This document provides an overview of basic physics concepts relevant to anaesthetists, including the physics of fluid flow and gas laws. It describes key determinants of laminar and turbulent flow such as pressure, tube diameter, length and viscosity. Flow is determined by the pressure difference and resistance according to equations like Hagen-Poiseuille. Gas laws including Boyle's law relating pressure and volume are also introduced. Understanding these physical principles helps ensure safe use of anaesthetic equipment.

1. Basic Physics for
Anaesthetists
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2. Outline
 Objectives
 Introduction
 The physics of flow
 The gas laws
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3. Objectives
• After completion of this course students will be able to
describe:-
– Basic concept on physics of fluid
– Basic concepts on gas laws and its implications
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4. INTRODUCTION
 For the safe & efficient use of anaesthetic apparatus, the
anaesthetist must have a clear concept of the physical aspects of
the equipment in use.
 Understanding of basic concepts may avert unnecessary
accidents & near misses.
 Measurable terms and the physical laws apply to all states of
matter (i.e. solids, liquids and gases).
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5. I. The Physics of Flow
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6.  Flow is defined as the quantity of liquid(gas, vapour or
sublimate) that passes a point per unit time
 A simple equation to represent this is:
• Flow is sometimes written as ∆Q (rate of change of a
quantity)
• Due to the number of different fluids that are given to
our patients during a routine anesthetic, flow is
obviously an important area of physics to understand.
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7. Biophysics flow
 Flow is determined by 2 factors
1. Pressure difference (P) -
 Flow(Q) from high pressure to low pressure.
 Q depends on P1-P2 (P) (pressure gradient)
2. Resistance (R)
R= measure of friction between
 tube/blood vessel and moving fluid
 Fluid molecules within themselves
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8.  As fluid flows through tubes there is resistance, between
the fluid and vessel wall that opposes the flow.
 For any given system, this resistance is constant and can be
expressed as:
 Q = P (Ohm’a law)
R
R Depends on type of fluid and dimension of tube and on:
 Viscosity of fluid ( ) – directly proportional
 Tube length (L)- directly proportional
 Tube radius (r) - indirectly proportional (R as radius
)
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10. Hagen– Poiseuille equation
 where p is pressure drop along the
tube (p1 − p2).
 r is radius of tube
 l is length of tube and
 η is viscosity of fluid.
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11. • Determinants of laminar flow
A. Pressure across tube
B. Diameter of tube
C. length of tube
D. Viscosity of tube
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12. A. Gradient pressure (ΔP)
 For flow to occur, there must be a pressure difference (ΔP)
between the ends of a tube.
 ΔQ is directly proportional to ΔP.
• The greater the pressure difference, the greater the flow.
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13. B. Tube diameter /Radius
• flow is proportional to the 4th power of the radius.
• If the diameter of the tube is halved the flow through it reduces
to one sixteenth.
• If the radius doubles, the flow through the tube will increase by
16 times
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15. C. Length
 Flow is inversely proportional to the length of the tube.
 If the length is doubled the flow is halved
 A central line is much longer than a cannula, and for the same
diameter fluid flows more slowly.
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16. D. Viscosity
 Viscosity of fluid also affects the flow of fluid
 viscosity increase in following condition
- Polycythemia , Increased fibrinogen level
- Hypothermia , cigarette smoking
- Age
 Increased viscosity leads to increase risk of vascular
occlusion .
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17.  Anaesthetic implication
• During fluid resuscitation, a short wide bore cannula
e.g.14G is superior to a 20G cannula or a central line.
• Intubating patients with very small tube increases
resistance to flow and thus pressure increases to deliver
the same amount of flow through the tube.
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18.  The physics of flow
o Flow can be divided into Three different types
1. Laminar flow
2. Transitional flow
3. Turbulent flow
o A number of different physical characteristics determine
whether a fluid obeys the principles of one or the other.
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19. 1.Laminar flow
• Normal and noiseless.
 All elements of the fluid move in a stream line, that are parallel
to the axis of the tube.
 No fluid move in radial or circumferential direction.
 Layer of fluid in contact with the wall is motionless (thin layer,
adherent to wall, hence motionless)
 Fluid that move along the axis of the tube has maximum Velocity.
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20. • Laminar flow
• Reynold’s number < 2000
• 'low' velocity
• Fluid particles move in
straight lines.
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21. fig. Diagrammatic representation of laminar flow
• If the mean velocity of the
flow is v, then the molecules
at the centre of the tube are
moving at approximately 2v
(twice the mean),
• the molecules at the side of
the tube are almost
stationary.
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22. 2. Turbulent blood flow
 Various elements of fluid move irregularly in axial,
radial and circumferential direction.
• More pressure required to drive blood than laminar
• Energy wasted in propelling blood radially and axially
• Often accompanied by noise(murmurs).
• Occur at valves and aorta (normal), at site of blood
clot(pathological).
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24. 2/15/2019 24WUBIE B

25. Turbulent Flow
• The fluid particles do not move in orderly manner and
they occupy different relative positions in successive
cross-sections.
• The flow is unpredictable
• High velocity
• Average motion is in the direction of the flow
• Changes / fluctuations are very difficult to detect.
• Most common type of flow.
• Particle paths completely irregular
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26. • Laminar flow change to turbulent flow if constriction is
reached
– Velocity of fluid increases
– Fluid is no longer in a smooth fashion
– Resistance is higher than for the same laminar flow .
– Flow is no longer directly proportional to pressure
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27. Occurrence of turbulent flow depends on:
• Velocity of blood flow (v)- directly proportional
 Diameter of the tube (d) – directly proportional
 Density of the tube (p)-directly proportional
 Viscosity of the fluid ()- indirectly proportional.
Turbulence in normal circulation:
 Heart chambers -- - - -v & d large
 Big arteries near heart ----v & d large
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28.  Reynold’s number (Re).
• There is a number that can be calculated in order to
identify whether fluid flow is likely to be laminar or
turbulent and this is called Reynold’s number (Re).
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29. Re : Reynold’s number,
ρ: Density of fluid
v: Velocity of fluid
d: Diameter of tube and
η : viscosity of fluid.
• Reynold’s number is dimensionless (it
has no units).
• Re < 2000 flow is likely to be
laminar
• Re > 2000 flow is likely to be
turbulent.
• Reynold’s number of 2000 delineates
laminar from turbulent flow.
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31. 3. Transitional flow
• Transitional flow
• Reynolds's number-2000-4000
• 'medium' velocity
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32. Clinical Aspects Of Flow
• Laminar flow is present in bronchioles, smaller air passage as they are
narrower than trachea.
• Turbulent flow is present in corrugated rubber tubing .
• Sharp bend or angles increase turbulence
• In quiet breathing , the flow in resp tract is laminar, while speaking ,
coughing or taking deep breath turbulent flow tends to occur .
• A lining layer of mucus may affect the flow .
• In circulatory system, bruit and murmur can be heard due to
turbulence of flow.
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33. Bernoulli’s Principle
• Describes the relationship between the velocity and pressure
exerted by a moving liquid.
• Applied to both liquids as well as gases.
• Venturi effect is based on the Bernoulli’s principle.
• Venturi effect is entrainment of fluid (gas or liquid ) due to the
drop in pressure
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34. Applications of Venturi effect
• Venturi masks used for oxygen therapy.
• Sander’s jet injector.
• Nebulisation chambers.
• Atomizers that disperse perfumes or spray paints.
• Water aspirators.
• Foam fire fighting nozzles and extinguishers.
• Modern vaporizers.
• Sand blasters to mix air and sand.
• Vehicle carburetors.
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35. Fig. Venturi effect
venturi face mask
venturi face mask
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36.  Coanda effect
 is the tendency of the fluid jet to be attached to a nearby
surface.
 When a narrow tube encounters a Y junction of the wide bore,
because the flow tends to cling to one side, the flow will not
evenly divide between the two outlets, but flows through only one
limb of the Y piece. This behavior is called Coanda Effect.
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37.  Coanda Effect
 If a constriction occurs at bifurcation because of increase in
velocity and reduction in the pressure, fluid (air, blood) tends to
stick to one side of the branch causing maldistribution.
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38. Coanda Effect
Application:
1. Mucus plug at the branching of tracheo-bronchial tree may
cause maldistribution of respiratory gases.
2. Unequal flow may result because of atherosclerotic plaques in
the vascular tree
3. Fluid logic used in ventilators employs this principle to
replace valves or mobile parts.
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40. II. The Gas Laws
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41.  Introduction
• As anesthetists we deal with liquids & gases under pressure at
varying temperatures and volumes.
• These inter-relationships are simple, measurable and their
understanding ensures a safe outcome for the patient.
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42. • Definitions
• What is a gas?
 Gas : is a substance that is in its gaseous phase, but is above its
critical temperature.
 Vapour: a substance in the gaseous phase but is below its critical
temperature.
 Critical temperature : the temperature above which a gas cannot be
liquefied no matter how high the pressure.
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43. • What is pressure?
• Defined as "the force per unit area acting at
right angles to the surface under consideration”
• Pressure = Force/Area
• The unit of pressure is the Pascal.
 Units ,1 Barr = 1 Atm = 100Kpa
=760mmHg=760torr = 14.7Psi = 1000cmH20 .
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44.  What is temperature?
 Measures heat, a form of energy, kinetic energy which comes
from movement of the molecules.
 Temperature is measured in the Celsius or Fahrenheit scales.
 In Physics it is the Kelvin.
 The divisions of the Kelvin and Celsius scale are the same but
the start points differ.
 0oC is 273K, so body temperature is 310K on this scale.
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45. Basic Concepts of Gas Laws
 The relationship between the three variables,
1. Volume
2. Pressure
3. Temperature is explained by the gas laws
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46. Definitions of the gas laws
1. Boyle’s Law
o At constant temperature (T) , the volume(V) of a fixed amount of
a gas is inversely proportional to its pressure(P).
o V α 1 / p
o PV = Constant ( if T is kept constant).
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47. 2. Charle’s Law
• At constant pressure, volume of a gas is directly proportional to
the temperature. V α T or V / T = K (constant)
• APPLICATION:
• Respiratory gas measurements of tidal volume & vital capacity
etc are done at ambient temperature while these exchanges
actually take place in the body at 37 OC.
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48. 3. Gay Lussac’s law/ the third gas Law
• At constant volume pressure is directly proportional to the
temperature. P α T or P/T = K (constant)
 Application
• Medical gases are stored in cylinders having a constant volume and
high pressures (138 Barr in a full oxygen / air cylinder).
• If these are stored at high temperatures, pressures will rise causing
explosions
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49. The gas laws
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50.  Perfect gas
• A gas that completely obeys all three gas laws.
• Or A gas that contains molecules of infinitely small size,
which, therefore, occupy no volume themselves, and
which have no force of attraction between them.
• No such gas actually exists still.
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51. The end!!!
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