8 5 2 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



I 25 mm Hg 



fig. 1 8. Blood flow in the aortic branches. All flow ordinates 

 are scaled equally. The contours of the various flow pulses here 

 may be considered characteristic of the flow in the indicated 

 branches. Carotid flow and renal flow characteristically pulsate 

 around a mean value representing considerable continuous 

 forward velocity. Blood flow in the femoral artery, iliac artery, 

 abdominal artery, and brachial artery may, in the resting condi- 

 tion, oscillate through zero in early diastole but are also, under 

 the conditions of muscular exercise, or metabolic demands, or 

 vasodilator drugs, raised to a level corresponding to considera- 

 ble mean forward velocity. 



taneity of the peaks and troughs of the abdominal and 

 descending thoracic aorta, iliac, and femoral flow 

 pulses. The time of the initial rise is delayed according 

 to the transmission time between the two points under 

 comparison. 



Function of the Resonant Wave 



The finding of a large backflow component to the 

 flow wave in the descending aorta and vessels of the 

 extremities is at first surprising when viewed from 

 the point of efficiency needs of the circulation. This 

 finding, however, observed in the resting state of 

 dogs (31), sheep (F. C. Greiss, unpublished observa- 

 tions), and in humans (48), disappears upon exercise 

 of the extremities as the muscle vascular beds dilate 

 to accommodate a greater flow. 



The normal terminal impedance (peripheral 

 resistance) of the arterial transmission line is ap- 

 parently greater than the characteristic impedance 

 during the resting state. The vasodilator mechanisms 

 of exercise bring the terminal impedance down to and 

 below that of the line, thereby eliminating positive 

 reflections. Negative reflections do not arise because 

 they are damped out by the resistance of the larger 

 channels made more effective by increased flow. The 

 circulation is thus brought up to more efficient 

 operating conditions when the demands are increased. 

 All the pulse energy passing to the periphery is 

 completely absorbed without reflections when the 

 peripheral resistance is decreased by exercise, injection 

 of vasodilator drugs, and in peripheral A-V fistulas. 

 Figure 1 9 illustrates the action of lowered terminal 

 impedance in increasing the more efficient transfer 

 of energy. Reflection from the bed beyond, seen in 

 the control blood flow of a small artery in the dog's 

 paw, disappears under vasodilation conditions caused 

 by an intra-arterial injection of acetylcholine. The 

 resonant flow wave disappears and the flow is a 

 simpler function of the arterial pressure. Okino 

 [see (42)] has also recorded these changes. 



Renal Blood Flow 



The renal vascular circuit may be, as a first approxi- 

 mation, compared to a simple parallel RC circuit 

 (30). The dominant hydraulic elements of the renal 

 artery flow are resistance and compliance, and the 

 equation relating abdominal aorta pressure (Pi), and 

 renal artery flow (F) is: 



dt R 



(16) 



