1078 



PHYSIOLOGY 



contraction will be found to be associated with a rise of pressure which 

 may be double or treble the normal extent, the arterial pressure 

 being kept at its normal height at the expense of additional work on 

 the part of the heart-muscle. This adaptation takes place even after 

 division of all the nerves which run from the central nervous system 

 to the heart, and is due entirely to the reaction of the muscle fibres 

 of the ventricles to the resistance which they have to overcome, i.e. 

 to the tension to which they are subjected. 



We have already seen that increased venous pressure leads to 

 increased diastolic filling of the heart, and that a distended heart 



is mechanically in a less favourable 

 condition for expelling its contents 

 (v. p. 1032). Nevertheless a healthy 

 heart reacts to increased diastolic 

 filling by increased systolic output. 

 The muscle fibres, stimulated by 

 their increased tension during dias- 

 tole, contract more forcibly and to 

 a greater extent ; so that the residual 

 volume of blood in the heart at the 

 end of systole may be very little or 

 no greater than it is in the undis- 



FIG. 440. Isometric contractions of fonrlorl VAarf 

 frog's ventricle. The initial ten- 



sion was continually increased This property of the cardiac muscle 



is responsible for the power of 

 'compensation' possessed by a dis- 

 eased heart. We may take as an 



example the destruction of one aortic valve, a lesion which can be 

 produced experimentally in a dog. In this case, immediately after 

 the lesion is established, no additional resistance is offered to the 

 expulsion of the blood, and the ventricle will send the normal amount 

 into the aorta. During the succeeding diastole the blood at a high 

 pressure in the aorta will leak back into the ventricle through the 

 damaged valve. The arterial pressure therefore falls rapidly, and 

 the ventricle receives blood from two sides, i.e. by regurgitation 

 through the aortic valves, and in the normal way from the auricles and 

 veins. At the end of diastole the ventricle is therefore overfilled. 

 Increased stretching of its fibres, however, has the effect of exciting 

 an increased contraction, and the heart at its next systole throws out 

 not only the normal quantity of blood but also that which it has 

 received back from the aorta. The arterial system thus receives at 

 each beat the normal quantity of blood plus the amount which leaks 

 back into the ventricle between each systole ; so that the amount 

 of blood remaining in the aorta and available for passage on to the 



energy of contraction. (V.FRANK.) 



