Principles of Haemodynamics Flashcards
Define haemodynamics
The relationship between blood flow, blood pressure and resistance to flow
Which factors affect haemodynamics
1) Force- controlled by cardiac contraction
2) Work- isovolumetric contraction then ejection
3) Pressure- difference in pressure between aorta and veins
4) Compliance- ability of arteries able to stretch
5) Resistance- arterioles
6) Flow Velocity- how blood slows down when it reaches the capillaries
Central venous pressure
Blood pressure in the vena cavae, near the right atrium
CVP reflects the amount of blood returning to the heart
CVP also reflects ability of heart to pump blood back into arteries
How can you increase cardiac output in terms of the venous system
Blood reservoirs in veins are mobilized through innervation from ANS
Veins contract
Pulmonary vein contracts》more blood in left ventricle》stretch》Starling’s law 》increases stroke volume so more blood ejected per heart beat
Darcy’s law
Role of pressure in blood flow
Darcy’s law is flawed; blood flow is not just determined by pressure as blood can flow against a pressure gradient, as well as flow when there is no pressure at all
Equation to calculate blood flow
Pressure difference
Blood flow= ——————————‐———
Resistance
Q = (P1-P2) / R
If resistance goes up, amount of blood flow goes down
Bernoulli’s law
Role of pressure, kinetic energy, potential energy in blood flow…not just pressure!
Note: kinetic energy= momentum of blood
Note: potential energy= effect of gravity
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
How does cross sectional area affect the velocity of blood flow
High CSA means slow velocity (speed)
Capillaries have a high CSA
Arterioles lead up to capillaries, their CSA begins to gradually increase thereby slowing down blood flow/velocity
In contrast, venules unite to form a vein post capillary, CSA begins to decrease again, this increases blood flow and velocity
Equation for volume flow
Volume flow = Velocity x Area
Q = V x A
Laminar blood flow pattern
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- laminar blood flow is mainly found in arteries, arterioles, venules and veins
- blood flow is fastest at the centre (max velocity)
- RBC move through centre
- blood flow slow at walls (zero velocity) due to friction
Turbulent blood flow pattern
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- turbulent blood flow is too fast, generates whirlpools and vortexes due to increased pressure and velocity
- can hear it using a stethoscope
- turbulent blood flow seen in ventricles, aorta, and sites of atheroma where extra pressure is needed
Bolus blood flow pattern
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- RBC have a larger diameter than the diameter of capillaries so therefore move in single file and deform slightly
- uniform velocity, little internal friction, low resistance
- bolus blood flow found in capillaries
Reynolds number
The number laminar flow needs to exceed to become turbulent flow (2000)
Re = (fluid mass density x velocity x diameter) / viscosity
Arterial blood flow
- blood exerts pressure onto vessel walls
- left ventricle contracting also exerts pressure on vessel
- arterial pressure decreases in systemic circulation with distance from left ventricle
Name 4 factors that affect arterial blood pressure
1) cardiac output
2) peripheral resistance
3) properties of arteries
4) blood viscosity
Name 4 interactions in arterial blood pressure
1) systolic pressure during ejection
2) diastolic pressure during relaxation
3) pulse pressure (radial artery in forearm)
4) mean blood pressure
How does the aorta minimize pressure difference
- it has elastin which is able to expand and recoil
- 60 to 80% of stroke volume is stored in aorta during LV ejection (contraction)
- energy stored in stretched elastin then released into blood during LV relaxation
How to calculate pulse pressure
stroke volume / compliance
what is dicrotic notch
aortic valve closes
How to calculate compliance
change in volume / change in pressure
Atherosclerosis
- disease where plaque builds up inside arteries
- aorta becomes less stretchy, loses its compliance
- low compliance during exercise would result in pressure being too high (systolic pressure and pulse pressure is increased disproportionally)
why are elderly people prone to hemorrhagic strokes
- as we get older, our compliance decreases
- any modest increase in stroke volume can lead to a huge increase in arterial pressure
- artery wall becomes weakened leading to formation of an aneurysm
- aneurysm bulge ruptures, blood supply to brain is cut off
why is pulse pressure at the aorta smaller than the pulse pressure further down the arterial tree?
aorta has a high compliance, as the distance from the aorta increases, the compliance decreases
areas which have a low compliance, such as further down the arterial tree, have a high pulse pressure
name 2 ways to detect pulse pressure
radial artery in wrist
carotid artery in side of neck
what is the mean blood pressure?
93.3 mmHg, area under curve
diastolic pressure (80) + 1/3 of pulse pressure (13.3)
influenced by various factors e.g. age, disease, exercise, emotions like anger and fear, and even sleep
