HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



INTERNAL-VOLUMES 

 (VIS A PARTE INTERIORE) 



(a) Blood volume 



vs 



(b) Vascular volume 

 ( i ) Rese r voirs- veins 

 (ii)Minute vessels - venules 



(VIS A FRONTE ) 



(VIS ATER^Q)^ 1^ oncrcCi in [T^I^fa) Normal frictional 



CAPILLARr^^^VENOUS PRESSURE <-^ resistance 



/ ;r 7 (b)Cajdlac 



/ X * II competence 



/ EXTERNAL-PRESSURES I J^ 



Vlecho nical 

 obstructions 



FIG. 4. Factors affecting venous pres- 

 sure. [After Landis & Hortenstine (41).] 



Normally, blood is propelled through the collect- 

 ing system by a pressure gradient of about 6 to 8 

 cm of water between the peripheral veins and the 

 right atrium [Brecher (5)]. The various coefficients 

 that determine the gradient are nicely summed up 

 in the diagram of Landis (41), reproduced as figure 4. 



Only a few points deserve editorial comment: the 

 author has been impressed with the major extent to 

 which the venous pressure gradient is controlled by 

 cardiac activity. For instance, when the cardiac 

 output increases, more blood is drained from the 

 atrium per unit time, central venous pressure de- 

 creases, and the pressure gradient augments through 

 reduction of the vis a fronle. Evidence clearly indicates 

 that the small and large veins contract as a result of 

 venomotor activity [Franklin (20), Gollwitzer- 

 Meier (Q2),McDowall (44)]. Further, veins participate 

 in vascular contractions reffexly induced from the 

 carotid sinus [Heymans & Neil (32)]. Despite con- 

 trary opinions, the present author believes that the 

 extent to which such reduction in venous capacity 

 augments venous return has not been established 

 experimentalh'. 



ADJUSTMENT OF CARDIAC OUTPUT TO 

 METABOLIC REQUIREMENTS 



It has long been recognized that enhanced metab- 

 olism is associated with increase in heart rate and 

 presumably with a greater cardiac output. The ques- 

 tion whether stroke volume also increases has been 

 debated for the past 60 years. At the beginning of tlie 



EXTERNAL- PRESSURES 

 (VIS A LATERE 

 (a)Cont r acti II ty of veins 

 (b)Pressure by skeletal muscle 

 (i) (Tone?) 



(i i) Pump and vol ves 

 (c) Ti ssue pressure 

 (d) Respirator y movements 

 (i)lntrat h or acic pressure 

 (i I) Intra - abdominal pressure 



present century, technical procedures were essentially 

 limited a) to metering aortic flow in rabbits, calcu- 

 lating volume flow per unit body weight, then ex- 

 trapolating results to man; and b) to estimation of 

 cardiac output in dogs and horses by use of the Fick 

 principle (1870, 1886). Nevertheless, Tigerstedt 

 (62) in his Lehrbuch (1897) ventured the guess that 

 the stroke volume in man ranged from 50 to too ml, 

 values that have subsequently proven correct (14). 

 But data acquired by such imperfect methods showed 

 such inconstant ratios between heart rate and cardiac 

 output that the two were considered independent 

 variables. This interpretation fitted in with Engle- 

 mann's concept that cardiac rhythmicity, excitability, 

 conductivity, and contractility could be altered 

 independently by humoral and nervous agencies. 

 On the other hand, experiments of Howell and 

 Donaldson (1884) on N. Martin's limited circulation 

 preparation had indicated that cardiac output varies 

 consistently with venous supply and right atrial pres- 

 sures [for details, see Tigerstedt (63, 64)]. 



During the first decade of the present century 

 physiologists began to question the accuracy of data 

 obtained by available methods and the applicability 

 to intact animals and man of values obtained from 

 use of partially or totally isolated hearts. Progress 

 during the next 30 years was beset with many frustra- 

 tions in attempting to develop better procedures 

 (46). The eff'orts inade to understand the laws of 

 cardiac performance and to estimate changes in 

 cardiac output in itian under different conditions 

 illustrate admirably that a goal in research is reached 

 only through development of a great number of 

 methodologies which, though inadequate, do offer 



