CVS 5 Blood Flow Flashcards
Define flow
The volume of fluid passing a given point per unit time
Define Velocity
The rate of movement of fluid particles along the tube
What is laminar flow?
Flow where there is a gradient of velocity from the middle to the edge of the vessel. Velocity being highest in the centre and fluid stationary at the edge. The flow in most blood vessels is laminar.
What is turbulent flow?
This occurs once mean velocity exceeds a certain point as velocity gradient breaks down due to layers of fluid trying to move over each other faster than physics will allow. The fluid tumbles over, greatly increasing flow resistance
What is viscosity?
The extent to which fluid layers resist sliding over one another
How does viscosity affect blood flow?
The higher the viscosity the slower the central layers will flow and the lower the average velocity. In low viscosity fluid the difference between the centre and edge is large and in high viscosity fluid the difference is small
How does diameter of the tube affect velocity?
mean velocity is proportional to the cross sectional area, the wider the tube, the faster the middle layers move as there are more layers of fluid.
What determines the slope of the gradient of velocity?
the viscosity
What equation compares flow along tubes to electricity along wires?
V=IR
pressure=flow x resistance
When viscosity increases, resistance…
increases. The thicker the blood, the harder to push through vessels
With what power of the radius does resistance change?
4th so a small change in tubes radius causes big resistance change. i.e. its much more difficult to push blood through small vessels than big ones
If flow is fixed, how does resistance affect pressure change?
the higher the resistance, the higher the pressure change from one end of vessel to the other
If pressure is fixed, how does resistance affect flow?
Higher the resistance, lower the flow
How is resistance determined in vessels in series?
Resistances add together
How is resistance determined in vessels in parallel?
Effective resistance lower being 1/number of vessels in parallel of original. e.g if two vessels 1/2 original resistance
Does flow change over the circulation?
No it is the same at all points
Are arteries high or low resistance? How does this affect pressure change?
Low so low pressure drop over arteries
Are arterioles high or low resistance? How does this affect pressure change?
High so large pressure drop over arterioles
What is the resistance in capillaries like?
Individual capillaries are high resistance but many connected in parallel so overall low resistance and so low pressure drop over capillaries
Are venules and veins high or low resistance? How does this affect pressure change?
Low resistance so low pressure drop
What keeps the pressure in arteries high?
The high resistance of arterioles
If the heart pumps more blood and the resistance of arterioles remains the same, what happens to arterial pressure?
it rises
What is the transmural pressure?
The pressure across the vessel wall which stretches the distensible walls of the vessel
How does increasing the pressure in a distensible vessel change the resistance and flow?
Increasing the pressure, stretches the vessel, increasing the size of the lumen so decreasing the resistance and increasing the flow.
In distensible vessels why does blood flow cease before pressure reaches 0?
The walls of the blood vessel collapse with low pressure due to the outside pressure pushing in on the vessels
What quality allows blood vessels to show capacitance and how?
Distensibility. As vessels widen with increasing pressure, more blood transiently flows in than out allowing the vessel to ‘store’ blood (capacitance)
Which vessels are most distensible and what proportion of blood is stored in them at rest?
Veins. 67%
What is systolic pressure?
The maximum arterial pressure, typically 120mmHg
What factors affect systolic pressure?
How hard the heart pumps
Total Peripheral Resistance
Compliance (stretchiness) of arteries
What is diastolic pressure?
The minimum arterial pressure, typically 80mmHg
What factors affect diastolic pressure?
Systolic pressure
Total Peripheral Resistance
What is pulse pressure?
The difference between systolic and diastolic pressure
typically 40mmHg
How is average pressure calculated?
diastolic +1/3 pulse pressure (as systole shorter than diastole)
What is total peripheral resistance?
Sum of resistance of all the peripheral vasculature in the systemic circulation
How would pressure in arteries be different if they had rigid walls?
Pressure would rise enough in systole to force whole stroke volume through total peripheral resistance then drop to 0 in diastole.
What is the pulse wave?
Wave created by contraction of the ventricles which propagates along arteries faster than blood and can be felt where arteries come close to the surface
What is the dicrotic notch?
The slight dip in the pulse wave due to pressure in LV falling below that in aorta and the subsequent backflow of blood that closes the aortic valve
What is the dicrotic wave?
The slight rise in pulse wave directly after the dicrotic notch due to recoil of blood off closed aortic valve.
How do arterioles control blood flow to tissues?
Variable flow restriction via vasoconstriction and vasodilation
What is vasomotor tone?
The continuous level of contraction of blood vessels
What part of the nervous system produces vasomotor tone?
Sympathetic branch of ANS (alpha 1 adrenergic receptors)
What antagonises the contraction in vasomotor tone?
Vasodilator factors. Actual resistance being a balance between the two
What are some examples of vasodilator metabolites?
H+, K+, adenosine
What is reactive hyperaemia?
After circulation to an organ or limb is cut off for a minute or two then restored, a large amount of blood enters as limb has ocntinued metabolising and creating vasodilators while circulation cut off with no blood flow to remove so when blood restored the local arterioles dilate maximally and blood flow high
What is auto-regulation?
The ability of most organs to automatically take the blood flow they need as long as the pressure in the arteries supplying them is kept within a certain range
What is central venous pressure?
The pressure in the great veins supplying the heart
What is venous return?
Rate of blood flow back to heart, limits cardiac output
