Principles of haemodynamics Flashcards
What is haemodynamics
Relationship between blood flow, blood pressure and resistance to flow
Describe Darcy’s law
Difference between arterial pressure and venous pressure is the driving force causing blood to flow along the vessels. Flow is limited by the resistance of the vessel, or total peripheral resistance.
Flow = P1 - P2 / R
Flow = Pa - CVP / TPR
What other factors affect flow
Potential energy and kinetic energy
(Bernoulli’s Law)
Define blood flow
Volume of blood flowing in a given time (ml/min)
Define perfusion
Blood flow per given mass of tissue (ml/min/g)
Define velocity of blood flow
Blood flow (cm/s) affected by the cross sectional area through which the blood flows, so flow may remain the same but velocity changes if there has been a change in cross sectional area
Velocity in a vessel is equal to
Flow divided by cross-sectional area. Velocity can increase just by making cross-sectional area of the vessel smaller
How fast is blood flow at the capillaries and why
Velocity is very low because total cross sectional area is very huge, lot of small capillaries.
Describe laminar blood flow
Occurs in most arteries, arterioles, venules and veins
Concentric shells, zero velocity at walls, maximum velocity at centre
Moves RBCs towards centre, speeds up blood flow through narrow vessels
Describe turbulent blood flow
Occurs in ventricles, aorta and atheroma
Blood does not flow linearly and smoothly in adjacent layers, due to increased pressure and velocity
High resistance to flow
Describe bolus blood flow
Occurs in capillaries
RBCs have a larger diameter than diameter of capillaries so move in single file. Plasma columns are trapped between RBC
Uniform velocity, little internal friction and very low resistance
What is the Reynolds number (Re)
Way of predicting when turbulent flow will occur, in the circulation Re above 2000 are associated with turbulent flow.
REynolds number eqation
Re = pVD / u (Viscosity)
