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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



volume, much as the functional residual capacity of 

 the lungs comprises the expiratory reserve volume 

 and the residual volume. Correspondingly, the 

 amount of blood accumulated in the ventricle at the 

 end of ventricular diastole (often called the end- 

 diastolic volume) could be referred to as '"diastolic 

 capacity," including the stroke volume plus systolic 

 reserve volume plus residual volume. Since the stroke 

 volume varies with changes in activity, the diastolic 

 capacity is not a fixed amount but functionally 

 variable. It becomes larger as the stroke volume 

 increases and smaller as the stroke volume decreases. 

 There are two more terms which can be described in 

 parallelism with the nomenclature used in respira- 

 torv physiology. The volume level reached at the end 

 of systolic ejection is termed '"end-systolic level" and 

 corresponds to the expiratory level of the lungs. The 

 volume level reached at the end of diastolic filling is 

 called ""end-diastolic level" in analogy to the in- 

 spiratory level of the lungs. 



The importance of a clear terminology for the 

 description of the dynamic shifts of ventricular 

 volumes under varying conditions of activity has 

 been pointed out by Rushmer (139) and many 

 others have followed his lead. The parallel with the 

 lung volumes also permits useful analogies. For in- 

 stance, an increase in residual volume of the lungs 

 in emphysema diminishes the ventilatory efficiency. 

 In a somewhat similar manner an increase in the 

 ventricular residual volume, such as occurs in ex- 

 cessive ventricular dilatation, diminishes the pump- 

 ing efficiency of the heart. 



To compare with the cardiac ventricles, a piston 

 pump would need the following features. The course 

 of the piston, which defines the stroke volume, would 

 have to be limited in order to leave fluid in the pump- 

 ing chamber at the end of ejection (functional 

 residual capacity). If a greater output were needed, 

 the piston would have to push farther and increase 

 its stroke volume by encroaching upon the systolic 

 reserve volume. Yet the volume filling the dead space 

 of the pump (residual volume) could never be ejected. 

 The diastolic reserve volume would be represented 

 by a farther pulling back of the piston to allow greater 

 filling of the pump chamber. In this mechanical 

 system, the need for a greater output could be met 

 instantaneously by the ejection of part of systolic 

 reserve volume. However, the diastolic reserve 

 volume could not be utilized instantaneously because 

 the pump chamber has first to be filled to a greater 

 extent before more can be ejected. The situation 

 seems to be the same in the heart. The left ventricular 



stroke volume can be increased from one heart beat 

 to the next by drawing upon the systolic reserve 

 volume, as occurs for instance when the organism 

 passes abruptly from rest to exercise (33, 139, 146). 

 On the contrary, the mechanism of greater diastolic 

 filling (Starling'., law) always involves a brief delav 

 brought about by the need for greater venous return 

 before increased ejection. Apparently, onlv in 

 strenuous exercise and in some pathological condi- 

 tions is the diastolic reserve volume called upon. 



Indeed the ventricular stroke volume varies almost 

 continuously and is not identical from beat-to-beat 

 even under resting conditions. Some of the variations 

 are probably caused by the fluctuating play of poorly- 

 known neural feedback processes. Others are caused 

 by mechanical forces such as those which accompanv 

 respiration. In fact the respiratory variations of the 

 ventricular stroke volume are remarkable even under 

 resting conditions (21). Figure 22 shows the typical 

 changes in right ventricular output during five 

 heartbeats modified by the action of one respiratory 

 cycle. After the onset of inspiration, .-1, there is first 

 an increase in venous inflow (second heartbeat) and 

 then in ventricular stroke volume (third heartbeat). 

 Similarly, the drop in venous return during expira- 

 tion (at the fourth heartbeat) is reflected by a de- 

 crease in stroke volume one beat later. From this 

 record it appears that the right ventricle temporarily 

 accommodates part of the large inspiratory inflow of 

 venous blood, and releases it into the pulmonary 

 circulation during the respiratory pause. This in- 

 dicates that with respiration not only the stroke 

 volume varies but also the functional residual capacity 

 (or end-systolic level ) . 



The functional residual capacity cannot be meas- 

 ured directly in the intact organism. Most of the esti- 

 mates obtained with indirect methods display con- 

 siderable variation according to the technique 

 emploved. The volume curves shown in figures 16 and 

 1 7 were obtained with multiple plane high-speed 

 X-ray cinematography. They show not only the 

 volume changes throughout the cardiac cycle, but 

 also the end-systolic level. The functional residual 

 capacity of the left ventricle of these 12-kg dogs 

 amounts to approximately 5 to 6 ml. In unanes- 

 thetized, quiescent dogs, the values measured were 

 of the same order of magnitude. Gribbe et al. (56, 57) 

 estimate that on the average the stroke volume of 

 dogs is 60 per cent of the diastolic capacity. Thus 

 functional residual capacity amounted to 40 per 

 cent of the diastolic capacity. It should be pointed out 

 that Gribbe's values are much smaller than tht 



