Blood Flow Flashcards
Define velocity
The rate of movement of fluid particles along the tube
Define flow
Volume of fluid passing a given point per unit time
What is laminar flow?
Where there is a gradient of velocity from the middle to the edge of the vessel. Velocity is highest at the centre and stationary at the edge.
What is turbulent flow?
After a certain velocity, the velocity gradient breaks down and fluid tumbles over.
What effect does turbulent flow have on flow resistance?
Increased flow resistance
Define viscosity
The extent to which fluid layers resist sliding over one another.
Relationship between tube diameter and velocity?
At a constant gradient, the wider the tube, the faster the middle layers move, so mean velocity is proportional to the cross sectional area of the tube.
Relationship between mean velocity and viscosity?
Velocity is inversely proportional to the viscosity
Equation relating mean velocity, flow and cross-sectional area?
Flow = mean velocity x cross sectional area
Relationship between resistance and velocity?
As resistance increase, velocity increases (thicker the blood, harder it is to push around vessels - think of velocity gradient)
Relationship between cross sectional area and resistance?
As diameter increases, resistance decreases.
What is the effect of combining vessels in series on flow resistance?
The resistances add together
What is the relative pressure change in arteries, arterioles and capillaries
Arteries - pressure drop is small
Arterioles - pressure drop is large
Capillaries - pressure drop is small
Why is the pressure in arteries high when their resistance is low?
Resistance of arterioles
What is the effect in resistance and flow if the vessel wall stretches? Therefore what is the effect on increasing pressure on flow?
Resistance falls
Flow increases
As pressure increases, the easier it is for blood to flow through the vessel
How does distensibility of blood vessels produce capacitance?
As vessels widen, more blood transiently flows in than out.
What is the systolic arterial pressure?
120mm Hg
What is systolic arterial pressure affected by?
How hard the heart pumps
Total peripheral resistance
Compliance of arteries
What is a typical diastolic pressure?
80 mm Hg
What affects the diastolic pressure?
Systolic pressure
Total peripheral resistance
What is the pulse pressure?
The difference between diastolic and systolic pressure
What is a typical pulse pressure reading?
40 mm Hg
How do you work out the average pressure?
Diastolic ➕ (1/3 ✖️ pulse pressure)
Define total peripheral resistance
The sum of the resistance of all the peripheral vasculature in the systemic circulation.
In the pulse wave, what is the dicrotic notch due to?
Pressure in the left ventricle falling below aortic pressure and subsequent backflow which closes the aortic value.
What is the dicrotic wave due to?
This is the slight ride immediately following the notch. Slight increase in pressure is due to the recoil of blood off the aortic valve
What is the role of arterioles as resistance vessels?
They control bloodflow to tissues by variable restriction
Define vasoconstriction
Increase in tonic contraction of smooth muscle
Define vasodilatation
Decrease in tonic contraction of smooth muscle
Define vasomotor tone
The tonic contraction of smooth muscle
What branch of the ANS mostly produces vasomotor tone and what antagonises it?
Sympathetic nervous system
Antagonised by vasodilator factors
What are the vasodilator metabolites?
H+, K+ and adenosine
What is the central venous pressure?
The pressure in the great veins supplying the heart
What is venous return?
The rate of blood flow back to the heart
It limits cardiac output
Equation for total peripheral resistance?
Blood pressure ➗ cardiac output
