Haemodynamics Flashcards
What trends follow the progression of vessels from the heart
BP drops from large arteries to small arteries and arterioles
Increase of total cross sectional are
Velocity also decreases with this. (inverse relation v inversely proportional to flow)
Max CSA and minimal velocity in the capillaries
What is ohms law in terms of circulation
Flow = change in pressure / resitance
How is the circulatory system set up?
In parallel, so a chnage in resistance at one organ will not alter the TPR. (note that main reistance is in the precapillary vasculature)
What factors influence flow
Pressure gradient; pi; radius
inversely- viscosity; length
equation
What is important about the radius?
That radius is raised to the power of 4 when related to flow, so a r x 2 will mean q x 16.
Thus vasodilators will impact blood flow a lot
What factors alter blood viscosity
Temp: increase temp, decrease viscosity
haemotocrit (more RBCs means more viscous)
Shear rate: at low flow velocities, more viscous as cells aggregate, so faster blood, apparent decrease in viscosity
vessel diameter: in a branch, apparent viscosity decreases as lower haemotocrit
What is the idea of distensible vessels?
That when you increase arterial presure, the vessels walls expand, further increasing flow, less resistance. vice versa so decreased pressure means less stretch, more resistance, less flow
What is shear stress?
If you increase flow, the stress causes endothelium to release NO, which causes vasodialtion. As we know increasing the area will decrease velocity! good as reduces the shear stress on the vessel
When does turbulent flow occur and what does it mean
high flow velocities or low fluid viscosities. (hundies flow)
Changes so that a greater pressure (square in fact) must be achieved to garner the same flow
Reynolds number eg stenosis or aneurysms
Bernoulli principle
Due to energy conservation
Increasing velocity somewhere (narrower vessel) will decrease the pressure.
may cause turbulent flow afterwards- aneurysms, e bad
What is the Laplace law
Larger vessel radius, larger wall tension required to withstand an internal pressure.
So tension is proportional to radius and pressure, inversely proportional to wall thickness (so increasing wall thickness will reduce tension)
Think of capillaries being alg, aneurysms are more likely to rupture, hypertension increasing wall thickness as a means to reduce tension.
For an enlarged heart, the increased wall size means more tension must be generated to develop the same pressure
What vessels are more and less complaint?
Arteries are less compliant (a small change in volume needs a large pressure) and veins the opposite
