Im not able to get this concept of haemodynamics. Constriction of blood vessels increases arterial pressure but it also increases resistance, so how does cardiac output increase? How can constriction of vessels increase venous return? It incteaes systolic filling pressure but resistance also increases.? Can anyone relate all this to sympathetic stimulation.? Im really confused. Thabk you.
I know nothing of biology, but from the verbal mathematical model that states: [tex]R \propto \frac{1}{r^4}[/tex] where: R = resistance r = radius of vessels R is bound to increase as r decreases (which is what happens when vessels constrict, their radius decreases) Plug in some numbers in r and you will see it matches the mathematical model. From here (about half a page down) cardiac output is defined as: [tex]CO = HR x SV[/tex] where: CO = Cardiac output HR = Heart rate SV = Stroke volume The stroke volume may not increase with increased resistance, but the heart rate will increase to try to keep everything working smoothly. Therefore, CO increases as well. (Of course, that inference is assuming HR increases way more than what SV decreases. Otherwise they just balance each other out.) Pressure increases due to the lower pipe diameters causes some of the blood to flow back (and other forward, depending on where the constriction took place and what we define as "back" and "forth"). That at the same time triggers an increase in the heart rate to prevent this from destabilizing the flow which answers the previous question as well.