Principles of Haemodynamics Flashcards
what is haemodynamics?
the relationship between blood flow, blood pressure and resistance to flow (flow of blood through the heart and vessels)
name 6 key factors involved in haemodynamics?
- Force (cardiac contraction)
- Work (isovolumetric contraction-when pressure gets to a certain level ejection occurs, the longer this period, the stronger the contraction)
- Pressure (the difference in pressure from the aorta to veins helps blood flow)
- Compliance (arterial stretch)
- Resistance (in arterioles)
- Flow velocity (slowing down blood flow in capillaries, increasing efficiency for gas exchange)
is the CVS is a closed or open system?
The CVS is a closed system
where is the majority of blood located?
Majority of blood is in the venous system
what is the venous system?
Low pressure reservoir system which expands a lot
what can the reservoir of venous blood be used for?
The reservoir of venous blood can be used to increase CO by increasing he force of contraction (starlings law). This happens as when more blood is needed, veins can contract to give more blood to the heart.
why do we constrict/dilate arteries?
To send blood to different places
what is darcy’s law, and the equation involved with it?
The role of pressure in energy flow. The flow in a vessel is equal to the difference in pressure across vessel divided by resistance of flow
Q=(P1-P2)/R
Q= flow
P1-P2= pressure difference. In this case, it is Pa-CVP (arterial pressure-central venous pressure)
R= resistance to flow. In this case it’s TPR (total peripheral resistance)
what is Bernoulli’s Law?
As the speed of a moving fluid increases, the pressure within the fluid decreases.
There’s more than just pressure controlling blood flow:
The role of pressure, kinetic and potential energies in flow
Blood is moving so means in terms of energy?
Blood is moving and so has kinetic energy and gravity pulling it so also potential energy
Blood having kinetic energy means what?
Kinetic energy means blood can travel against the concentration gradient
what is the definition of blood flow?
volume of blood flowing in a given time (ml/min, l/min)
what is the definition of perfusion?
blood flow per given mass of tissue (ml/min/g). Takes into account mass of tissue.
what is the definition of 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 flow determined by?
Flow is determined by arterial blood pressure and resistance.
what is the relationship between volume flow, velocity and area
Volume flow (Q)= Velocity (V) x Area (A)
Is the velocity of blood flow in the aorta high or low?
The velocity of blood flow in the aorta is high
When the blood reaches the arteries what happens to the speed?
Speed starts to increase at arteries
What slows the velocity of blood?
The branching of arteries into capillaries slows velocity
Describe the capillaries:
- 1 cell thick
- very narrow
- in total, they have a large cross-sectional area
what is the relationship between cross-sectional area and flow?
The greater the cross-sectional area, the slower the flow
So, where is flow slowest?
flow is slowest in the capillaries
veins coming together increases what?
veins coming together increases the velocity
name the 3 patterns of blood flow?
laminar, turbulent and bolus
laminar flow is present in which vessels?
most arteries, arterioles, venues and veins
describe laminar flow:
-smooth, layers
with laminar flow, what is the velocity like at the walls compared to the centre and why is this?
- Zero velocity at walls due to friction and molecular interactions, as near the walls of the vessel there is some friction between water molecules in the blood and the walls of the vessel, which slows down the blood
- Maximum velocity is at centre
(laminar flow) having maximum velocity at the centre means what?
- Moves RBCs towards centre
- speeds up blood flow through narrow vessels
laminar flow can be disrupted to become what?
turbulent flow
when does turbulent flow occur (examples)?
- Ventricles (mixing)
- aorta (peak flow)
- atheroma (bruits- abnormal sound generated by turbulent flow of blood)
- with high bp
- with obstructions
why does turbulent flow happen?
Turbulent flow happens when blood flows too quickly above a certain speed, no longer laminar
with turbulent flow, why does the blood not flow linearly and smoothly in adjacent layers?
because of increased pressure and velocity
(turbulent flow) instead of flowing smoothly how does it flow?
whirlpools, eddies, vortices
can you hear turbulent flow when listening with a stethoscope?
yes
name the third type of flow?
bolus
bolus flow occurs in only 1 type of vessel-which one?
capillaries
describe bolus flow (efficiency, velocity, friction etc.)
- Very efficient flow
- uniform velocity
- little internal friction
- very low resistance
with bolus flow in the capillaries, why do RBC’s move in single file?
RBCs have a larger diameter than capillaries and so move in single file, squeezing their way through.
what are trapped between the RBC’s in bolus flow?
Plasma columns (bolus of fluid) are trapped between RBCs
what is Reynolds Number (Re)?
Determines at what point the flow will change from laminar to turbulent flow
what does Re measure?
Flow against pressure, providing there is constant resistance and area
what does an increase in pressure subsequently increase?
Increase in pressure increases flow up to a point where it becomes turbulent – after that, further increase in pressure doesn’t increase flow but just goes to turbulent state
when does turbulence occur (in terms of Re)?
Turbulence occurs when Reynolds number exceeds a critical value (>2000)
give reasons why this value could be exceeded?
- bruits
- ejection murmur
- increased blood velocity
how does diameter affect Re?
Bigger diameter = bigger Reynolds number
does Re have units?
Reynolds number doesn’t have units
minor clotting can occur due to changes in what?
changes in viscosity
where is blood flow highest?
Blood flow is highest in aorta
what is the pressure in aorta during systole and diastole?
120mmHg during systole 80mmHg during diastole
when does arterial pressure fall?
Arterial pressure falls steadily in systemic circulation with distance from LV
Resistance occurs in arterioles by what?
constriction
name some factors that affect arterial blood pressure
- Cardiac output (SV, HR)
- Properties of arteries
- Peripheral resistance
- Blood viscosity (increase in viscosity, increases blood pressure)
Arterial blood pressure involves interaction between which 4 key relationships?
o Systolic pressure – pressure when ejecting
o Diastolic pressure – pressure when relaxing
o Pulse pressure – difference between diastolic and systolic pressure (radial artery for pulse is difference)
o Mean blood pressure – average pressure (as when measuring blood pressure)
why can’t you detect a pulse in veins?
You can’t detect a pulse by the time you get to veins because its smooth flow, and there isn’t much change to the pressure
role of elastic fibres in the movement of blood?
- The recoil of elastic fibres of the aorta and larger arteries helps to propel the blood into the circulation
- Maintains blood pressure even during diastole
during LV ejection, where is SV and energy stored?
- 60-80% of stroke volume is stored in aorta and arteries as these structures expand
- Energy stored in stretched elastin
during LV diastole what happens?
- Energy is returned to the blood at the walls of the aorta and arteries contract
- This sustains diastolic blood pressure and blood flow when the heart is relaxed
what is compliance?
how much the aorta is stretched
what is pulse pressure?
what the finger senses, eg. at the wrist (radial artery).
what does the pulse pressure tell you about?
Tells you about stroke volume and arterial compliance (stretchiness)
how is pulse pressure affected when there is a higher SV?
With a higher stroke volume, pulse pressure goes up
how will a less compliant aorta affect pulse pressure?
A less compliant aorta will give higher pulse pressure for the same cardiac output
Rest Vs Exercise- compare the SV, stretch, compliance and systolic volume
exercise: o Greater SV o Greater stretch of arteries o Less compliant o Relatively greater systolic pressure
how does exercise affect the SV?
During exercise the stroke volume increases. Each time the heart beats, we expel more blood. Squeezing the blood in the legs and veins so more is going back to the heart
when the brain lacks o2 what does it do?
sends sympathetic messages to veins, which squeeze in so more blood goes back to the heart.
increasing CO does what to the aorta?
aorta stretches more
why is there greater stretch of the arteries?
because more blood is ejected
greater stretch of the arteries causes what?
Greater stretch of the arteries as more blood is ejected causes less compliance and less recoil and the difference between systole and diastole increases i.e. pulse pressure increases
at a normal SV, there is a linear relationship between what?
pulse pressure has a linear relationship with stroke volume
is there a limit to how much the aorta can stretch?
yes
given there is a limit to how much the aorta can stretch, how does this affect the curve?
Increase SV, increase in steepness of curve
– caused as reaching upper limits as it is how much aorta can stretch
as you get older how is arterial compliance affected?
there is a decrease in arterial compliance because the arteries get stiffer
when you have a decrease compliance, SV increases which pressures disproportionally?
Stroke volume now increases systolic and pulse pressure disproportionally
having a high pulse and a low compliance increases what?
increases after-load
having a high pressure means what?
damages vessel but also reduces CO
what is the consequence of reducing CO?
makes it harder for the heart to eject blood, so it now needs more oxygen, pumps harder, heart muscles grow to match demand
-hypertrophy of the ventricle wall
age increases the stiffness of vessels (particularly the aorta) how does this affect the arterial tree?
it means the large pulse pressure is present throughout arterial tree, usually this pulse will decrease
when looking at the arterial tree, when can the pulse pressure no longer be detected?
- once you get into the arterioles the pulse pressure can’t be detected
- the flow is more continuous
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 – Decrease in BP 80/50mmHg
