Define Pressure (P)
Force exerted per unit area (surface area)
Define Pressure Gradient (∆P)
Difference in forces exerted (per unit area) at either end/side of an object
Define Flow (Q)
The volume of liquid passing a given level of the circulation/airways per unit time
A force that tends to oppose the flow of a substance
The rate of movement of fluid particles along a vessel/airway
The property of resistance to flow in a fluid or semifluid
How is a pressure gradient calculated? How can this be applied clinically?
∆P=QR (BP= CO x SVR)
What is the relationship between flow and pressure gradient?
Flow is directly proportional to pressure gradient assuming all other things are equal
Describe the relationship between resistance and radius if the flow is constant in laminar flow.
What is the clinical application of this?
Laminar flow means that the resistance is greatly affected by the radius of the tube (if flow is constant)
Narrowing of vessels/valves or airways means that the pressure gradient (i.e. blood pressure) has to be greatly increased to maintain constant flow. The velocity will also be increased, causing a wheeze in airways or bruits in blood vessels.
What affects resistance? Give a clinical example
Resistance depends on the radius of the vessel/airway and the viscosity of the fluid
E.g. Bronchoconstriction in asthma, reduced flow of air entry.
How is flow (Q) calculated? (Laminar flow)
What is the result of this?
Flow = pressure gradient/ resistance
Flow is directly proportional to r4
- Radius doubled: flow increases 16 times
- Radius halved: flow decreases 16 times
Describe the relationship between flow, velocity and cross sectional area of the circulation/airways if the flow is constant.
If the flow is constant, velocity is inversely proportional to the cross-sectional area (V=Q/A) (A=𝜋𝑟2) (decrease in area= increase in velocity)
What is laminar flow?
Fluid moves in organised layers through tube. Velocity will be highest in the centre of the tube and lowest at the outer edges.
Describe the relationship between flow, velocity and cross sectional area if the pressure gradient (not the flow) is constant (in laminar flow)
If the pressure gradient is constant, a reduction in cross sectional area results in decreased velocity in laminar flow and a decrease in flow due to increased resistance from smaller vessel.
What is Pouiselle's Law?
The resistance of the vessel depends on its radius, its length and the viscosity of the fluid:
Resistance = 8nl/πr4
What would happen to the velocity and pressure gradient if the radius of a tube was halved and the flow is kept constant?
Velocity would increase x4
Pressure gradient would increase x16
What would happen to the velocity and flow if the radius of a tube was halved and the pressure gradient was kept constant?
Velocity would reduce x4
Flow would reduce x16
What is turbulent flow?
What are the consequences of this clinically?
Breaking up of layers of laminar flow causing flow to become disordered.
Turbulent flow means greater resistance, can result in damage to the endothelium of blood vessels
What makes flow more likely to be turbulent?
Increased velocity (from narrowed vessel)
Low viscosity (less adhesion between layers)
High blood vessel diameter (difficult for layers to maintain ordered flow)
What is the effect of branching of the circulation on resistance?
Resistances combine in vessels that are in parallel rather than in series; the overall resistance is reduced.
The flow through each vessel/airway will be inversely proportional to its resistance.
(Arterioles have the highest resistance as they are arranged more in series)
What happens if the resistance of resistance vessels (small arteries and arterioles) increases?
The heart must increase the pressure gradient by increasing stroke work to maintain cardiac output
What is transmural pressure?
Transmural pressure = intravascular pressure - extravascular pressure.
Transmural pressure has to be >0 in order to keep blood vessels open
What is the effect of resistance from increasing pressure in blood vessels?
Increased pressure causes decrease in resistance in blood vessels as they are distensible rather than rigid. Therefore increased pressure causes an increase in radius which reduces resistance.
What is the advantage of distensibility of blood vessels?
Distensibility of blood vessels gives them capacitance, allowing them to store blood.