5 Flow through tubes Flashcards
What is the relationship between pressure and flow?
The higher the pressure difference, the higher the flow
What is the definition of ‘flow’?
The volume of fluid passing a given point per unit time (L per min)
What is the definition of ‘velocity’?
The rate of movement of fluid particles along the tube (meters per second)
How does velocity vary if the flow remains constant?
Varies with cross-sectional area
What is the relationship between velocity and cross-sectional area?
Inversely proportional
Small cross-sectional area, high velocity eg. aorta
Large cross-sectional area, low velocity eg. capillaries
What is ‘laminar flow’?
Velocity is highest in the centre and fluid is stationary at the edges
What is ‘turbulent flow’?
There is no velocity gradient. Fluid moves in various directions at various speeds.
What is ‘viscosity’?
The extent to which fluid layers resist sliding over each other.
What is the relationship between viscosity and velocity?
Inversely proportional
The higher the viscosity, the lower the velocity
At a constant velocity, what is the relationship between viscosity and cross-sectional area?
Proportional
The larger the cross-sectional area, the
What causes flow of blood around the vessels?
The gradient of pressure
How does the resistance of vessels combine when those vessels are:
i. ) In series
ii. ) In parallel
i.) Add together
ii.) R1 x R2
———-
R1 + R2
Where does the main pressure drop occur in the CVS?
The arterioles
Why is pressure in the arteries high?
The arterioles are high resistance vessels
What happens to the resistance inside a vessel when the wall distends?
The pressure increases so the resistance falls
The blood flow will stop before the pressure reaches zero. At what point will it stop?
At the critical closing pressure
What is ‘capacitance’ in a vein?
The ability to store blood
What is the consequence of blood cells congregating in the middle of the vessel?
Blood cells travel quicker than the plasma
What is ‘cardiac output’?
The product of stroke volume and heart rate
The heart ejects blood intermittently but blood flow is continuous. Why is this?
In systole, arteries stretch so more blood flows IN than OUT. This means the pressure does not rise significantly. As the arteries recoil in diastole the flow can continue as normal.
What is the arterial pressure in:
i. ) Systole?
ii. ) Diastole?
i. ) 120mmHg
ii. ) 80mmHg
What is the ‘pulse pressure’?
The difference between the systolic and diastolic pressures (40mmHg)
What is the ‘average blood pressure’?
The diastolic pressure plus 1/3 of the pulse pressure
What is ‘vasomotor tone’?
Tonic contraction of smooth muscle
What happens when the circulation to an extremity is cut off for a short period of time?
When there is no blood flow, metabolites accumulate so the arterioles dilate fully. When flow returns, the resistance is low and so the flow is very high. The high flow removes the metabolites and the smooth muscle constricts.
What is ‘central venous pressure’?
The pressure in the great veins which fills the heart in diastole.
