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HANDBOOK OF PHYSIOLOGY 



CIROl'LATION II 



sancc who recognized the heart as a hollow muscle 

 and probably as a pump was the artist-engineer, 

 Leonardo da Vinci (1452-1519 A.D.), who stated: 

 "The heart is a principal muscle, in respect of force, 

 and it is much more powerful than the other muscles" 

 [Keele (90)]. However, it remained to William 

 Harvey ( 1 578—1657) to prose that the heart, and not 

 the liver, is the center of the vascular system and 

 that it propels the blood unidirectionally by its 

 rhythmical contractions as would the repeated 

 strokes of a man-made pump. The microscopic proof 

 of the muscular nature of the heart was brought by 

 Niels Stenson (1 638-1 686), who demonstrated that 

 the substance of the heart is composed of fibers, 

 membranes, arteries, veins, and nerves just as is the 

 substance of other muscles. Once this important point 

 had been firmly established, it became customary to 

 consider the heart as a pump, to develop analogies 

 with mechanical systems of fluid transfer, and to 

 apply to the myocardium the increasing knowledge 

 about skeletal muscle contraction. The present 

 chapter is a rather general and classically oriented 

 treatment of the mechanical function of the heart. It 

 attempts to provide an understanding of the anatomi- 

 cal structures, while avoiding teleological oxer- 

 simplification as well as useless controversies about 

 functions. 



The role of the heart consists of providing the body 

 tissues with a continuous stream of blood. The heart 

 fulfills this function by converting potential energy 

 (primarily chemical energy, secondarily energy of 

 position) into kinetic energy, as movement is imparted 

 to the blood ejected from the ventricular cavities. 

 From the standpoint of cellular function at large, it 

 does not matter whether tissue perfusion is brought 

 about by alternate contraction and relaxation of 

 myocardial cells, or by the action of an artificial 

 pump. This concept has been established on a firm 

 experimental basis by the advent of extracorporeal 

 circulation techniques, whereby a mechanical pump 

 substituted for the human heart can fully support the 

 circulation. Thus the heart can be looked upon as a 

 pump inserted in the circulatory system and its 

 function can be described by analogy with purely 

 mechanical systems. 



Mechanical pumps are divided into two main 

 classes: kinetic pumps and positive displacement 

 pumps. In the former class, kinetic energy is added to 

 the fluid by the forced rotation of an impeller (fig. 

 lA). In the latter class, the fluid is progressively 

 displaced from a suction inlet to a discharge opening. 

 Two kinds of positive displacement pumps need to 



fig. i . Mechanical analogues for some pumping principles 

 embodied in the heart. A: kinetic pump in which energy is 

 added to the fluid by the rotation of an impeller. B: rotary 

 pump in which fluid is propelled through squeezing a resilient 

 tube by means of rollers mounted on a rotating arm. C: recipro- 

 cating pump in which fluid is displaced by the back and forth 

 movement of a diaphragm while valves give direction to the 

 stream. 



be mentioned here. In rotary pumps (fig. \B), 

 moving members trap a portion of the fluid in a 

 chamber of pliable tubing and conduct it toward the 

 outlet. The segment of tubing occluded acts as a 

 valve to prevent backflow. In reciprocating pumps 

 (fig. 1 C) a cavity limited by two valves is subjected to 

 the action of a piston or diaphragm. As the piston 

 moves back and forth, fluid is drawn in through the 

 suction valve and forced out through the discharge 

 valve. 



The action of the heart in some invertebrates can 

 be compared to that of rotary pumps, since forward 

 movement of fluid is obtained by peristaltic move- 

 ments of the walls. In the mammalian heart also 

 some degree of blood propulsion may be accomplished 

 on the ''progressive cavity principle" as in rotary 

 pumps, particularly the displacement caused by the 

 wringing action of the ventricles. However, cardiac 

 action in vertebrates most closely resembles that of 

 reciprocating pumps. It is characterized by pulsatile 

 action, by the presence of valves, and by the capa- 

 bility of the pump to be adjusted in terms of either 

 speed, or volume displacement, or of speed and 

 volume displacement simultaneously. Although the 

 design of the heart has nothing in common with that 

 of kinetic (centrifugal) pumps, its control displays 

 two characteristics for which kinetic pumps are 

 appreciated in technology: namely that the volume 



