6.1 Haemodynamics Flashcards
define flow
- Volume of substance over time (cm3/s)
define velocity
Distance substance is displaced over time
(cm/s)
define vascular resistance
Force within the circulatory system opposing
the flow of blood
define laminar flow
Flow of a substance (blood) through a tube (blood vessel) parallel to the tube’s edges (vessel walls) whereby velocity increases towards the centre of the lumen
define turbulent flow
Disorganised flow of substance (blood) through a tube
(blood vessel) in a wide variety of antegrade and retrograde velocities simultaneously
define critical velocity
The velocity at which a fluid will switch from a laminar
flow to a turbulent flow
define viscosity
- Resistance of a fluid to deformation
Most important takeaway of poiselle-hagan formuala ?
small changes to radius have a big impact on flow
Poiselle-Hagan formuala expresses relationship between ?
flow, viscosity and tube (blood vessel) radius
What is tension on wall of a (blood vessel) cylinder equal to ?
the product of transmural pressure and radius divided by wall thickness (P x r)/ w
most important implication of law of laplace ?
explains why capillaries don’t rupture despite having very thin walls
- Since the smaller the radius of a vessel, the less tension is required to withstand the internal pressure
What is flow (volume of fluid moved per unit time) affected by ?
- Pressure – fluids flow from high to low pressure
-> therefore ∆P as pressure difference between one end of the tube and the other is taken - Resistance – High resistance reduces flow
what is the flow of blood flow through vessels ?
laminar
Explain blood flow through vessels:
- Flow is organised and parallel to the cylinder (blood vessel) walls
- Velocity is highest in the centre of the lumen and lowest where blood is in contact with the cylinder (blood vessel) walls
until when will blood flow remain laminar?
Until it reaches a critical velocity, at which point flow will become turbulent
Velocity = distance over time , what is this affected by ?
[Assuming that ∆P is constant]
the flow rate through a vessel and the cross-sectional area of that vessel
why does aorta have highest velocity ? [check!]
- high pressure
- low surface area
Why is the lowest velocity within the capillaries ?
largest total area is at the level of capillaries
what does area refer to in average velocity ? [alter and change!]
total area of all tubes running in parallel within the system
turbulent flow…
Viscosity is degree to which …?
a fluid resists deformation
INC Viscosity = DEC flow
What is viscosity in blood produced by ?
- friction between molecule and cells (INC molecuels = INC V and INC haematocrit = INC V)
- compressibility of cells (ING rigidity = INC V e.g. hereditary spherocytosis)
- composition of plasma e.g. conditions leading to INC immunoglobulin concentration
what is hereditary spherocytosis (change!)?
red blood cells have deficiency gene mutation expression of protein messes up cytoskeleton of red blood cell
- what is shear stress ?
- proportional to ?
- force exerted onto the endothelium parallel to blood flow
- viscosity
….1… flow produces regular ..2.. this impacts ..3… cells and …4… of athero-protective factors
- pulsatile laminar
- shear stress
- smooth muscle
- increased expression
What’s problem with turbulent flow ?
- can damage endothelium and INC risk of atherogenesis
- often occurs at arterial bifurcations and curvature => INC plaque formation
- more likely to occur after a constriction (stenosis)
why bifurcation -> plaque ? [what did he mention !!!]
orientation change ?
One problem of turbulent flow is it’s more likely to occur after a constriction (stenosis), why ?
- Increased velocity through stenosis due to decreased cross sectional area of lumen
- Further damage directly after plaque/stenosis
- Further increases risk of atheroma proliferation
Poiseuille-Hagen is a formula that shows what ?
radius has a HUGE impact on Flow and Resistance
Poiseuille-Hagen, what happens when:
1. double diameter
2. half diameter
- 16x flow
- 16x resistance
Effects of small increase and decrease in diameter?
INCREASE = massively INC flow
DECREASE = massicely INC resistance
What cannulae are used for IV drug administration ?
peripheral
Poiseuille-Hagen relationship to peripheral cannulla sizes ? [he mentioned in lecture ]
measured in gauge
higher the guage = smaller diameter / the less fluid you can give in given time
french catheter scale ?
one french unit = 0.33 mm therefore as they INC diameter gets bigger
urinary catheter
give 12 - 16 inch catheters
Arterioles have:
..1… in their walls so a …2.. area this makes them …3. at INC flow or ….4… which is very effective at controlling ..5.. to organs
- smooth muscle
- very large total
- very effective
- resistance
- blood supply
What is critical closing pressure ? = ALTER Q + A
In small vessels when luminal P < tissue pressure the vessel will collapse
- due to surrounding tissues exerting a small amount of pressure
- there is a point where flow = 0 , even though pressure > 0
Although capillary walls are 1 cell thick they don’t burst, why don’t they rupture ?
law of laplace [T = Pxr/w]
T = tension
P = transmural pressure
r = radius
w = wall thickness
**they don’t burst because of their small radius ! **
law of laplace tension ?
required to withstand pressure INC in T = INC in chance of rupture
law of laplace transmural pressure ?
Transmural pressure = inside pressure – outside pressure
* Outside pressure is so small it can be ignored
* Therefore, Transmural pressure = inside pressure
law of laplace wall thickness ?
Capillary wall so thin that this can be ignored so law of laplace equation becomes T = Pxr
Law of laplace when pressure in an individual capillary stays very low ?
P = T/r (smaller value of r = DEC tension requirement)
what type of vessels are veins considered to be ?
capacitance vessels
Why are veins referred to as capacitance vessels ?
they can hold a large volume of blood without a significant INC in venous pressure due to their distensibility
% of body’s circulating blood volume is contained within veins ?
50%
How do veins differ from arteries in terms of their ability to store blood?
Veins have a greater capacity to store blood than arteries due to their high distensibility, which allows them to expand and accommodate more blood without raising pressure significantly.
How does the distensibility of veins benefit the circulatory system?
The distensibility of veins allows them to act as reservoirs that can accommodate excess blood, helping to regulate blood volume and pressure throughout the circulatory system.
What would likely happen if veins were not as distensible?
If veins were not as distensible, even a small increase in blood volume could cause a significant rise in venous pressure, potentially leading to problems like venous hypertension or impaired blood return to the heart
