FUNCTIONAL ANATOMY OF CARDIAC PUMPING 



7 6. 



output is directly related to the input pressure, and 

 is inversely related to the pressure head against which 

 the pump works. Like centrifugal pumps, the heart 

 has the tendency to deliver a higher flow as more 

 blood is fed into it at the atrial level; it also provides 

 a lower flow when the resistance to ejection in the 

 vascular system increases. 



A close look at mechanical pumps for cardiac 

 substitution throws a light on built-in features of the 

 natural heart that one easily takes for granted. 

 Adequate perfusion of an adult human organism 

 under all possible conditions requires that : 



/) The heart be able to move blood volumes 

 ranging from 3 to 30 liters per min and to pump 

 against pressures up to 300 mm Hg. 



2) Even at maximal cardiac output, the flow 

 velocity must not exceed the limit of tolerance for 

 mechanical trauma to blood corpuscles through 

 turbulence, friction, or cavitation (1-2 m/sec). 



3) The relationship between stroke volume and 

 stroke rate must not deviate much from an optimum 

 which is set by the elastic properties of the cardiac 

 walls, the time needed for efficient transformation of 

 potential into kinetic energy and by the lowest flow- 

 velocity compatible with the output required. 



4) The valves must easily open during their flow- 

 phase, yet be competent and prevent regurgitation 

 of blood during their holding period. 



5) The regulation of the pumping action must be 

 automatically controlled through sensing elements 

 with feedback mechanisms which adapt the output 

 to the tissue demands [see also Wagner (153)]- These 

 control mechanisms must integrate hemodynamic 

 data (e.g., perfusion flow, arterial and venous pres- 

 sures) and metabolic data (e.g., arteriovenous oxygen 

 difference) to maintain viable conditions. 



Considering these points in more detail, one must 

 first emphasize the pumping capacity of the heart. 

 As 3 to 30 liters per min of blood is pumped by the 

 left ventricle into the systemic circulation, practically 

 the same amount is ejected by the right ventricle into 

 the pulmonary vascular bed. Furthermore, the atria 

 have some pumping function of their own, so that 

 the combined pumping of all the chambers of the 

 human heart is in the order of 7 to 70 liters per min, 

 depending upon the state of muscular activity. A 

 range of this magnitude (1:10) is not easily obtained 

 in artificial pumps and, when it is reached, it is at the 

 price of considerable sacrifices in mechanical efficiency 

 (ratio of work produced to fuel consumed). On the 

 contrary, the mechanical efficiency of the heart does 

 not seem to be very closely related to cardiac output. 



The extended scale of activity over which the heart 

 can perform is certainly facilitated by the elastoviscous 

 properties of the cardiac walls. The cavities are 

 distensible over a wide range of volume increments 

 without much increase in intraventricular or intra- 

 atrial pressures [see fig. 2, and Little (99)]. Therefore 

 the heart can easily accommodate and deliver 

 varying stroke volumes even if the stroke frequency 

 remains unchanged. Furthermore the time needed 

 for the transformation of chemical into mechanical 

 energv apparentlv comprises only a fraction of the 

 systole. At a constant stroke volume the heart can 

 increase its minute output simply by beating faster 

 and shortening the pause between the strokes without 

 affecting the energy conversion processes. The limiting 

 factor of cardiac output at high heart rates is not an 

 encroachment on the time needed for energy con- 

 version but an encroachment on the time needed for 

 filling the pump chambers (ventricular filling phase). 

 Another fundamental difference between artificial 

 pumps and the heart is that in the former a force is 

 applied from the outside to activate a part or the 

 entire wall of the pump chamber, whereas in the 

 latter the force is developed within the wall of the 

 pump chamber itself by small elements, the muscle 

 fibrils, which alternately shorten and lengthen. 

 Furthermore, since the heart is surrounded by other 

 resilient structures in the thorax, there is an inter- 

 action of the physical forces developed in the myo- 

 cardium and those developed either passively or 

 actively in these structures [Pfuhl (129, 130), Blair & 

 Wedd (12)]. For example, during ventricular con- 

 traction and ejection the elastic forces of the lungs 

 oppose to a small extent the diminution of the 

 ventricular size, whereas during ventricular relaxation 

 the same forces of the lunsjs enhance slightly the 

 expansion of the ventricles. These forces are said to 

 be negligible as compared with the intravenous 

 filling pressures (60, 64). Mechanical effects are 

 exerted upon the rhythmical form changes of the 

 heart by such structures as the pericardium, the 

 attachments of the heart to the large vessels, the 

 sternum, the mediastinal tissues, and the diaphragm 

 through its changes in position during respiration or 

 because of varying degrees of abdominal filling. The 

 complexity of these forces, in terms of direction and 

 magnitude, and their continuous changes during the 

 cardiac and the respiratory cycle make it presently 

 impossible to evaluate quantitatively the contribution 

 of extracardiac structures to cardiac pumping. 

 Nevertheless, their importance is demonstrated by 

 the possibility of pumping blood solely by the action 



