What is the total volume of blood in the systemic circulation?
Flow of blood will be 5L per min at any level of the circulation
Which vessels produce the majority of resistance to flow?
What are the two main factors that affect blood flow in a vessel?
Pressure gradient: blood flow is generated by a pressure gradient. Flow is proportional to the difference in pressure between teh ends of a vessel
Resistance: flow is determined by the resistance of the vessel for a given pressure gradient
flow = pressure gradient/resistance
The volume of blood passing though the circulation per unit time (ml per second)
The distance blood travels along a vessel per unit time (cm per second)
What factors affect the velocity at which blood travels?
Cross sectional area: velocity will vary along the length of the vessel if the cross sectional area changes (inversely proportional)
Flow: flow must be kept constant at all points along a system of vessels (e.g. veins, arterioles)
flow = velocity x area
How does the resistance to flow change with the radius?
If flow is kept constant, then a vessel with a smaller radius will have a higher velocity.
However normal blood flow is laminar, so reducing the radius reduces the velocity in the middle of the vessel (mean velocity is reduced). This increases the resistance.
Resistance is inversely proportional to the radius
Note: To keep the flow constant, the pressure gradient will have to increase.
What is transmural pressure?
The difference between intravascular pressure and extravascular pressure.
This affects the distensibility of the vessel (promotes stretching)
Describe laminar flow
Fluid does not move with the same velocity across the width of a tube.
Laminar flow means that velocity is lowest at the edges (slmost stationary) and highest at the centre where the effect at the edges is weakest.
Cells tend to travel in the centre of blood vessels, increasing viscosity of the blood
This means that the width of the tube greatly affects resistance.
What are the factors that determine the resistance to flow in vessels?
Viscosity of the fluid - greater viscosity increases resistance. Viscosity of the blood is dependent on hematocrit (and plasma proteins)
Radius of the vessel - smaller radius increases resistance
Length of the tube - the longer the vessel the greater the resistance
Name 3 factors which increase the chance of turbulent flow
Velocity is high
Viscosity of blood is low
Blood vessel diameter is high
Describe turbulent flow
In turbulent flow the layers of laminar flow break up and flow becomes disordered.
The resistance to flow is increased.
Turbulent flow results in damage to the endothelium and will give rise to murmurs/bruit
How is pressure of the blood affected as it flows through the circulation?
Cardiac output flows through each part of the circulation.
In the major arteries the pressure is high, however this decreases in proportion to the resistance of each vessel that it passes through.
The flow through each vessel is therefore inversely proportional to resitance (low resistance, higher flow)
Greatest drop in pressure is seed in the arterioles which have the highest resistance
What happens when resistance in arterioles rise?
The stroke work of the heart increases to maintain cardiac output. This increases the pressure gradient so that a constant flow can be maintained
How is flow affected in distensible vessels?
Distensibility of blood vessels gives them capacitance
In distensible vessels, an increase in pressure stretches the vessel walls, which lowers resistance. This means that as pressure is increased, the flow of blood in the tube is increased. If the pressure falls towards zero, the vessel collapses and flow stops.
Describe the relationship between pressure, resistance and flow
Flow = pressure gradient/ resistance
Flow is directly proportional to the pressure difference between two points
Flow is inversely proportional to the resistance