Chapter 5: Fluids Flashcards
What is a fluid and how can it be quantified?
Fluid is any material that has the ability to flow
Liquids and gases are fluids
F = Q/t
Flow = Quantity / time
Flow is the volume of fluid passing at a given point per unit time
How does pressure affect fluid?
Fluid only flows when there is a pressure gradient from the original to final destination. Ex. Water towers create pressure for faucets.
What is the difference between hydrostatic and hydrodynamic?
Hydrostatic: study of fluids not in motion
Hydrodynamic: study of fluids in motion
What is Pascal’s Principle?
An external pressure applied to a confined fluid is transmitted unchanged to every point within the fluid.
Applies to confined, incompressible fluids.
P2 = P1 + ρgh = Patmosphere + ρgh
If you increase pressure on top of the fluid by 5 Pa, then P2 will now be:
P2 = P1 + ρgh = (Patmosphere + 5 Pa) + ρgh
‒ The pressure at point P2 is the original pressure plus 5 Pa
What are some trends in volume flow rate?
Flow is directly proportional to the speed, the radius/cross-sectional area of the tube and change in pressure.
Flow is inversely proportional to viscosity and length.
Poiseuille’s law? Components and manipulation?
For laminar flow only.
Flow rate: volume of fluid that flows per unit time
Flow rate (Q) = V (volume) / (time)
can also be written as
Q = A (cross sectional area) x v (speed of the fluid in that section)
Manipulating formulas example: Raise A to A5, what is v? (1/5)v
Flow rate = directly proportional to the radius of the tube raised to the fourth power
Flow rate = inversely proportional to the viscosity of the fluid and the length of the tube
Describe viscosity and how it is measured
Viscosity: resistance of a fluid to flow
High viscosity = slower flow; ex. honey
Can be determined by measuring the amount of time it takes for a fluid to flow a known length through a tube of known diameter.
Viscosity in blood:
Increased in: dec temp, inc hct; inc. age, smoking, inc plasma proteins
Decreased in: warm and diluted blood; anemia, low molecular weight dextran
Bernoulli Principle
When velocity of flow increases = pressure decreases
P + ½△^2= pgh
P = pressure, △ = density, V = velocity, g = gravitational acceleration, h = height
Must apply the Work-Energy Theorem
Examples:
Airplane take-off
Roof coming off during a storm (velocity of storm air high = decrease in pressure on the roof; inside the house velocity is low = increase in pressure inside; high pressure tries to move to low pressure = roof flies off)
Venturi effect
The Venturi effect describes how the rate of fluid flow in an enclosed system changes as the flow enters a constricted channel.
Pressure drop occurs in narrowing of tube = increase in velocity in the tight sections of the tube.
If you go from A to 1/2A, then velocity = 2v
Examples: Squeezing a water hose = water speed increases
Venturi mask: when pt breathes in, oxygen travels at high velocity (low pressure) through a small tube and travels into the orifice device creating a suction/vacuum that also pulls in surrounding air = large total flow at a predictable FiO2
Coanda effect
Fluid that comes in contact with a curved surface will tend to follow the curvature of the surface rather than continue traveling in a straight line.
From her ppt:
A fluid that comes in contact with a curved surface will cling to the surface which alters the direction of the flow
After constriction fluid will flow along convexed surface
Increase in pressure from narrowing results in decrease of velocity
Greater flow will follow path of lower pressure and therefore receive more volume
LaPlace Variables
In an enclosed structure, the fluid pressure everywhere in that structure will be the same; however, tension on the walls of that structure will vary according to the shape.
LaPlace describes the relationship of wall tension (T) to pressure (P) and radius (r) in cylinders and spheres.
LaPlace: How are cylinders and spheres different?
Cylinders: Tension (T) = Pressure (P) x Radius (r)
Spheres: 2T = Pr or T = (Pr) / 2
LaPlace: How is it related to physiological organs in the body?
Larger arteries = larger radius = larger tension; therefore, designed with stronger walls/reinforced with fibrous bands.
Capillaries have smaller radius = lower wall tension.
Aneurysms to some degree provide “relief” to tension as shape is more spherical, but radius increases -> increased wall tension and eventually ruptures.
LaPlace: Left Ventricle
Heart failure eventually causes dilation (increased radius) of LV = needing more tension to generate the same pressure
Chronic higher pressures cause the walls to hypertrophy to balance the equation.
LV Tension = (LV pressure x radius) / ( 2 x LV wall thickness)
Per LaPlace formula, if you increase thickness, you can stabilize tension.
LaPlace: Alveoli
2T = PR or P = 2T/R
P = the distending pressures within the alveolus T = surface tension of the alveolar fluid R = radius of the alveolus
From this we can see that the pressure needed to expand an alveolus is:
1- directly proportional to the surface tension, and
2- inversely proportional to the radius
The greater the surface tension and the smaller the radius, the greater the pressure needed to expand the alveoli.
Surface tension inside the alveoli will cause it to want to collapse, especially a smaller one communicating to a larger alveoli. Surfactant prevents this from happening as it reduces the surface tension.
Double whammy for premies: small alveoli and no surfactant. They need high FiO2 concentrations and pressures to overcome the dead space.
