9. Flow Through Tubes Flashcards
What is flow resistance determined by?
The nature of the fluid and the vessel.
Define flow.
The volume of fluid passing a given point per unit time.
Define velocity.
The rotate of movement of fluid particles along the tube.
How are flow and velocity linked?
Flow must be the same at all points in a vessel, velocity can vary if the radius changes. So at a given flow, velocity is inversely proportional to cross sectional area.
What is laminar flow?
The flow in most vessels, there is a gradient of velocity from the middle to the edge of the vessel from higher velocity in the centre to fluid stationary at the edge.
What is turbulent flow?
When the velocity gradient breaks down, fluid tumbles over and flow resistance is increased.
What does velocity of the fluid depend on?
Viscosity of the fluid and radius of the tube.
What is viscosity?
The extent to which fluid layers resist sliding over one another. Higher viscosity means central layers flow more slowly and the average velocity is slower.
How is mean velocity and cross sectional area of the tube linked?
They are proportional.
What is Poiseuille’s law in words?
When blood flow is steady and laminar in blood vessels larger than arterioles, then flow is proportional to the difference between the inflow and outflow pressures, proportional to the fourth power of the radius, inversely proportional to the length of the vessel and inversely proportional to the viscosity of the blood.
What can hyperviscosity syndrome be caused by?
Abnormally high plasma protein levels or abnormally high red or white blood cell count.
How can severe anaemia cause functional cardiac murmurs?
High blood flow velocities or reduced viscosity of the blood, caused by low red blood cell count.
How are pressure, flow and resistance related?
Pressure = flow X resistance.
What is the resistance of vessels aligned in series and in parallel?
In series - total resistance = sum of the individual resistances.
In parallel - total resistance = sum of the reciprocals of all the individual resistances.
Why is pressure in arteries high?
Because of the high resistance of the arterioles.
