Week 1: CTB Flashcards
Define Pressure
P - Force exerted per unit area
Define Pressure Gradient
DeltaP - Difference in forces exerted (per unit area) at either end/side of an object e.g. a tube or membrane
Define Flow (Q)
The VOLUME of fluid passing a given level of the circulation/airways per unit time (ml/s or l/min)
e.g. in Cardiovascular System remains constant throughout (cardiac output ~5l/min)
Define Velocity (v)
The rate of movement of fluid particles along a vessel/airway (cm/s)
Define Resistance (R)
A force that tends to oppose the flow of a substance
Explain the relationship between flow, pressure gradient and resistance.
- Flow is generated by a pressure gradient
- Flow proportional to pressure difference between the ends of vessels & airways if all other things are equal
- For a given pressure gradient, flow is determined by the RESISTANCE of the vessel/airway
- Pressure gradient = Flow x Resistance
- DeltaP = QR
Outline the factors determining resistance in a tube (3)
- Radius - Inversely Proportional
- Length - Proportional
- Viscosity of fluid - How easily layers of laminar flow move over each other
Explain the relationship between flow (q) , velocity (v) and cross-sectional area (A) in connected tubes
- At a given flow, velocity is inversely proportional to cross-sectional area (A) = πr²
- V = Q/A
- When the pressure gradient is constant, mean velocity is proportional to r² (radius)
What is Laminar flow?
- Layers of fluid moving over eachother parallel to tube
- Fluid does not move with same velocity across width of tube
- Velocity lowest at edges
- Velocity highest at centre of tube
- Laminar flow will mean that the width of the tube greatly affects its resistance
Explain the principle of Poiseulle’s Law and its relationship to flow
- Affect on flow
- Flow = Pressure gradient / Resistance
- Flow is directly proportional to the fourth power of the radius of the tube (r^4)
Explain ‘Flow is directly proportional to r^4’
This is Poiseulle’s Law, this means that if we double the radius, flow will increase by 16 times. Small changes in radius have a huge impact on flow
Define Viscosity
How easily the layers of laminar fluid move over each other
Outline the influences of particulates on flow e.g. blood
- Blood composition affects viscosity and thus flow.
- E.g. Haematocrit, % of RBC in blood volume
- Plasma proteins
- Laminar flow - RBC tend to get borne along in most rapidly moving stream in the centre of blood vessels
- Does not have a big impact on blood flow but can be altered in certain conditions
What can affect blood viscosity?
- Major determinant - Haematocrit - % RBC in blood volume. Increase in number of RBC can increase viscosity affecting flow.
- Can be caused by physiological conditions e.g. living at high altitude to increase oxygen carrying capacity of blood
- Pathological conditions e.g. Haematological malignancies, response to hypoxia
What is turbulent flow?
The layers of laminar flow break up and flow becomes disordered
What makes turbulent flow more likely?
- If Velocity is high (e.g. secondary to a narrowed tube)
- Viscosity is low
- Tube diameter is high
- Tube branching or irregular surfaces
In what forms are Turbulent flow relevant to clinical application?
- Turbulent flow is noisy
- Bruits (in blood vessels due to stenosis)
- Murmurs (turbulent flow around a heart valve due to stenosis / not closing properly)
- Wheeze
- Stridor (obstruction in the upper airways e.g. larynx/trachea)
Compare laminar and turbulent flow through a tube
- Laminar flow is flow parallel to the tube, whereby there is are layers of flow where the middle at which velocity is highest and sides where velocity is lowest. All fluid travelling in the same direction uniformly.
- Turbulent flow is when the layers of laminar flow break up and flow becomes disorganised
Compare the effects of resistances in series and resistances in parallel
- In series - Resistance is added for vessels/airways in series
- When many tubes arranged in parallel, effective cross-sectional area is greater, overall resistance is reduced.
Where are tubes arranged in parallel?
- Lower parts of tracheobronchial tree
* Capillaries
Where are tubules arranged in series?
Resistance vessels i.e. small arteries and arterioles
Describe the pattern of flow, resistance and pressure over the branching networks of the cardiovascular and respiratory systems
- Flow is constant
- Resistance is reduced at parallel branching sites e.g. capillaries and lower parts of tracheobronchial tree
- Resistance is highest at small arteries and arterioles arranged in series. Very tightly controlled via contraction and relaxation of smooth muscles within these vessels.
What is distensibility of tubes?
- Particularly veins.
- Blood inside vessels creating pressure (intravascular p.), external pressures acting on vessel outside (extravascular p.).
- Overall - Transmural p.
- Determines whether vessel stays the same size, distends, or collapses
What is Transmural pressure?
- Transmural pressure = Intravascular pressure - Extravascular pressure
- Determines distensibility of vessels, especially veins