Principles of Haemodynamics Flashcards
What is haemodynamics?
The relationship between blood flow, blood pressure and resistance to flow.
What controls the force, work, pressure, compliance, resistance and flow velocity?
- 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.
Effect of control of arterioles.
Control arterioles in different parts of the body and regulate the flow to different organs.
If the heart is more compressed, what happens to contraction?
Increases
How can the reservoir of venous blood increase cardiac output by Starling’s law?
When we squeeze the veins, and more blood arrives back to the right side of the heart, and when this reaches the left ventricles, the ventricles stretches and increases force of contraction (Starling’s law) – so soon balance both sides of the heart.
What happens when there is reduced blood flow to one area of the CVS?
Reduced blood flow to one area increases pressure upstream and alters flow to other areas.
What happens when we squeeze the muscular arterioles?
We can change resistance and direct flow wherever we want it. - blood spreads out and flows in various proportions around the body according to the amount of constriction and dilation in the arterioles.
What is the difference between Darcy’s law and Bernoulli’s law? and why is Darcy’s law not sufficient?
- Darcy’s law - role of pressure energy in flow
- Bernoulli’s law - role of pressure, kinetic and potential energies in flow – NOT JUST PRESSURE.
- Darcy’s law explains on a simple level but not stuff like how pressure is higher in legs than from blood that just left the heart?
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).
What is 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. (changes by constriction and dilation).
Changes in velocity within blood vessels
- Velocity of blood flow in aorta is high
- Branching of arteries slows velocity
- Slowest flow in capillaries
- Velocity increases with veins coming together
Volume flow equation
Volume flow (Q) = Velocity (V) x Area (A) - The flow volume may remain the same, but area and velocity may chage, depending on vasodilation (inc. A, dec. V) or vasoconstriction (dec. A, inc. V).
What are the 3 patterns of blood flow? Where are they common? and describe them.
- Laminar
– smooth BF
– in most blood vessels
– When blood flows through, the blood near the walls has more frictional contact w/ the edges and slows - maximum velocity at centre. ~ Concentric shells. - Turbulent
– Ventricles (mixing),
aorta (peak flow),
atheroma (bruits)
– Blood does not flow linearly and smoothly in adjacent layers
– Due to increase pressure, velocity, viscosity or when there are some other problems like obstructions.
– Sometimes you can get this in the heart, and you can hear it with a stethoscope. - Bolus
– In capillaries when the size is the same or smaller than RBC.
– RBC bends and slips through nicely in the capillary
– smooth and low resistant flow
– Fluid in front and fluid behind.
What is Reynold’s number?
Reynold’s number – Describes what determines change from laminar to turbulent flow
• Turbulence occurs when Reynold’s number exceeds a critical value (>2000). By e.g. bruits, ejection murmur and increased blood velocity.
• As pressure increases, flow increases (as constant resistance), until it reaches turbulent flow, where increase in pressure doesn’t affect the flow that much.
How can viscosity of the blood increase?
When stasis of the blood – we sometimes get minor clotting, which increases the viscosity of the blood.
When viscosity increases, pressure increases.
What are the factors that affect arterial blood pressure?
- Cardiac output (SV, HR)
- Properties of arteries
- Peripheral resistance
- Blood viscosity
What are systolic, diastolic, pulse and mean 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
Role of the aorta in maintaining arterial blood pressure.
Recoil of elastic fibers of the aorta and large arteries helps to propel the blood into the circulation
DURING LV EJECTION
• 60-80% of stroke volume is stored in aorta and arteries (energy stored in stretched elastin) as these structures expand. - stretching means the aorta is accommodating.
DURING LV 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.
Arterial BP - rest vs exercise.
- Greater SV
- Greater stretch of arteries
- Less compliant
- Relatively greater systolic pressure
How does SV effect pulse pressure during exercise?
During exercise as the stroke volume increases compliance curve gets very steep leading to very high pulse pressure.
Bigger pulse pressure when stroke volume raised
What happens to the pulse pressure when there is a greater stretch of the arteries?
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 is pulse pressure?
Difference between diastolic and systolic pressure. This is what the finger senses. Tells you about stroke volume and arterial compliance (stretchiness).
Pulse pressure equation
Pulse pressure = SV / Compliance
Compilance Equation
Compliance = (Change in volume) / (Change in pressure)
What controls mean blood pressure?
Age Disease Distance along arterial tree Blood volume - SV, CO Exercise - SV, CO Emotion - stress, anger, fear, apprehension, pain Wake/sleep - dec. BP 80/50 mmHg
Equation to find the mean BP
Mean BP = diastolic pressure + 1/3 (pulse pressure)
Arterial pulse pressure and arterial tree
Pulse pressure at the aorta is relatively small but further down the arterial tree you it increases slightly because vessels become less compliant.
But you are still getting the effects of the aorta so it doesn’t increase all that much.
This one reason to measure it in the radial artery quite far away from the heart.
Arterial pulse pressure and age
Age increases stiffness of vessels – particularly aorta – this means that large pulse pressure is present throughout arterial tree.
Pulse pressure in the arterioles
Once you get into the arterioles the pulse pressure can’t be detected and the flow is more continuous.
Affect of decreased arterial compliance on SV, and why is this important in the elderly?
Decreased compliance (steeper curve)
Stroke volume now increases systolic and pulse pressure disproportionally
- When SV is normal the pulse pressure is similar to younger person. – When CO and SV increases there is a much greater increase in pulse pressure, with the same SV. – so very high pressure per heart beat.
Increase in age – stiffer arteries (atherosclerosis) decreased compliance
What would happen during exercise (increase in SV) for the elderly?
High pulse pressure and low compliance of the aorta – we are INCREASING AFTERLOAD (Laplace’s law) – so we are decreasing CO, so heart needs to work more, gets bigger, demands more O2… vicious cycle continues..
