854 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



fig. 20. Computed renal blood flow. 

 P = the abdominal aorta pressure at 

 the level of the renal artery. dP/dt = 

 the first derivative of the abdominal 

 arterial pressure. F c = computed blood 

 flow using equation 16. The lower 

 tracing is measured blood flow using 

 the square-wave electromagnetic flow- 

 meter on the renal artery. 



dt 



Fc 



v\. w Vy 



500 



Femf 



0. 5 sec 



-^vv' 



ventricular pressure where coronary resistance 

 is a complex function of vasomotor tone, intra-arterial 

 pressure, and intramuscular pressure. The function of 

 intramuscular pressure which reduces inflow during; 

 systole increases venous outflow during systole and 

 may also increase capillars- flow at the same time. 



IV. FLOW IN THE SYSTEMIC VEINS 



Phasic variations in venous blood flow result from 

 three principle sources: /) the beat of the heart; 

 2) the respiratory fluctuations; and j) the contraction 

 of skeletal muscle. Severe changes in position and 

 acceleration of the body also may have profound 

 effects on venous flow. Pulsatile flow originating from 

 the heart beat may occur in the small peripheral 

 veins, as a result of transmitted oscillations from the 

 arterial system. These pulsations are generally small 

 in the normal condition because of the damping 

 action of the resistance of the small arteries and 

 arterioles and the elasticity of the capillary bed. They 

 may be accentuated, however, by vasodilatation, 

 either by reactive hyperemia or by means of drugs, 

 such as acetylcholine. Flow in the renal vein, normally 

 phasic presumably because of the low renal vascular 

 resistance, causes less damping than in most vascular 

 beds. Great variations in blood flow within the 

 thoracic vena cava have been recorded by Brecher 

 (5) and others. 



Effect of the Heart's Action on Vena Caval Flow 



It has been shown by Gauer and Sieker [quoted in 

 (5)] that there is an almost immeasurable gradient 



in the mean blood pressure along the venae cava 

 toward the heart. Since there is a net movement of 

 blood in that direction, some small gradient must be 

 present which may be sufficient, in view of the large 

 size of the channels, to move considerable blood. It 

 is also true, according to the principles of vascular 

 hydraulics discussed in sections II and III, this 

 chapter, that considerable blood may be moved by 

 an oscillatory pressure gradient without consideration 

 of a mean frictional gradient (fig. 23). 



Atrial contraction injects a late diastolic quota of 

 blood through the tricuspid valve and also causes a 

 pressure transient to pass along the vena cava away 

 from the heart. This pressure transient produces a 

 sharp reduction in flow which may or may not cause 

 a reversal depending upon its amplitude and the level 

 of mean flow (fig. 24). This impediment or reversal 

 is, however, overcome immediately by a large for- 

 ward flow caused by ventricular contraction. This 

 "vis a fronte" which draws blood toward the heart 

 during ventricular systole arises from movement of 

 the base of the heart (tricuspid valve closed) toward 

 its apex, producing a transient pressure gradient in 

 favor of flow toward the heart. Flow may be expected 

 to follow this differential pressure transient, approxi- 

 mately 90 degrees out of phase. 



As the base of the heart moves away from the apex 

 during diastole, the tricuspid ring dilates and filling 

 of the heart takes place as much by the ventricle 

 sliding over the atrial blood as by the atrial blood 

 flowing into the ventricle. From direct observations 

 of the heart and from slow motion movies, it can be 

 seen that the ventricle fills by a) dilation of the tri- 



