CVS - Flow Through Tubes Flashcards
What must happen to blood pressure to allow it to keep flowing?
The pressure must fall all the way from the aorta to vena Cavae
What is blood flow driven by?
What is flow proportional to?
Gradient of pressure
Proportional to the pressure difference between the ends of the vessel
Higher the pressure going to low pressure = increased flow
What is flow resistance?
What is it determined by?
Flow for a given pressure is determined by the resistance of the vessel
Nature of fluid and the vessel
Define flow
Define velocity
The volume of fluid passing a given point per unit time
The rate of movement of fluid particles along the tube
At given flow rate, In vessels with small cross sectional area e.g. Aorta what would the velocity of blood flow be?
High velocity
What is meant by laminar?
There is a gradient of velocity from middle to the edge of the vessel
Velocity is highest in the centre and fluid is stationary at the edges.
What is meant by turbulent?
What happens to flow resistance?
As mean velocity increases, flow eventually becomes turbulent.
Velocity gradient breaks down as layers of fluid try to move over each other faster than physics will allow
Fluid tumbles over –> greatly increasing flow resistance
What type of flow occurs in most blood vessels?
Laminar flow
In a vessel with constant pressure driving flow, how is flow determined?
What does that depend upon?
Mean velocity
Dependent upon - viscosity of the fluid, radius of the tube
What is viscosity?
What would happen to the flow in high viscosity?
What would the average velocity be?
The extent to which fluid layers resist sliding over one another
Central layers flow slower, lower average velocity
What does viscosity determine?
What is the effect of the radius of the tube on flow rate?
What is mean velocity proportional to?
Slope of the gradient of velocity
Wider the tube, the faster the middle laters will move
Cross sectional area of the tube.
What may cause hyperviscosity?
Abnormally high plasma proteins
Abnormally high RBC or WBC count
Underlying condition must be treated otherwise it re-occurs
What may cause cardiac murmurs in a severely anaemic patient?
High blood flow velocities
Decreased viscosity of the blood caused by low RBC
What is the relationship between pressure, resistance and flow?
What factors affect flow resistance?
Pressure = flow X resistance
Resistance increases as viscosity increase
Resistance decreases to the fourth power of an increase in radius
If pressure is fixed, what will happen to resistance and flow?
If flow is fixed, what is the effect of high resistance?
Higher resistance, lower the flow
Higher resistance, the greater the pressure change from one end of the vessel to the other
What happens to flow at all points in circulation?
What resistance are arteries?
Arterioles?
Veins?
Flow is the same
Low resistance
High resistance
Low resistance
Why is the pressure within the arteries high?
Due to the arterioles high resistance.
It is difficult to push blood into them, therefore pressure increases
When does flow become turbulent?
Flow velocity is high
Low viscosity
Lumen of vessel is irregular
If vessel irregularly narrowed ee.g atherosclerosis
Given each capillary is very small and has high resistance why is their collective resistance low?
Many capillaries are connected together in parallel which gives them low resistance
What is the transmural pressure?
What happens when pressure increases?
The pressure difference between the inside and outside of the tube
Pressure increase –> walls stretch –> resistance falls
What is capacitance?
Distensible vessels being able to store blood
What is the function of the: Heart Arteries Arterioles Capillaries Veins?
Heart - cyclic muscular pump, enable circulation
Arteries - gross conduction and distribution of blood supply
Arterioles - local distribution of fine control to defined tissue volume
Capillaries - micro diffusion and filtration
Veins - collection, return and capacitance
Define compliance
Define capacitance
Ability to distend and increase volume due to pressure increase
Effectively same as compliance - measure of relative volume increase per unit increase in pressure. (reservoir)
Define pressure
Measure of mechanical energy gradient in blood that drives its flow around different parts of the system
How do you measure cardiac output?
What is it about?
Stroke volume X heart rate
5l.min to the -1
Define the total peripheral resistance
The sum of all Arteriolar resistance
The sum of the resistance of all the peripheral vasculature in the systemic circulation
Describe the arterial pressure in systole
And diastole
Maximum arterial pressure
Minimum arterial pressure
What drives flow at all times in the cardiac cycle?
Blood pressure gradient
What is the typical systolic and diastolic pressure?
120 mmHg
80 mmHg
What factors affect systolic and diastolic pressure
How hard the heart pumps
Arterial compliance ‘stretchiness’ of elastic arteries
Total peripheral resistance
What happens to the pulse pressure down the arterial tree?
Variation in pulse pressure decreases
Approaches smoothed mean prior to flowing capillary circulation
What happens to the pressure gradient by the time it gets to the capillaries?
Decreases substantially by the time it gets to capillaries - but there is enough pressure to drive it through otherwise you would get Ischaemia
Name two resistance vessels
What is the resistance in these vessels?
Arterioles and pre capillary sphincters
High resistance - narrow lumen due to proportionately large amount of smooth muscle
Why do Arterioles have high resistance?
Narrow lumen due to the large amount of smooth muscle in tunica intima
What is vasomotor tone?
What is one of the main influences?
Continuous contraction of the muscle.
As muscles do not actively relax, expect under maximum flow there must always be some vasoconstriction.
Vasodilation is reduced vasoconstriction
Sympathetic branch of the autonomic nerves system
What happens to vasomotor tone at rest?
Why?
High vasomotor tone - tonic contraction of smooth muscle
Modest resource demand, no need to employ large functional reserve, increase SNS activity driving alpha 1 GPCRs - contraction leads to further vasoconstriction
What type of metabolites do metabollically active tissues produce?
Name four.
What effect do they have on blood flow?
Vasodilation metabolites
H+, K+, Adenosine, Lactate, CO2
Relax vascular smooth muscle, lowering resistance and increasing blood blow
What would decrease vasomotor tone?
How are metabolic factors removed?
Metabolic activity and release of vasodilation factors (usually acute)
Increased blood supply removes metabolic factors with gradual return to vasomotor tone dominated by SNS
In reactive hyperthermia when blood flow is restored why are the local arterioles dilated maximally and blood flow very high?
As blood to an organ has been cut off and then restored.
Organ/limb has continued metabolising and producing vasodilators during the period of no circulation.
When circulation is restored, arterioles dilate maximally and blood flow very high
What is compliance?
The ability of a blood vessel wall to expand and contract passively with changes in pressure
E.g. Veins - high compliance - accommodate change rapidly
What is the pressure in veins determined by?
Volume of blood they contain
Which is determined by:
Cardiac output
Body metabolic demand
Back pressure from pressure waves
What is central venous pressure?
What is it usually?
What does it depend on?
Pressure measured in the great veins supplying the heart
Required for filling RA in diastole
2-6 mmHg
Return of blood from body, pumping of the heart, gravity and muscle pumping
How do arterioles control variable blood flow to tissues?
Walls contain much smooth muscle whose state of contraction determines lumen diameter and therefore flow resistance
What are the two major subdivisions of the vascular system?
How are they arranged?
What is each subdivision composed of?
How are most vessels of a given type arranged?
Systemic and pulmonary circulations
In series with one another
Number of types of vessels (arteries, arterioles, capillaries, venules and veins)
In parallel
