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increase in aortic blood pressure because of the increase in cardiac output. For reasons described later in this chapter, this will lead to a small increase in end-systolic volume; the net effect, however, will still be an increase in the width of the pressure-volume loop (i.e., increased stroke volume). The normal ventricle, therefore, is capable of increasing its stroke volume to match an increase in venous return. The increase in the area within the pressure-volume loop, which represents the ventricular stroke work, will also be increased.

Effects of Preload Length on Tension Development (Length-Tension Relationship)

The mechanical or biophysical basis for the Frank-Starling mechanism can be described by the length-tension relationship for cardiac myocytes. The length-tension relationship examines how changes in the initial length of a muscle (i.e., preload) affect the ability of the muscle to develop force (tension). To illustrate this relationship, a piece of cardiac muscle (e.g., papillary muscle) is isolated and placed within an in vitro bath containing an oxygenated, physiologic salt solution. One end of the muscle is attached to a force transducer to measure tension, and the other end is at tached to an immovable support rod (Fig. 4-11, left panel). The end that is attached to the force transducer is movable so that the initial length (preload) of the muscle can be fixed at a desired length. The muscle is then electrically stimulated to contract; however, the length is not permitted to change and therefore the contraction is isometric.

If the muscle is stimulated to contract at a relatively short initial length (low preload), a characteristic increase in tension (termed "active" tension) will occur, lasting about 200 m/sec (Fig. 4-11, right panel, curve a). By stretching the muscle to a longer initial length, the passive tension will be increased prior to stimulation. The amount of passive tension depends on the elastic modulus ("stiffness") of the tissue. The elastic modulus of a tissue is related to the ability of a tissue to resist deformation; therefore, the higher the elastic modulus, the "stiffer" the tissue. When the muscle is stimulated at the increased preload, there will be a larger increase in active tension (curve b) than had occurred at the lower preload. If the preload is again increased, there will be a further increase in active tension (curve c). Therefore, increases in preload lead to an increase in active tension. Not only is the

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