OUTPUT OP HEART AND VENOUS INFLOW 219 



to the relation between active and passive diastole prior to the exercise. The venous 

 inflow also increases because of the muscular activity, so that the diastolic filling is 

 more complete, the initial tension higher, and the systolic discharge greater. As the 

 muscular activity continues, the heart rate continues to accelerate, partly because of 

 increase in the temperature and in C H of the blood (see pages 162 and 168) and 

 partly because of increase in venous pressure (Hooker an d Bainbridge). This in- 

 creased pressure, probably by the tension produced on the walls of the ventricle (right) 

 during diastole, sets up afferent impulses which act on the vagus and sympathetic 

 cardiac centers. 



Dilatation and Tonus 



We have seen that in the healthy heart the extent of dilatation and tension 

 during diastole determines the discharge during systole. When the dilation is not 

 accompanied by a large discharge (i. e., the volume does not diminish properly during 

 systole), "pathological dilatation" is said to exist. The essential difference between 

 physiological and pathological dilatation depends on the presence or absence of tonus, 

 by which is meant a sustained partial contraction ' ' by virtue of which the muscle 

 fibers resist stretching more than they would by virtue of inherent elasticity." In 

 other words, for a given volume of venous inflow, a greater pressure will exist in the 

 ventricle when the tonus is high than when it is low. Being, therefore, already partly 

 contracted at the end of diastole the tonic heart will develop greater force than the 

 atonic, during systole; it has less slack to take up, as it were. 



The Dynamics of the Circulation in Heart Disease 



When the myocardium is weakened, as after debilitating diseases or in poisoning 

 by chloral, chloroform, etc., the systolic force is diminished so that all the blood is 

 not discharged with each beat, and the ventricle consequently becomes overdistended 

 by the inflow of the venous blood. Although the elongation of the fibers and the 

 greater tension to which they are subjected does not at first succeed in calling forth a 

 sufficiently greater systolic contraction to maintain an adequate discharge, as would 

 be the case in health, a compensation may ultimately set in and the heart contract 

 more powerfully so that the intraventricular pressure increases sufficiently to maintain 

 the normal discharge. If this compensation does not occur the pressures rise in the 

 pulmonary circuit without, however, causing as a rule serious congestion of the lungs 

 because the systolic discharge of the right heart is lessened to a corresponding degree 

 to the left. The congestion therefore usually occurs further back in the venous sys- 

 tem and in the liver. Both first and second sounds are weak until compensation oc- 

 curs. Cardiac stimulants such ;is strophanthin and digitalis assist in the development 

 of compensation. 



When the aortic pressure is decidedly increased, the pressure maximum of the suc- 

 ceeding systoles is insufficient to expel all the blood that has collected in the ventricle 

 during diastole. The resulting greater stretching and initial tension within the ven- 

 tricle immediately stimulate more powerful contractions and if the pressure be not too 

 great the discharge returns to the normal in a few beats. If the pressure be excessive 

 however, or if it be maintained for too long this compensation begins to fail, the iso- 

 metric phase of systole becomes more gradual, the first sound becomes feebler (while 

 the second sound is still intense) and the venous bloods dams back in the left auricle 

 and pulmonary veins in which the pressure becomes raised so that, with the right 

 ventricle meanwhile discharging its full load of blood, extreme congestion of the lungs 

 occurs. Only later does the right ventricle fail to maintain its output and when this 

 occurs venous congestion a IK! fall of pulmonary pressure become evident. 



