44 THE MECHANISM OF THE CIRCULATION. 



metres (or 1000 kilogramme-metres per hour). 1 The day's work of the 

 heart is therefore equivalent to 56-6 kilo-calories, or about one-fiftieth 

 of the total amount of heat produced in the body, in addition to which, 

 it must be remembered that there is a constant production of energy 

 in the form of heat in the heart muscle, as in other muscle, as a result 

 of its metabolic changes, and this heat is also taken up by the blood. 

 Thus the heart is an important mechanism for the production as well 

 as for the distribution of heat in the body. 



The Diastolic Filling of the Heart. 



In the excised heart no evidence of any suction power has been 

 observed ; indeed, the heart will only fill when supplied with blood under 

 a positive pressure. Similarly, when the thorax is opened, the heart 

 cannot fill Itself by its own power of suction ; this force must be aided 

 by a positive pressure in the thoracic veins. 



If the pressure in the vense cavre be not positive, then any negative 

 pressure which occurs in the heart cavities leads to a collapse of the thin- 

 walled venaz cavas, and not to suction of blood from the veins. In hydraulic 

 engineering the efforts of engineers are directed towards making the 

 water enter without shock. If the negative pressure had any decided 

 action, the blood would enter the heart with considerable shock, and 

 consequent loss of energy. This is contrary to what we should 

 expect in nature. The driving force of the heart is sufficient by itself 

 to complete the circulation and return the blood to the heart, when the 

 action of the respiratory pump is abolished by injection of curari, and 

 the thorax is opened. 



Normally, the filling of the heart is largely under the control of the 

 respiratory pump. In the closed thorax the pressure is less than that of 

 the atmosphere by the amount of tension which is required to overcome 

 the pulmonary elasticity, and expand the lungs to the size of the thoracic 

 cavity. In ordinary inspiration this tension is equivalent to 9 mm. Hg ; 

 in the position of deepest inspiration it may be 30 mm. Hg. On the one 

 hand, the extrathoracic veins are under a pressure which is nearly that of 

 the capillaries, and is thus somewhat above the atmospheric pressure ; on 

 the other hand, the intrathoracic veins and the heart are under a slight 

 but constant negative pressure, which in inspiration may be — 9 to 

 - 30 mm. Hq;. The venous blood is by these alterations in thoracic 

 pressure aspirated into the heart. If, in sequence to, or synchronously 

 with, the inspiratory movements of the thorax, the abdominal muscles 

 be thrown into contraction, then the abdominal veins are compressed, 

 and the respiratory muscles act on the venous circulation, not only as a 

 suction- but also as a force-pump. If the pressure in the thorax be 

 made sufficiently great, the efficiency of the heart as a pump ceases, 

 for the heart is unable to fill. Thus the diastole of the heart can be 

 prevented by raising the intrapericardial pressure. 



To carry out this experiment, a pressure bottle filled with oil is 

 connected by a "["-piece with an oil manometer, and a tube tied into the 

 pericardial sac. So soon as the pressure of the oil rises to 60-70 mm. oil, 

 the arterial pressure falls by 20-30 mm. Hg, while the vena cava pressure 



1 Estimates of more than double this amount are frequently given, but they have been 

 based upon a far larger output and a higher aortic pressure than (for reasons subsequently 

 to be given) have here been adopted. 



