CVS Blood flow Flashcards
Define:
Flow
Velocity
What is the relationship between them?
Flow is the volume of fluid passing a given point per unit time (flow = velocity x cross sectional area)
Velocity is the rate of movement of fluid particles along the tube
Flow must be constant at all points along the vessel, whereas velocity can change
At a given flow, velocity is inversely proportional to cross sectional are
What is laminar and turbulent flow?
In laminar flow there is a gradient of velocity from the middle to the edge of the vessel with the highest velocity in the centre (type of flow in most BV)
As velocity increases flow becomes turbulent because layers of fluid try to move over each other faster than physics will allow, causing tumbling fluid with increased flow resistance
What is viscosity and how does it effect flow?
Viscosity is the extent to which fluid layers resist sliding over one another
Higher viscosity = slower the flow
How does tube diameter effect flow rate?
The wider the tube the faster the middle layers of fluid will move so:
Mean velocity is proportional to cross sectional area
Define resistance to flow
What factors effect flow resistance?
The flow for any given pressure gradient is determined by the resistance of the vessel, and the resistance is determined by the nature of the fluid and vessel
- resistance increases as viscosity increases
- resistance decreases with the fourth power of the radius (much harder to push blood through small vessels than big ones)
What is the relationship between pressure, resistance and flow?
Pressure = flow x resistance
What happens to flow resistances in series and parallel?
For vessels in series, resistances add together
For vessels in parallel the resistance is lower because there is more than one path for the current to flow done
e.g. 2 vessels in parallel, the resistance of one of the vessels is half the original because the blood has 2 paths
Describe the pattern of flow and resistance over the systemic circulation
Flow is constant over the whole circulation
Arteries have low resistance (small pressure drop)
Arterioles are high resistance (large pressure drop)
Venules and veins are low resistance (small pressure drop)
The pressure is high in arteries because of the high resistance in arterioles - it is difficult to push blood into them
How does the distensibility of blood vessels affect the relationship between flow and pressure?
The pressure within a vessel generates a transmural pressure across the wall which stretches the vessel
When the vessel stretches the lumen diameter increases so resistance falls and flow increases
As the pressure falls the walls eventually collapse and the blood flow stops before driving pressure falls to 0
How does the distensibility of blood vessels produce capacitance?
As vessels widen with increasing pressure, more blood flows in than out. This allows the vessels to ‘store’ blood. Veins are the most distensible vessel.
