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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



this concept until they come to recognize its profound 

 implications. While the application of Poiseuille's 

 law to collapsible tubes requires only minor modifica- 

 tions in arithmetic, the degree of determinacy of the 

 respective variables is radically different. For example, 

 there is a vast body of older literature which demon- 

 strates that venous pressure is not altered by a host 

 of physiological factors known to alter the regulation 

 of the cardiovascular system. The conclusion of these 

 authors, that the venous system therefore played no 

 part in cardiovascular regulation, may be a signifi- 

 cant reason why most textbooks are devoid of positive 

 statements in regard to venomotor mechanisms. 

 What these experiments actually proved was that 

 the various experimental maneuvers had no effect on 

 the extravascular tissue pressure which is the major 

 determinant of venous pressure; such measurements 

 are completely meaningless in reference to venous 

 tone. 



It would be a serious error, moreover, to regard the 

 collapsibility of veins as a structural defect in the 

 system which serves no better purpose than to com- 

 plicate the understanding of venous hemodynamics. 

 Since the venous system operates in the same pressure 

 ranges as the gravitational forces and tissue pressures 

 to which it is exposed, drastic disturbances would 

 result if veins were rigid tubes. Consider, for example, 

 an individual turning a handspring. If veins were 

 rigid, there would be drastic surges in venous blood 

 flow and chaotic alterations in venous return to the 

 heart. More conventional running and jumping move- 

 ments would seriously tax the homeostatic adjustment 

 of a low pressure system of cylindrical tubes. The 

 collapse mechanism serves to check such hydrostatic 

 shifts of venous blood. As soon as pressure in the 

 veins becomes reduced to tissue pressure levels, col- 

 lapse occurs to throttle flow and maintain the periph- 

 eral bed at more nearly normal functional levels. 



Duomarco and associates (22-24) have extended 

 this concept of venous hemodynamics to claim that 

 the design of a collapsible venous system guarantees 

 that extravascular factors capable of altering pressure 

 relationships can have no influence on venous flow. 

 Duomarco's enthusiasm for the teleological magnifi- 

 cence of such a scheme apparently exceeds the actual 

 facts. It must be appreciated that, with normal 

 blood volume, a significant fraction of the venous bed 

 is distended so that it does behave as a system of 

 cylindrical tubes. This will hold for most of the 

 extrathoracic veins which are below heart level and 

 which are not in regions subjected to significant 

 extravascular compression. Furthermore, the work 



of Brecher (10) has established that phasic pressure 

 changes are capable of producing phasic changes in 

 flow during the intervals when geometric adjustments 

 in the degree of collapse are taking place. Such 

 phasic pressure changes are conspicuous in intra- 

 thoracic and intra-abdominal veins in association 

 with respiration, and also seem to be a characteristic 

 manifestation of venous vasomotion in the small 

 peripheral veins (44). 



An additional word of caution should be appended 

 to emphasize that there are some important excep- 

 tions to generalizations as to the collapsibility of 

 veins. This is particularly true of venous structures 

 that are bound by connective tissue to rigid skeletal 

 elements which prevent their collapse, such as the 

 sinuses of the dura mater and the vertebral venous 

 sinuses. In these vessels, gravitational or respiratory 

 forces may lower the intraluminal pressure to values 

 significantly below the pressure existing on the out- 

 side of the vessel. A clinical consequence is the danger 

 of air aspiration into the vascular system if these 

 vessels are opened to the atmosphere during surgical 

 procedures or by accidental trauma. A similar prob- 

 lem exists to a lesser degree at the point where veins 

 enter the chest. The thyroid surgeon is well aware 

 that veins near the base of the neck have sufficient 

 connective tissue attachments so that traction may 

 pull open an incised vein that has not been securely 

 ligated, and aeroembolism result when inspiratory 

 pressure changes lower the central venous pressure 

 below atmospheric pressure. 



To qualify generalizations about venous collapse, 

 however, should not obscure the importance of this 

 phenomenon in venous function as a whole. Any 

 approach to the venous circulation which neglects 

 the collapsibility of veins will lead to serious distor- 

 tions of the hemodynamic factors which control the 

 flow of venous blood. 



Venous Distensibihty 



In view of the collapsible nature of veins, a vein 

 segment will empty freely from cut ends, the vessel 

 will flatten and all blood will leave the lumen except 

 for a minute amount retained within the folds on 

 opposite sides of the vessel. If fluid is now added to 

 this collapsed vessel, two theoretically distinct proc- 

 esses will occur. The first phase will be '"filling," 

 during which the geometry of the vessel wall is 

 restored to the cylindrical shape without increasing 

 in circumference. The succeeding phase will repre- 



