Fluid Dynamics/Gas Exchange

Question 1

Ohm’s Law of Fluid Dynamics

Answer 1

Q = ∆P / R
Defines how a system resists flow. Relates a driving force (pressure difference) to flow through a resistance.
States that fluids flow easier with greater pressure (overcomes resistance) or less resistance (allows flow). Pressure difference between two ends of a vessel is necessary for flow to occur.

Question 2

Bernoulli’s Law

Answer 2

Describes the ideal fluid, focusing on energy conservation. States in an ideal, streamline flow with no viscosity/friction, the total energy of a fluid per unit volume remains constant and no energy is lost, and velocity remains constant throughout vessel. Kinetic, pressure, gravitational energies. One changes, so do the others to keep total energy constant. Lower pressure = higher velocities.

Question 3

Reynold’s Number

Answer 3

Dimensionless number defining if a fluid is laminar or turbulent. Compares inertia (moving force) to resistance (viscous force). If resistance is higher, flow remains laminar. If inertia is higher, flow is turbulent. Laminar <2000, turbulent >4000

Question 4

Poiseuille’s Law

Answer 4

Compared to Bernoulli’s law, this refers to a real fluid. Describes fluids flowing easier through wider, shorter tubes. Viscosity slows flow due to resistance and drag against wall creating friction and energy loss. This causes pressure gradient as pressure ↑ which is necessary to overcome resistance and maintain flow. Radius affects resistance most = r^4

Question 5

Incompressible fluid

Answer 5

Density of the fluid remains constant and unchanged, despite pressure

Question 6

Continuity of Flow

Answer 6

Relates to conservation of mass in a laminar, incompressible fluid inside a cross-sectional closed loop. States that whatever enters the vessel must be equal to whatever leaves it, despite its area. Fluid must have a constant density and there must be no addition/removal of fluid in the system. It equates to the average velocity of fluid across a cross-section. If the cross-section ↓ then velocity ↑ to preserve flow rate and continuity throughout.

Question 7

Newtonian fluid

Answer 7

Fluid with constant viscosity and obey Newton’s law of viscosity
Example water, air, most gases

Question 8

Non-Newtonian fluid

Answer 8

Does not follow Newton’s law of viscosity. Viscosity changes with shear rate and shear stress, the relationship of which is non-linear.

Question 9

Shear rate

Answer 9

A measure of how quickly the velocity changes between fluid layers across a distance (from vessel wall to centre). Velocity is highest in the centre of the vessel (parabolic)

Question 10

Is blood Newtonian or non-Newtonian? Why?

Answer 10

Non-Newtonian - shear thinning/pseudoplastic.
Blood does not have a constant viscosity, it behaves differently in different conditions. RBCs deform depending on shear rates, plasma proteins, and haematocrit (volume of RBCs)

Question 11

How does blood behave in low shear rates?

Answer 11

Gradual change in velocity across fluid layers = higher viscosity as RBCs aggregate into stack-like clusters and fall out of line with the flow, ↑ resistance to flow

Question 12

How does blood behave in high shear rates?

Answer 12

Fast change in velocity across fluid layers = lower viscosity as RBCs de-aggregate and disperse, and align with the flow, ↓ resistance to flow
(Think rapidly shaking a ketchup bottle makes it flow out easier - less viscous)

Question 13

Shear stress

Answer 13

Occurs when layers of fluid slide past each other at different speeds, creating friction between them. It is the “sliding force” that makes more viscous fluids flow slower than non-viscous fluids

Question 14

Strain

Answer 14

The response to stress. A measure of deformation in a material (solid or fluid) caused by an applied force/stress. It reflects the continuous deformation of fluid elements under flow, without returning to their original shape.

Question 15

Young’s Modulus

Answer 15

Measures the stiffness/elasticity of a material, by relating stress to strain. It measures stiffness by comparing how much it stretches or compresses under force

Question 16

High Young’s modulus

Answer 16

Very stiff material (steel)
Blood - lower deformability of RBCs, affecting their ability to fit through smaller vessels. High volume of stiff RBCs ↑ viscosity

Question 17

Low Young’s modulus

Answer 17

Flexible material (rubber)
Blood - higher deformability of RBCs (due to their elastic membrane), easier to change shape and fit through small capillaries, and to absorb pressure waves from pulsatile flow from the heart to maintain a smooth flow

Question 18

Bulk Modulus

Answer 18

Version of Young’s modulus for fluids.
Describes fluid’s resistance to compression (i.e ability to maintain density). Blood is mainly water so it is highly incompressible (constant density despite pressure) and therefore has a HIGH bulk modulus, meaning pressure changes result in flow changes instead of volume changes

Question 19

Vasoelastic nature of blood

Answer 19

Blood acts as both a fluid (viscosity) and a solid (elasticity).
Its viscosity is dependent on shear rate dependence (viscosity ↑ as shear rate ↓), haematocrit (volume of RBCs), and plasma viscosity.
Its elasticity is related to the deforming ability of RBCs and their elastic membrane allowing them to stretch and compress

Question 20

Laplace’s Wall of Tension

Answer 20

Tension (force) inside a wall helps to prevent collapse or excessive expansion.
Tension increases with increased internal pressure and increased radius - needs more force to maintain integrity of the wall in these cases.
Tension decreases with increased wall thickness (not as much is needed, thick walls are strong enough)

Question 21

Laplace’s Law in Respiratory Mechanics

Answer 21

Describes the alveolar pressure required to overcome excessive surface tension of the alveolar walls.
Alveoli need to be small with thin walls for surface area and efficient gas exchange, so Laplace’s Law states that because of this, the alveolar pressure needs to be very high to prevent collapse from excessive surface tension (i.e extreme elastic recoil, coming back to original shape after stretching and then some)

Question 22

Surface tension

Answer 22

Elastic force causing lungs to recoil after stretching during inspiration, allows alveoli to return to original shape/size and prevent over-expansion/rupture. Too strong - cause collapse.
Induced by liquid layer inside alveoli at the air-liquid interface - water molecules attract each other forming a sphere, creating a pressure/pull/tension, dragging the wall inward

Question 23

What regulates surface tension in alveoli

Answer 23

Surfactant
Positive transpulmonary pressure (alveolar pressure is higher than pleural pressure)

Question 24

Surface tension v compliance

Answer 24

If surface tension is too high, compliance is low and alveoli cannot stretch/expand enough for sufficient inspiration.
If surface tension is not high enough, compliance is very high and alveoli cannot recoil to original shape.
A balance of the two is needed for sufficient expansion to accommodate air influx and recoil to original shape/size and maintain integrity of walls.

Question 25

Critical radius

Answer 25

The perfect radius needed to allow for balance between surface tension and internal pressure while maintaining the integrity of the wall (prevent collapse/expansion)

Question 26

What happens to flow at a curvature?

Answer 26

Centrifugal forces interact with viscous forces.
Centrifugal forces push the flow outward toward the wall, viscous forces try to resist this to maintain steady flow.
Determines whether flow will remain laminar or become more turbulent. If centrifugal forces dominate, the distribution of flow can cause secondary flow patterns to occur alongside normal flow in a spiral

Question 27

What is the Dean number?

Answer 27

Dimensionless number predicting the formation and strength of secondary flow patterns at a curvature by giving a ratio of centrifugal to viscous patterns. High = centrifugal forces dominate, potential for turbulence and secondary flow patterns to occur

Question 28

Laplace’s Law in circulatory system

Answer 28

Wall tension resists the outward push of blood pressure inside the vessel. High BP ↑ wall tension, higher risk of rupture (aneurysm)

Question 29

Transmural pressure

Answer 29

The difference between pressures inside the vessel (BP) and pressure outside (interstitial pressure).
When negative/low = vessel may collapse, particularly in veins

Question 30

How does vessel radius relate to Laplace’s Law

Answer 30

Larger vessels (aorta) have higher wall tension for the same amount of BP due to their larger radius - need more tension to maintain vessel integrity and contain BP in the larger surface area.
Smaller vessels (capillaries) have lower wall tension

Question 31

Main differences in Laplace’s Law between Respiratory and Circulatory systems

Answer 31

Resp features surface tension which is the dominant pressure to be resisted and it involves surfactant and transpulmonary pressure to regulate.
CV features wall tension which resists the dominant pressure of blood pressure inside the vessel. Positive transmural pressure ensures this is higher than interstitial pressure (if not there would be oedema). Wall tension can be reduced (helped) by vascular smooth muscle, NO, baroreceptors to control lumen size

Question 32

Fick’s first law of diffusion

Answer 32

States that diffusing capacity is directly proportional to the concentration gradient

Question 33

Diffusion v effusion

Answer 33

Diffusion = movement of gas molecules from area of high to low concentrations
Effusion = movement of gases through small openings without collisions between molecules

Question 34

Factors affecting deffusion

Answer 34

1. Solubility of the gas
2. Size and weight of the gas
3. Concentration gradient
4. Surface area
5. Membrane thickness

Question 35

How does concentration rate affect diffusion?

Answer 35

A steeper concentration gradient of gases (a large difference between the concentration of a gas across two areas) increases efficiency and rate of diffusion.
Example PO2 in blood = ~40mmHg but is ~100mmHg in alveoli so this steep concentration gradient is ideal

Question 36

Which is more soluble: O2 or CO2?
How does this affect diffusion?

Answer 36

CO2 is more soluble than O2, this increases the efficiency and rate of diffusion.
PCO2 in blood = ~45mmHg and is ~40mmHg in alveoli which is not a steep gradient but as it is more soluble, the efficiency and rate of diffusion is not compromised

Question 37

Ideal Gas Laws

Answer 37

Boyle’s, Charles’, Gay-Lussac’s, Avogadro’s

Question 38

Boyle’s Law

Answer 38

States that at constant temperature, pressure and volume are inversely proportional.
“Boyle is a bouncer” - more people are allowed in so there is less volume available in the room (vol ↓) but the pressure from all the people inside ↑ and temperature stays the same

Question 39

Charles’ Law

Answer 39

States that at a constant pressure, volume and temperature are directly proportional.
“Charles is chilling in a hot air balloon” - the balloon expands with temperature - the volume in the balloon ↑ as temperature ↑ because the gas molecules inside move faster and need more space

Question 40

Gay-Lussac’s Law

Answer 40

States that at constant volume, temperature and pressure are directly proportional.
“Gay-Lussac grilling soda cans” - a soda can gets hotter and so the pressure inside of it increases as the gas molecules move faster, colliding against the walls. Volume remains the same inside the can

Question 41

Avogadro’s Law

Answer 41

States that at constant pressure and temperature, number of molecules (moles) and volume are directly proportional.
“Avogadro adds air to balloons” - a balloon is filled with more air (moles) and so the volume inside the balloon increases while temperature and volume remain constant

Question 42

How is Boyle’s law applied to respiratory devices?

Answer 42

Boyle = bouncer in a club = ventilator is needed!
Mechanical ventilators - volume changes lead to corresponding pressure changes in the lungs, helping push air out.
Spirometers - assessing how pressure in the lungs change with volume changes during breathing cycles

Question 43

How is Charles’ law applied to respiratory devices?

Answer 43

Charles = hot air balloon = inhaler is needed!
Inhalers - compressed gases expand slightly when they release into the atmosphere, causing them to cool. Charles’ law must be considered to ensure meds/gases delivered to patients are at the right temperatures for optimal delivery and efficient breathing

Question 44

How is Gay-Lussac’s law applied to respiratory devices?

Answer 44

Gay-Lussac = soda can = CPAP!
CPAP/BiPAP devices - pressure of gases delivered changes with temperature. Important to understand how pressure ↑ with temperature to regulate gas supply and ensure correct pressure delivery

Question 45

How is Avogadro’s law applied to respiratory devices?

Answer 45

Avogadro = balloon = ventilator needed!
Mechanical ventilator - volume of air delivered is adjusted based on the amount of O2 molecules needed for sufficient supply. As volume ↑ so too does the number of O2 molecules to match that volume

Question 46

Venturi effect

Answer 46

Describes the reduction in fluid pressure as it passes through a narrow section of a pipe. Describes both Bernoulli’s law and the continuity equation.
Bernoulli - conservation of energy - velocity ↑ so pressure ↓
Continuity of flow - conservation of mass - what enters must equate to what exits so velocity must ↑