1 424 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



inadequacy of current concepts of cardiovascular 

 regulation and stimulating numerous new investiga- 

 tions of the hepatic circulation. 



Physical Determinants 



Neural, neurohumoral, and local chemical influ- 

 ences operate for the most part reactively to alter 

 vascular perfusion and content by changing arteriolar 

 cross section and venous distensibility. As such they 

 mediate adjustments but do not actually produce 

 them. In contrast, external physical forces to which 

 the abdominal viscera are exposed affect the hepatic 

 and splanchnic vessels directly, frequently eliciting 

 corrective or compensatory responses in which all the 

 mechanisms discussed above are called into play. 

 Movements and gaseous distention of the gastroin- 

 testinal tract compress, stretch, and variously deform 

 the mesenteric vasculature with resultant change in 

 flow and volume that must affect the delivery of blood 

 to the liver by the portal vein. To what extent shifts 

 within the mesenteric circuit, repartitioning of the 

 total hepatic inflow, and redistribution of the splanch- 

 nic blood volume depend upon activity of the mus- 

 culature of the gut remains uncertain. These problems 

 are dealt with at length in Chapter 42 but must be 

 touched upon here in order to indicate the potential 

 importance of extravascular forces upon the hepatic 

 blood supply. In this instance a reciprocal relationship 

 obtains in that digestion, absorption, and the other 

 functions of the gastrointestinal tract depend in turn 

 upon the integrity of the mesenteric vasculature. 

 Moreover, the integrated function of the liver and 

 intestine hinges upon movement of absorbed material 

 through the portal system to the liver. Exercise and 

 respiration also impinge directly upon the hepatic 

 circulation by raising intra-abdominal pressure and 

 often by changing the position of the body. Both 

 activities are associated with widespread cardiovas- 

 cular changes affecting all parts of the body. The 

 splanchnic vasculature participates in these reactions 

 but it may actually be more markedly influenced 

 directly by the associated physical effects. 



intra-abdominal pressure. The roughly cylindrical 

 container holding the abdominal viscera and their 

 vasculatures is almost completely muscular and 

 capable of actively increasing the pressure within the 

 peritoneal cavity. Moreover, the gastrointestinal 

 system is periodically filled with fluids, solids, and gas, 

 which occupy space and raise the pressure. As long 

 as the pressures distending the vessels are in excess 



of the circumambient pressures, there is little effect 

 upon the resistance to flow or upon the forces con- 

 ducive to blood flow. When the external pressure 

 equals or exceeds the intraluminal pressure, by an 

 amount determined by the elastic properties of the 

 vessel, instability develops and collapse occurs at 

 some critical pressure, provided the contents can be 

 displaced. Any pressure increment is uniformly dis- 

 tributed throughout the abdomen, and blood, not 

 being compressible, must move from an area of high 

 pressure to one of low pressure to permit collapse of 

 those vessels in which the intraluminal pressure is 

 less than the external pressure. This occurs most 

 readily at the diaphragm, across which the usual 

 pressure drop is augmented. It may be inferred that 

 with a rise in intra-abdominal pressure, blood in the 

 veins close to the diaphragm is expressed and the 

 vessels collapsed, thus producing a dam at the point 

 of outlet. Since the arterial perfusing pressure is little 

 affected, blood continues to pour into the system, 

 but with the cessation of outflow, pressures gradually 

 rise and inflow begins to diminish. When the local 

 pressure at the point of collapse exceeds the critical 

 opening pressure, outflow is restored and a new equi- 

 librium established. Whether the expected increase 

 in resistance so induced would tend to result in pool- 

 ing of blood within the splanchnic circuit in the face 

 of the forces operating to prevent distention must 

 depend upon the relationship between the distribu- 

 tion of resistances and pressure gradients within the 

 bed. Little information is available on which one may- 

 base further speculation. Increased intra-abdominal 

 pressure has been found to reduce hepatic blood 

 flow in man and experimental animals (47, 221), 

 but precise localization of the sites of increased re- 

 sistance and the character of volume capacity changes 

 have not been satisfactorily settled. 



gravity. Gravitational force is also constantly opera- 

 tive in affecting intravascular pressures within the 

 abdomen. Change of position results in a change in 

 the hydrostatic pressure at every point in the vascula- 

 ture by means of an addition or subtraction of a 

 column of blood, the height of which depends upon 

 the elastic properties of the vascular system. In com- 

 puting the hydrostatic head at any point in the vessels, 

 it is customary to take the center of the right atrium 

 as the zero base line in recumbency. All pressures 

 above that level in the supine position are negative 

 with reference to it, and all below are positive. With 

 a shift in position, the levels at which pressure in the 

 arteries and veins remains unchanged mav be taken 



