Dynamic Flow Flashcards
How are fluids defined?
Fluids are defined by their response to stress.
What is stress?
the distribution of force per unit area
What is strain?
the deformation caused by stress.
What two ways will fluids responds to shear stress or perpendicular forces?
Resist compression (e.g., liquids) Become compressible and easily expandable (e.g., gases)
Both liquids and gases are _______.
Fluids
What are forces associated with fluids (4)?
Gravity, Pressure, Friction, Viscosity
What is friction?
Friction is resistant to flow from surface interaction and is proportional to viscosity.
What is viscosity?
A physical property of a fluid that relates to shear stress to the rate of strain (thickness/ stickiness of a solution)
What is flow?
Is the result of pressure forces in a fluid established by differences in pressure from one point to another, which creates a pressure gradient
What creates a pressure gradient?
Flow
How does flow move?
All flow moves from higher pressure/resistance to lower pressure/resistance.
What is a compressible fluid?
Gas
What is an incompressible fluid?
Liquid
What is the equation for flow?
Defined as the quantity of a fluid passing a point per unit of time. F = the mean flow Q = the quantity t = time. F=Q/t
What are three types of flow?
Laminar, transitional, and turbulent flow
Who describe laminar flow?
Jean Louis Marie Poiseuille
Laminar flow can be thought of as thin layers, laminae, which are all _____ to each other.
parallel
What is laminar flow?
The fluid/gas in contact with the horizontal surface is stationary and al other layers slide over each other with increasing speed, but none of the layers mix
Another name for laminar flow is _______.
Smooth flow
What are examples that show laminar flow?
Substance of high viscosity, slow speed, and in smooth narrow tubes, such as capillaries.
Laminar flow: Where is flow at is greatest point?
Toward the center of the tube (about twice the mean flow rate) and approaches zero at the tube wall.
What must be available to drive laminar flow?
Pressure gradient
What is viscosity a measure of?
Friction “within the fluid”
Laminar flow: where is the viscosity?
Since laminar flow is viewed as thin layers that do not interact with one another, the viscosity will be the friction between the layers. This friction will resist the movement of the layers and therefore resist flow.
What rules laminar flow?
Viscosity
What has the most dramatic effect on flow?
Radius or diameter of the tube
What amount of flow increase will be seen by doubling the radius according to Poiseulille’s Law?
16-fold increase
What amount of flow increase will be seen by tripling the radius according to Poiseulille’s Law?
81-fold increases
Which has greater flow a 16-gauge catheter or a 20-gauge catheter?
16-gauge
If the viscosity of a fluid is increased, flow _______
decreases
What happens to flow when you lengthen the tube?
Flow rate will decrease
What happens to flow when viscosity increases?
Flow rate will decrease
Applying Poiseuille’s Law: We can increase the infusion rate of a unit of packed red blood cells by _________, which will lower viscosity.
diluting the blood with normal saline
Applying Poiseuille’s Law: An anemic patient will have a _____ viscosity of the blood r/t _____and therefore a faster flow rate.
low; fewer RBC
Applying Poiseuille’s Law: Patients with _______ have decreased blood flow due to increased blood viscosity
polycythemia
Applying Poiseuille’s Law: We can increase the IV flow rate by ____________ or _________ and thereby increasing the pressure gradient (ΔP ).
raising the IV pole or applying a pressure bag
The difference in the flow through equally long 18-gauge catheter and 20-gauge catheter is almost ________ through the 18-gauge.
2 times faster
If the length of a tube is decreased by 50%, there will be a corresponding ______ of the flow.
doubling
If the length of a tube is doubled, flow decreases by ______.
half
True or false. Laminar flow can change to turbulent flow.
True. Laminar flow may change to turbulent flow if the conditions for creating the laminar flow changes
What can cause turbulent flow?
When flow reaches a constriction, the fluid/gas velocity increases and the flow may become turbulent
What is turbulent flow?
Chaotic with the thin layers of flow beginning to swirl in eddies throughout increasing the resistance.
Turbulent flow: When is the flow velocity highest?
The flow velocity is no longer highest in the center, but becomes even throughout the tube.
Who created the calculation for turbulent flow?
Osborne Reynolds
What is Reynolds number?
An index that incorporates the factors of Poiseuille’s law with the addition of a fluid’s density to determine whether a given flow will be laminar or turbulent.
How is the Reynolds number influenced by fluid density?
directly proportional
How is the Reynolds number influenced by linear velocity of flow?
directly proportional
How is the Reynolds number influenced by tube diameter?
directly proportional
How is the flow influenced by fluid viscosity according to Reynolds number?
flow is inversely proportional to fluid viscosity.
What will the type of flow be when the Reynolds number is <2,000?
Laminar
What will the type of flow be when the Reynolds number is >2,000?
Turbulent
What rules turbulent flow?
Density
What is the relationship between flow and perfusion pressure in the vessel? Why?
Linear relationship- Laminar flow has a constant resistance
What happens to flow, when it becomes turbulent?
the resistance to flow increases.
What happens to perfusion pressure as turbulence increases?
The perfusion pressure required to generate a given flow increases as well
What causes murmurs?
Dysfunctional heart valves create restrictions to flow, which will cause turbulence and audible vibrations.
How can a helium and oxygen mixture effect flow during a severe asthma attack or epiglottis?
Helium has a significantly lower density than nitrogen and oxygen. A helium/oxygen mixture can improve flow conditions by somewhat restoring laminar flow conditions through the significantly narrowed airways during a severe asthma attack or epiglottitis.
What are the five characteristics that turbulent flow is often present in?
- An orifice
- A sharp bend in the tube (>25 degrees)
- Increased fluid/gas velocity
- Corrugated (rough walled) tubing
- Reynolds number >2,000
What effect do ET size have on flow?
Larger endotracheal tubes increase flow of gas for ventilation
How does albuterol (B2 receptor agonists) improve ventilation?
Delivery of β2 receptor agonists such as albuterol increase the diameter of the bronchial tubes of the lungs to improve flow.
How is ventilation improved by increasing peak inspiratory pressures?
Establishes a higher pressure gradient, which improves flow and delivered tidal volumes
Increases in the pressure gradient and flow velocity may ____________, but risk converting that flow to turbulent flow.
initially improve flow
Where does turbulent flow often occur?
often occurs in the medium to large airways of the lung and predominates during periods of peak flow, coughing, and phonation.
What can cause laminar flow to become turbulent?
Orifice constriction, such as glottic closure
Why is laminar flow maintained in smaller bronchial tubes?
have slower velocities
What is transitional flow?
A mixture of laminar flow along the walls of a tube with turbulent flow in the center.
When will flow become fully turbulent according to Reynolds Number?
When Reynolds number exceeds 2,000, flows tend to become turbulent, but will not be fully turbulent until Reynolds number exceeds 4,000.
Reynolds number describes transitional flow as _________
2,000 and 4,000
if the value is >2,000 and transitional flow is not an option, it would be considered ________ flow
Turbulent flow
What are the walls like during transitional flow?
Laminar
What is Bernoulli’s Principle?
Describes the effect of fluid flow through a tube containing a constriction.
Who describes Bernoulli’s Principle?
Daniel Bernoulli
According to Bernoulli’s Principle, an increase in the velocity of the fluid occurs simultaneously with a __________.
decrease in the pressure within the fluid
What is the foundation of Bernoulli’s Principle?
The total energy must remain the same at all times (law of conservation of energy) so when the velocity increases, the pressure must decrease for the internal energy to remain unchanged.
Bernoulli’s Principle: The relationship of the pressure gradient to mass flow requires the pressure to _______ when the velocity increases
decrease
According to Bernoulli’s Principle:, how is flow influenced by velocity?
The velocity of a fluid is equal to the flow rate divided by the area of flow.
Bernoulli’s Principle-Velocity: What is the equation?
Velocity= Quantity of flow per unit of time/ area
If given flow is 4 L/min over an area (tube) with volume of 2 L, the fluid velocity would be ________.
2 L/min.
If the flow then meets constriction that decreases the cross-sectional volume to 1 L, the fluid velocity would increase to
4 L/min
What is an example of application of the Bernoulli Principle?
Metered-dose inhalers (MDIs)
What is metered-dose inhalers (MDIs)?
use the Bernoulli principle to create a jet past a constriction that aerosolizes a drug in the expanding flow of gas.
What is the venturi flow?
By placing an orifice at the narrowed region of flow, air or fluid is allowed to be entrained and enter the flow.
The Venturi Effect utilizes _______
Utilizes the pressure drop across a narrowing in a tube.
How does jet ventilation apply the venturi effect?
Uses this entrainment of air to augment lung ventilation volumes
What type of medications use the venturi effect?
Nebulizers-deliver both humidification and medications
What has replaced nebulizers?
Deliver medications into fluid paths, such as ventilator circuits, but have been replaced to a large extent by MDIs
What does the coanda effect explain?
Explains the tendency of fluid flow to follow a curved surface upon emerging from a constriction.
What can the coanda effect cause?
This may cause preferential flow to one tube at a bifurcation just past a narrowing in a tube.
Describe the coanda effect.
When fluid flows through a constriction, it will speed up and the increased velocity will cause a diminished pressure (Bernoulli’s). After the constriction, the velocity will again diminish, and pressure will increase. If a bifurcation is present, there will be a tendency for the flow to adhere to the wall of the branch where the velocity falls the least and therefore the pressure increases the slowest. This will be the side with the smallest angle at the bifurcation.
Why can the coanda effect be problematic?
The path with the greater flow will receive a higher volume of fluid or gas at the expense of the path with lesser flow. In situations where this flow is blood in vessels or gas in the lungs, this diversion of flow could be consequential.
What is Laplace’s Law?
Describes the relationship of wall tension (T) to pressure (P) and radius (r) in cylinders and spheres.
What is Laplace’s formula for cylinders?
T=Pr
What is Laplace’s formula for Spheres?
2T = Pr
What is tension?
a stress force exerted over a given area.
How is tension measured?
Measured in newtons per centimeter (N/cm)
What law demonstrates why smaller-diameter capillaries do not burst during hypertension compared to larger aneurysms?
Laplace’s Law in Cylinders
What is maintained in abdominal aorta even in a aneurysm?
MAP along its length
What happens to aneurysm according to Laplace’s law?
Aneurysms, which have a greater radius than the rest of the aorta, have a corresponding greater tension and are more likely to rupture.
Laplace’s formula, when rearranged, reflects the direct relationship of ________ to radius in both the aorta and an aortic aneurysm at a constant pressure:
Tension
P=T/r
Example 1—Cylinders: If the mean aortic pressure is 100 mm Hg with a normal radius of 2 cm and an aneurysm radius of 4 cm, the tension is calculated as follows:
Normal aorta: 100 mm Hg (P) × 2.0 cm (r) = (T)
1.33 N/cm2 (100 mm Hg) x 2.0 cm = 2.66 N/cm
Aortic aneurysm: 100 mm Hg (P) × 4.0 cm (r) = (T)
1.33 N/cm2 (100 mm Hg) x 4.0 cm = 5.32 N/cm
What could cause a dissection or rupture?
Any increase in blood pressure will increase the already high wall tensions of an aneurysm- Laplace Law
In a sphere, wall tension is _____ that of a cylinder of the same radius.
half
What can Laplace’s law be applied to in health care?
Saccular aneurysms
If the mean pressures of two saccular aneurysms are 100 mm Hg, one with a radius of 0.5 cm and the other with a radius of 1 cm, the tension for each is calculated as follows:
- Small saccular aneurysm:
100 mm Hg (P) x 0.5 cm (r) / 2 = T
1.33 N/cm2 (100 mm Hg) x 0.5 cm / 2 = 0.3325 N/cm - Large saccular aneurysm:
100 mm Hg (P) x 1.0 cm (r) / 2 = T
1.33N/cm2 (100 mm Hg) x 1 cm / 2 = 0.665N/cm
________ would be present in a large saccular aneurysm (0.665 N/cm) than in a small saccular aneurysm (0.3325 N/cm), and any increases in pressure would risk further increases in wall tension and rupture.
Greater wall tension
What is the rationale for decreasing blood pressure in patients with aneurysm?
Decreasing pressure will decrease wall tension in both cylinders and spheres
How is Laplace’s Law applied to cardiac ventricle?
Sphere- to a cardiac ventricle of increasing size, explains the necessary inotropic response but eventual failure of contractility with increasing wall tension and pressure
Increased pressure=Increased Wall tension, Increased radius = increased wall tension= increased wall tension= increased contractility
What law represents the change in contractility of the heart?
Frank-Starling Curve
What creates surface tension in alveoli?
created by a layer of water
What effect does increased surface tension on the alveoli?
alveoli to collapse
As surface tension increases, collapsing pressure _______.
Increase
What is surfactant?
is a substance that lowers surface tension in the alveoli and prevents the effects observed with Laplace’s law.
What happens as a result of lowering surface tension?
the pressure required to open alveoli is lowered
What effect does surfactant have on surface tension?
Surfactant lowers surface tension more in smaller alveoli than in larger alveoli, owing to the concentration effect that occurs when an alveolus contracts.
What would happen with out surfactant?
small alveoli would collapse, as they would require higher pressure to open compared to larger alveoli.
What can surfactant create?
Surfactant has the ability to equilibrate surface tension among different-sized alveoli and create stabilized alveolar pressures.
