Factors affecting flow through tubes Flashcards
Define pressure.
Force exerted per unit area
Define pressure gradient.
The difference in forces exerted (per unit area) at either end / side of an object e.g. a tube or membrane
Define flow.
The volume of fluid passing a given level of the circulation / airways per unit time
Define velocity.
The rate of movement of fluid along a vessel / airway
Define resistance.
A force that tends to oppose the flow of a substance
What is flow generated by?
A pressure gradient
Describe Ohm’s law for fluid dynamics.
Flow = pressure gradient / resistance
What does Boyles Law state?
At a given temperature, the pressure and volume of an ideal gas are inversely proportional
How is pressure gradient calculated?
Pressure gradient = flow x resistance
Apply the formula ‘pressure gradient = flow x resistance’ to the cardiovascular system.
Mean arterial pressure – central venous pressure = cardiac output x systemic vascular resistance
Where is velocity at the lowest and highest in laminar flow?
Lowest at edges
Highest at centre
Which 3 factors affect resistance?
Radius
Length
Viscosity
Flow is directly proportional to which power of the radius of a tube?
Fourth
List 4 factors which make turbulent flow more likely.
High velocity (e.g. secondary to a narrowed tube)
Low viscosity
Wide tube diameter
Tube branching or irregular surfaces
In which blood vessels is resistance highest and why?
Small arteries and arterioles, because they are arranged in series
The distensibility of blood vessels gives them which characteristic and what does it mean?
Capacitance – the ability to increase blood volume without significant increase in pressure
Why does turbulent flow occur on narrowed vessels?
Because flow must remain constant, so increased velocity is needed to move the same amount of blood / air through a narrowed tube.
Increased velocity causes turbulent flow
Describe the relationship between velocity and radius when flow is constant.
When flow is constant, velocity is indirectly proportional to radius
Describe the relationship between velocity and radius when the pressure gradient is constant.
When the pressure gradient is constant, velocity is directly proportional to radius
Which factor affecting flow is the most significant?
Radius
How does dehydration affect blood viscosity?
Increases blood viscosity
How does polycythaemia affect blood viscosity?
Increases blood viscosity
Why id resistance lower in capillaries than arterioles if they have a narrower radius?
Because of the effect of branching - capillaries are arranged in parallel, meaning there is increased area for flow
What is meant by the capacitance of blood vessels?
The ability of increase blood volume without significant increase in pressure
Which blood vessels are the main capacitance vessels?
Veins - hold around 70% of circulating blood volume
