Week 1 - Physics Fluid and Fluid Flow

Question 1

Define Fluid

Answer 1

Any material that has the ability to flow when acted upon by outside forces

Has no fixed shape and conforms to the shape of its container

Liquids (resist compression) Gases (easily compressible/expandable)

Question 2

Adhesion vs Cohesion

Answer 2

Adhesion = force of attraction between DIFFERENT types of molecules
Adhesive forces draw water inside of thin tubes (causes capillary action)

Cohesion = force of attraction between the SAME molecules
Causes surface tension

Question 3

Define Surface Tension

Answer 3

The elastic tendency of a fluid surface which makes it acquire the least surface area possible

The force required to increase the surface area of a fluid in dynes/cm

The surface molecules align can act as a “skin”

Increase surface tension, increase force

Question 4

How does the surface tension of alveolar fluid affect the alveoli?

Answer 4

Surface tension of alveolar fluid (water) creates a polar force that promotes alveolar collapse

Question 5

What is surfactant?

Answer 5

A lipoprotein complex formed by type II alveolar cells

the proteins and lipids that surfactant comprises have both a hydrophilic region and a hydrophobic region

Hydrophilic ends attach to the water and the hydrophobic ends pull away from the water, breaking up the surface tension (prevents alveoli collapse - decreases pressure in the alveoli by decreasing wall tension)

Question 6

Law of LaPlace

Answer 6

Relationship between wall tension, pressure, and radius and wall thickness

• Wall tension is directly related to the radius of the sphere/cylinder
• Wall tension is directly related to pressure inside the sphere/cylinder
• Wall tension is inversely related to the wall thickness

Cylinder: T = PR / wall thickness
Sphere: T = PR / 2 wall thickness
(wall tension in a sphere is half the wall tension of a cylinder with the same pressure, resistance, and wall thickness)

Question 7

What is critical closing pressure or volume?

Answer 7

In very small vessels or alveoli, wall tension is low collapsing force of external pressure may be greater than distending force

if critical close pressure is reached, vessel or alveoli may collapse

Question 8

How does positive end expiratory pressure (PEEP) affect alveoli?

Answer 8

Application of pressure above atmospheric throughout expiration

Recruit alveoli and keep vulnerable ones from collapsing (prevents alveoli from reaching critical closing pressure/volume)

Especially important in neonates and children who have immature alveoli and surfactant production

Question 9

Define Transmural Pressure

Answer 9

Internal pressure minus external pressure or the net pressure across the walls of the vessel

(pressure from the inside acts to expand or increase vessel or airway diameter

external pressure acts from the outside to try and collapse the vessel or alveoli)

Question 10

How is the Law of LaPlace related to an aneurysm?

Answer 10

Radius is increased so wall tension is increased
Wall thickness is decreased which increases wall tension
Internal pressure is reduced which also increases transmural pressure

Question 11

Hagen-Poiseuille Law

Answer 11

Describes the relationship of laminar flow through a tube to pressure and resistance

F = π (radius)^4 (pressure gradient) / 8 (viscosity) (length)

• Flow is directly proportional to the 4th power of the radius and the pressure difference
• Flow is inversely proportional to the length of the tube and the viscosity
• Flow is directly related to the pressure gradient across the system and inversely related to resistance

Ex: administering blood (large IV, pressure bag, dilute with NS)

Question 12

Define Laminar Flow

Answer 12

all molecules of a fluid travel in a parallel path within the tube

center encounters the least adhesive force (move at a velocity 2x that of mean flow)

More efficient (uses less energy so fluid can travel faster leading to more output)

Flow rate is proportional to 1/viscosity

Question 13

Define Turbulent Flow

Answer 13

chaotic with irregular eddies throughout

increased with high velocity, high density, large tube diameters, and low viscosity

Flow rate is proportional to 1/density

Question 14

Define Transitional Flow

Answer 14

mixture of laminar flow along the walls of a tube with turbulent flow in the center

Question 15

Define Viscosity

Answer 15

Measure of the internal friction of a moving fluid (thickness)

flow rate is inversely related (the more viscous the less flow)

viscosity of a liquid increases as temp decreases

viscosity of a gas increases as temp increases (particles bound around in all directions more, layers don’t slide as easily)

Question 16

Velocity vs Flow Rate

Answer 16

Velocity = the DISTANCE the fluid travels with respect to time (think speed) - cm/sec

Flow Rate = the VOLUME of fluid that is moving per unit of time - mL/min

Question 17

Define Critical Velocity and the Reynolds number

Answer 17

Critical Velocity = the velocity above which flow changes from laminar to turbulent

Reynolds Number = an index that is used to predict when flow will become turbulent (density now becomes important in turbulent flow)

Flow turns turbulent when Reynolds number is above 2000

Question 18

What conditions increase turbulent flow?

Answer 18

High velocity
High density
Large tube diameters
Low viscosity

Question 19

At flows _____ the critical flow rate, laminar flow is replaced by turbulent flow.

Answer 19

Below

Question 20

Bernoulli’s Principle

Answer 20

Describes the relationship of velocity on the pressure of an ideal fluid as it flows through a constriction in a pipe

Assumes the volume flow rate is constant and energy must be conserved

Pressure is greatest where the velocity is lowest
Pressure is lowest where velocity is greatest (at the constriction)

Ex: Aortic stenosis decreases flow to the coronary arteries – narrowing of the valve decreases the pressure thus lowers the amount of blood that reaches the coronary arteries

Question 21

Venturi Effect

Answer 21

Describes and applies the effect of the decreased pressure at constriction in a pipe (based on the Bernoulli Principle)

A tube inserted at the constriction will draw fluid into the tube due to the drop in pressure at the constriction. The fluid will be injected into the pipe stream (Ex: Atomizer)

Air will be dragged into a tube if there is a hole placed in the constriction (Ex: Venturi mask)

Question 22

Coanda Effect

Answer 22

Describes the tendency of fluid flow to follow a curved surface upon emerging from a constriction

Ex: deviated trachea points ETT to the right so right lung inflated more than left

Question 23

Ohm’s Law

Answer 23

Describes the flow of current in an electrical system

I = V / R
(Current = voltage / resistance)

Analogous to fluid flow in a system (Flow=P/R)

Question 24

Describe the effect surfactant has on alveoli

Answer 24

Without Surfactant: decrease in the radius of alveoli increases the pressure within. when pressure increases in small alveoli the pressure is higher than the larger alveoli and air moves from small to large collapsing the small

With Surfactant: allows for the surface tension to increase as radius increases, maintaining wall tension keeps pressure within the alveoli the same. the gas doesn’t move from small to large alveoli thus preventing collapse

Question 25

What is the equation for resistance to flow?

Answer 25

Resistance = pressure gradient / flow
For laminar flow – R = 8(viscosity)(length)/ π (radius)^4

• Resistance is inversely proportional to r^4 (greater radius = less resistance)
• Resistance is directly proportional to viscosity (thicker fluid = more resistance)
• Resistance is directly proportional to length (longer tube = greater resistance)
• Turbulence increases resistance to flow