the pause between two beats. 

 Supposing that the heart muscle always contracted to the same 

 extent, then the fact that there was more blood in the ventricular 

 cavity would not result in more being expelled. In fact, the 

 opposite would occur, on account of the less mechanical advantage 

 of the tangential force when the curvature is less, as it is in the more 

 distended ventricle. When, therefore, the arterioles of an extensive 

 region of the body dilate, as in the muscles in running, or the 

 alimentary canal in digestion, the larger quantity of blood entering 

 the heart from the veins would only be sent on incompletely, and 

 the benefit of the vaso-dilatation would only imperfectly be realised 

 on account of the large fall in arterial pressure. Moreover, the 

 blood flow through the lungs would not be increased, and the urgent 

 need for more oxygen would fail to be satisfied. In actual fact, 

 however, this is not the case. It is found that the ventricles of the 

 heart expel more blood per beat the more they contain to begin 

 with, unless the initial distension is excessive. 



Further, this behaviour is shown by the heart when separated 

 from the central nervous system, so that it is due to some inherent 

 property of the heart muscle itself. 



Again, it is found that when the arterial pressure is raised, the 

 amount of blood expelled by the heart in a given time is not 

 decreased, although to raise the same volume to a higher pressure 

 requires more work. How is this to be explained ? Suppose that 

 a particular heart is working with such an expenditure of energy 

 that the arterial pressure is kept at a mean value of 80 mm. of 

 mercury, and that to do this 8 c.c. of blood are expelled at each 

 beat. It is to be remembered that the pressure against which the 

 heart works rises from about 65 mm/at the beginning of the out- 

 flow to 100 mm. at the end. The peripheral resistance may now 



