Background: I am coming at this question from an electrical engineering background, and I feel like I am missing certain assumptions that are going into the statement found in my physiology textbook, "vasoconstriction increases blood pressure"."
Consider a simple series circuit and a parallel circuit run by a battery/heart *[you will find the parallel and series circuit in any physiology book description of the vasculature, yet I can't find any exploration of the assumptions made when applying these circuit models]:*****
In the series circuit, if I have an increase in resistance across one of my resistors, this will basically redistribute the pressure drops across the resistors, but it will not alter the total pressure drop across all the resistors [fixed by the heart].
In the parallel circuit, if I have an increase in resistance across one of my resistors, this will redistribute the flow to different branches, but the pressure drop will not change as again this is fixed by the heart.
This analysis seems to suggest that if the resistance across an organ [branch of parallel circuit] changes, the flow changes, not the pressure. The heart, I would assume, then responds by increasing the pressure to increase flow ie actually injecting energy into the system.
Here is the problem with the circuit model though:
- it assumes that the battery/heart is the only source of energy in the system, and the resistors are passive re distributors of that energy.
-the controllers of vascular resistance are smooth muscle which must actively put energy into the system to vasoconstrict. This could be a source of pressure increase as the smooth muscle would be actively constricting against an incompressible fluid, but I am really not sure.
- it does not account for the compliance of the vasculature.
-the tubing the heart is hooked up to modifies the blood pressure the heart has to generate to inject fluid into that tube. If the tubing was stiff, the heart would have to generate very high systolic pressures that would then rapidly decrease during the diastolic phase. The more compliant the tubing, the less pressure the heart has to generate to inject fluid into the tube. Intuitively though, there would seem to some relationship between the ability of a fluid to flow and vessel compliance. A highly compliant vessel with a fluid injection will simply expand and hold the fluid while a less compliant vessel will maintain a pressure necessary to push the fluid along.
Sparknotes in the form of questions:
*1.Is the only source of energy in the cardiac circuit the heart? Or does artiole smooth muscle actually inject energy into the system, and result in systemic increases in the pressure available in the closed circuit? 2. I don't think vascular compliance ie expansion of the artery walls due to volume filling results in any active injections of energy into the system..it should simply transfer the energy available to push fluid to elastic energy in the connective tissue of the artery walls. Is this correct? 3. Does vessel compliance partly determine the pressure the heart has to inject into the system? 4. What is the relationship between compliance and flow if there is one?*