Fluids & Pressure Flashcards
What is pressure?
Force applied per unit area
What is a fluid?
A substance that continually deforms under an applied shear stress
What is fluid pressure?
The pressure at some point in a fluid. Can be static (still) or dynamic (moving)
What is flow?
The quantity of fluid passing a point per unit time
Flow = Quantity/Time
What are the 2 types of flow?
- LAMINAR: Molecules pass smoothly & steadily, parallel to walls of vessel
- TURBULENT: Molecules swirl into eddies
What factors affect how a fluid flows?
- Viscosity
- Density
- Pressure gradient
- Tube features
What is viscosity?
Property of a fluid that causes resistance to flow
- Newtonian: Viscosity is constant
- Non-newtonian: Viscosity changes (blood)
What is density?
Mass of a substance per unit volume (kg/m3)
What is a pressure gradient?
The fall in pressure across the tube length/orifice
What equation can be used to describe Laminar flow?
Hagen- Poiseuille equation
Q = πPr^4 / 8ηl
Q = flow
P = pressure drop
n = viscosity
l = tube length
What does the Hagen - Poiseuille equation show?
Flow is proportional to:
- pressure gradient across a tube
- the radius to the power of 4
Flow is inversely proportional to:
- Viscosity
- Length of tube
What is the most important factor in turbulent flow?
Density
Flow is inversely proportional to the square root of density
Is heliox useful in bronchospasm?
No - similar viscosity to air, in bronchospasm flow is largely laminar
What does Reynold’s number tell you?
Whether flow is likely to be laminar or turbulent
< 2000 = laminar
> 2000 = turbulent
What is the equation for Reynold’s number?
Re = pvd / n
What is the equation for resistance?
R = P / Q
(Pressure gradient / flow)
How is resistance linked to laminar flow?
On a pressure/flow graph resistance is the gradient of the line
What is the equation for resistance in terms of turbulent flow?
R = P / Q2
What is Bernoulli’s principle?
In a closed system the amount of energy remains constant. Therefore as flow/velocity increases pressure must decrease by an equal amount
What does the Bernoulli principle give rise to?
Venturi effect
What is the Venturi effect?
When a fluid flows through a constriction within a tube the velocity increases and drop in pressure. This drop in pressure allows a second fluid to be introduced.
What is the equation for entrainment ratio?
ER = Entrained flow / driving flow
What devices use Venturi effect?
- Venturi O2 masks
- Nebulisers
- Sanders injector for ventilation
- Suction
What is the Coanda effect?
The tendency of a stream of fluid flowing close to a convex surface to follow the surface rather than its course.
What can the Coanda effect explain?
- Uneven ventilation at bronchiole
- Myocardial ischaemia distal to CA branching
How can gas flow be measured?
- Vitalograph
- Bell spirometer
- Rotameter
- Pneumotachographs
- Vane meters
- Mechanical flow transducers
- USS flow meter
- Hot wire anemometers
How can liquid flow be measured?
- Infusion pumps: Syringe drivers, peristaltic pumps
- IV drip counters
How does a rotameter work?
- Valve opened allowing greater flow rate of gas
- Bobbin rises in tube to position where force of gas pushing up is equal to gravity pushing down
How do low & high flows affect a rotameter?
LOW: Bobbin at narrower end of tube. Gas flows across greater length than diameter - laminar flow
HIGH: Bobbin at wider ed of tube. Gas flows across greater diameter than length - turbulent flow
What factors can affect a rotameters accuracy?
- Warmer gasa have lower density - may over-read
- Reduced atmospheric pressure reduces gas density- may over-read
