540 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



cardiovascular function is a characteristic shared by 

 healthy dogs and people. This aspect of cardiovascular 

 function should be enough by itself to discourage 

 attempts to explain cardiac control in terms of any 

 simple generalization. 



POSTURAL RESPONSES 



When a healthy dog is fully relaxed and recumbent, 

 its heart rate is relatively slow, ranging between 70 

 and 90 beats a minute, and pronounced sinus ar- 

 rhythmia is often present. Continuous recordings of 

 cardiac dimensions demonstrate that the left ventricle 

 expands rapidly during diastole to reach a steady 

 size (diastasis) early in the diastolic interval. This 

 size persists with little change, even during atrial 

 systole, until the next systolic contraction. Under 

 these conditions the left ventricle functions at or 

 near its maximal diastolic dimensions. The validity of 

 this statement is evidenced by the fact that the filling 

 pressure can be increased some 15 to 20 mm Hg by 

 intravenous infusions without the diameter of the 

 left ventricle increasing as much as a millimeter (36). 

 In the relaxed recumbent dog, the systolic changes 

 in the ventricular dimensions are greater than those 

 observed in this laboratory under any other condi- 

 tions, with the possible exception of the beats follow- 

 ing extrasystolic compensatory pauses. Confirming 

 this impression is the observation that the stroke 

 volume recorded at the aortic root by an ultrasonic 

 flowmeter is also as large as that under any other 

 condition observed in these studies (13). The peak 

 flow velocity is higher when contractility has increased 

 (figs. I and 2), but the area under the velocity flow 

 curve (stroke volume) is apparently maximal or 

 almost maximal in the recumbent dog. Any condi- 

 tion — spontaneous, induced, or pathological — which 

 produces a very slow heart rate would quite certainly 

 produce this maximal degree of distention and a large 

 stroke volume. It is recognized that stroke volume 

 may increase under extremes of exertion to values 

 well in excess of those observed during relaxed re- 

 cumbency (see below). 



When the animal shifts his position, sits up or 

 stands, the left ventricular dimensions promptly 

 diminish; at the same time the systolic deflections, 

 peak flow velocity, and stroke volume decrease (33). 

 Thus, the reduction in diastolic distention is accom- 

 panied by a reduction in the amount of energy 

 released by the ventricle in a manner predictable from 

 the Frank-Starling mechanism. In dogs, the trunk is 



vent'' * STANDING— •!♦ 



DIAM 

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J2Q_HEART RATE 



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Hq LEFT VENTRICULAR PRESSURE 



120-- r^ 



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DIASTOLIC PRESSURE 



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^*^^^ 50 mm /sec 1 mm /$ec 



FIG. 4. When a dog lies down, the left ventricular diameter 

 promptly increases to a higher level, associated with increased 

 systolic deflection and stroke work in accordance with the 

 Frank-Starling mechanism. This change in diameter is not 

 necessarily accompanied by a change in either heart rate or 

 effective filling pressure as nearly as this can be ascertained. 

 This observation suggests that ventricular "distensibility'" may 

 have changed, but this cannot be definitively established (see 

 text). 



horizontal in both the recumbent and standing 

 positions, and these changes in ventricular dimensions 

 frequently occur without changes in diastolic filling 

 pressure (fig. 4). Clearly, this response does not 

 depend upon a vertical orientation of the long columns 

 of blood even though it can be readily induced by 

 passive tilting of the animal (fig. 5). In fact, a similar 

 reduction in ventricular dimensions, stroke volume, 

 and ejection velocity may occur when a recumbent 

 dog merely lifts his head in response to a sudden 

 noise. 



In normal human subjects, the area of the cardiac 

 silhouette on roentgenograms has consistently been 

 observed to be maximal in the recumbent position 

 and to be reduced promptly when the subject stands 

 up (23, 27, 28, 30, 45). For example, Sjostrand (45) 

 reported that the heart volume is greater during 

 recumbency than during standing or sitting. The 

 diastolic volume was either unchanged or decreased 

 when adrenaline, noradrenaline, atropine, nitro- 

 glycerin, or digitalis in toxic doses was administered. 

 Consequently, under optimal conditions of heart 

 filling, as exist in recumbency, the heart appears to 

 to fill maximally with blood during diastole. 



When the subject stands up, blood accumulates 

 in the \eins of the dependent parts and the quantity 



