Factors Affecting Flow Flashcards
How much is human cardiac output (at rest)?
~5L/min
How does the pressure change as you move through the vascular system?
Pressure drops as move from left ventricle –> aorta + major arteries –> resistance vessels (arteries + arterioles) –> exchange vessels (capillaries) –> capacitance vessels (veins)
If increase volume then pressure will..
Drop
Pressure in lungs needs to be … atmospheric pressure to allow air to move in
Below
Pressure in lungs need to be … atmospheric pressure to allow air to move out
Above
Flow is proportional to
Pressure Difference
The resistance of vessel/airway depends on its…
Radius
Resistance is proportional to…
Pressure
What is the equation for pressure gradient?
Pressure gradient = flow x resistance
Although flow is ALWAYS the same, its … will vary
Velocity
Velocity is inversely proportional to…
Cross Sectional Area (if radius increase, velocity will decrease)
When velocity suddenly changes e.g. vessel stenosis, what will happen to the flow?
Turbulent flow
What is flow through tubes normally like? Does all fluid move within that tube the same velocity?
Laminar flow - parabolic curve. Fluid does not move with same velocity - lowest velocity at edges (almost stationary) whilst highest at centre
What affect does reducing the radius have on the velocity of laminar flow?
Mean velocity is reduced
At constant pressure gradient, mean velocity is proportional to…
Radius squared
When is flow more likely to be turbulent? Why?
- Velocity is high - layers struggle to remain ordered
- Viscosity of blood is low - less adhesion between layers so break apart
- Blood vessel diameter is high - layers separate with more space
What is Poiseuille’s Law?
Resistance of a vessel depends on its radius, length and the viscosity of fluid.
Define the term Viscosity.
Viscosity = layers don’t move over each other as freely
What is the equation for flow?
flow = pressure gradient x radius/8 x viscosity x length
Flow is directly proportional to…
4th power of radius of tube
How does blood flow through a tube?
Laminar flow - RBCs move along in most rapidly moving stream in centre
Will smaller vessels have lower or higher resistance than larger ones?
Yes - smaller vessels have higher resistance but only when individual
For circulation as a whole which vessel has the greatest resistance?
Arterioles
Is resistance lower when vessels are in series or in parallel?
Parallel - overall resistance is reduced whilst in series the resistances are added up
What pressures act on blood vessels?
Transmural pressure (Intravascular - Extravascular pressure)
Blood vessels are distensible tubes (particularly veins). Why is this beneficial?
- Increase pressure enables walls to stretch - lowers resistance
- Get better flow as radius is fixed
- Keeps vessel open (transmural pressure >0)
- Gives Capacitance - more blood flow in than out (stores blood, particularly veins)
