principles of haemodynamics Flashcards
what is haemodynamics ?
how your blood flows
the relationship between blood flow , pressure and resistance to flow
what are the key factors in haemodynamics ?
Force - cardiac contraction
Work - isovolumetric contraction & ejection
Pressure - difference aorta to veins
Compliance - arterial stretch
Resistance - arterioles
Flow velocity - slowing down blood flow in capillaries
what is Darcy’s law ?
role of pressure energy in flow
what is Bernoulli’s law ?
role of pressure, kinetic and potential energies in flow – NOT JUST PRESSURE
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.
what is the equation for blood flow and what does it describe ?
blow flow (Q) = Velocity x Area
Greater the cross- sectional area, slower the flow, slowest in capillaries
Velocity increases with veins coming together
describe the 3 patterns of flow
laminar - most arteries , arterioles , venules and veins. Zero velocity at walls (molecular interactions). Maximum velocity at centre. Moves RBCs towards centre. Speeds up blood flow through narrow vessels
turbulent - ventricles , aorta. Blood does not flow linearly and smoothly in adjacent layers (whirlpools, eddies, vortices) due to increased pressure & velocity. High resistance to flow.
bolus - 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 & very low resistance.
what is the reynolds number ?
point at which laminar flows changes to turbulent flow
turbulence occurs when reynolds number exceeds a certain value.
what are the 4 key relationships and interactions of arterial blood pressure ?
Systolic pressure
Pressure when ejecting
Diastolic pressure
Pressure when relaxing
Pulse pressure
Difference between diastolic and systolic pressure
Mean blood pressure
Average pressure
what occurs to the arterial blood pressure during left ventricular systole ?
60-80% of stroke volume is stored in aorta and arteries as these structures expand.
Energy stored in stretched elastin.
what occurs to the arterial blood pressure during left ventricular diastole ?
Energy is returned to the blood as the walls of the aorta and arteries contract.
This sustains diastolic blood pressure and blood flow when heart is relaxed.
what is pulse pressure ?
what the finger senses
tells you about stroke volume and arterial compliance ( stretchiness)
= SV/ compliance
what happens to arterial blood pressure when we excercise compared to rest ?
During exercise greater stretch of the arteries as more blood is ejected causes less compliance and less recoil and the difference between systole and diastole increases ie pulse pressure increases.
what happens to arterial compliance at rest
Decreased compliance
Stroke volume now increases systolic and pulse pressure disproportionally
why does compliance decrease in the elderly ?
Increase in age – stiffer arteries (atherosclerosis) decreased compliance
why does pulse pressure become more noticeable futher down the aterial tree ?
because vessels become less compliant.
This is one reason to measure it in the radial artery quite far away from the heart.
what controls mean blood pressure ?
Age
Disease
Distance along arterial tree
Blood volume – affects SV, CO
Exercise – increased SV, CO
Emotion - stress, anger, fear, apprehension, pain
Wake/sleep - decreases BP 80/50 mmHg
