1. THE AUSTRALIAN NATIONAL UNIVERSITY
Overview of Physical Principals in Physiology Christian Stricker Associate Professor for Systems Physiology ANUMS/JCSMR - ANU

3. At the end of this lecture students should be able to
Aims
At the end of this lecture students should be able to
state what the elementary SI units are;
explain the concept of SI units and give examples;
interpret relevant symbols for multiples/fractions of units;
define frequency, temperature, distance, velocity, acceleration, volume, mass density, concentration, force, weight, tension, pressure, torque, lever, energy and power and assign the correct units;
state which special dimensions are used in medicine; and
name and estimate some different forms of energy.

4. Contents Importance of the concepts covered
SI units, and derived dimensions
Multiples and fractions of units
Frequency, Temperature
Distance, velocity, acceleration
Volume, density, concentration
Force, weight, tension, pressure
Torque and lever types
Energy and power

5. Importance How to document a patient’s state? Patient data
Inspection
History
Measurements
Become more and more important as “objective”.
Examples: BP, Temperature, pH, cardiac output, hearing sensitivity, …
Without unit, a number is meaningless!
Patient data
We have to refresh
units / dimensions

6. Elementary Units (SI)
SI units refer to Système international d'unités”.
kg is special case as it has a “multiple” in front (see next).
Follows “meter-kilogram-seconds” system (MKS).

7. Some Derived Dimensions
List incomplete (some physiologically relevant ones…)
It is possible to describe all measurements using the 7 elementary SI units.
Dependency often established via the energy associated.

8. Multiples and Fractions of Units
Multiples are typically upper, fractions lower case.
Because of inability to print Greek character µ, sometimes people use ‘mu’ or ‘u’ for it.

9. How Often Does Your …?
Frequency (f) [Hz] used to describe a periodic event.
In medicine, as 1 s is mostly too short, the base is often 1 min (beats per min; bpm).
Useful to determine the time interval between the events: 1/f
10 Hz → 100 ms
80 bpm → 0.75 s = 750 ms (60/bpm)

10. Temperature Kelvin is SI unit. Conversion: ºC = K - 273.15
0 K = absolute zero (in space…)
Conversion: ºC = K
In medicine, typically ºCelsius.
0º: triple point of water (solid, fluid, gaseous) at sea level
100º: boiling point at sea level
37º: average body temperature

11. Distance, Velocity Distance [m] = length or equivalent
Velocity [m/s] = distance travelled per unit time.
How fast does a pulse wave travel?
Distance (heart → wrist) in m: 1.50 m.
Delay to periphery: 210 ms
V = 1.50 / 0.21 = 7.1 m/s.
You are going to measure this in context of the ECG in Block 2.
Similarly for nerve conduction velocity in Block 6.
Modified from Nam et al. (2013), Sensors 13:4714

12. Distance, Velocity, Acceleration
Acceleration = change in velocity per unit time [m/s2].
Acceleration +-ve; deceleration –-ve.
Velocity can be inferred from acceleration as area under that curve.
Distance can be inferred from velocity as area under that curve.
Acceleration can be inferred from the velocity as its differential in time.

13. Volume - Density
Volume in SI units [m3], however this is an impractical size.
Volume is typically in litres; i.e (l or L) = 1 m3.
In medicine, it is customary to abbreviate litre in L instead of l as the latter could be mistaken as number.
Density [kg / L] = mass per volume. At 4ºC, H2O density is 1 kg/L.
At this density, 1 mL = 1 g

14. Concentration
Molar concentration [ ] = amount of substance s per unit volume [mol/L ≡ M] = molarity.
Molal concentration = amount of substance per unit mass of solvent [mol/kg H2O].
At 4ºC molar conc. = molal conc.
Otherwise molar conc. ≈ molal conc.
Deviations at high concentrations
1 mol contains 6.022∙1023 molecules
Mass of a molecule expressed in daltons (Da)

15. Force – Weight – Tension
Force equals mass * acceleration [N]. Because acceleration has a direction, force is directional (vector).
Weight is a special case, where accele-ration is caused by gravity (9.81 m/s2).
For every force (“action”), there is a counter force (“reaction”, resistance, etc.)
Tension is a special force that develops in a muscle (string, cable, vessel wall, etc.) after a force has been applied to it (muscle contraction,…).

16. Pressure Pressure = Force per unit area
Does not have a direction, but if in a container (vessel), exerts force in all directions.
= fluid column height.
Unit Pa = N/m2; unfortunately area is very big. In medicine, mostly kPa for most, except body fluids, which are in torr (mmHg; BP) or cmH2O (for small pressures like CSF, lung).
1 torr = kPa.
Mercury manometers are unsafe.

17. Force – Torque – Lever Types
Head on spinal column.
Triceps on ulna
Achilles tend. / forefoot
Molar crushing
Biceps, biting with incisives, etc.
To explain, torque is introduced: Torque = Force * radius.
Torque causes a rotation / movement.
Fulcrum / pivot is at center of rotation.
Applies to muscles and sesamoid bodies, like patella, etc.

18. Muscle Tension
Modified after Rhodes & Pflanzer 2003
Large forces at work equivalent to weights of masses of several 100 kg.

19. Energy – Work – Power Energy = Work [J]
Energy can take different forms
Heat (“internal” work)
Kinetic energy
Potential energy
Electrical energy
PV work
Chemical energy …
Different forms sum to total energy; can be converted into other forms.
Metabolic energy in medicine still often measured in cal (old…)
1 cal = J
Power = Work per time unit [W]

20. Take-Home Messages Numbers without dimensions have little meaning.
In medicine, frequency is often based on minute.
Weight is a special force caused by mass * gravity.
Tension is a force that develops in a muscle.
Pressures are measured in kPa, except body fluids.
Torque is the product of radius and force to causes rotation.
Muscles produce large forces when contracting.
Energy takes different forms, summing to the total and can be converted into others.

21. MCQ
The force of 1 N on a standard mass causes which of the following accelerations?
9.81 m/s2
0.981 m/s2
10 m/s2
1 m/s2
0.1 m/s2
How large is the energy of 1 mg of blood moving at a speed of 20 µm/min?
20·10-12 kg*m2/s2
5.6·10-12 kg*m2/s2
5.6·10-20 kg*m2/s2
0.56·10-15 kg*m2/s2
5.6·10-20 kg*m/s2

22. That’s it folks…

23. MCQ
The force of 1 N on a standard mass causes which of the following accelerations?
9.81 m/s2
0.981 m/s2
10 m/s2
1 m/s2
0.1 m/s2
How large is the energy of 1 mg of blood moving at a speed of 20 µm/min?
20·10-12 kg*m2/s2
5.6·10-12 kg*m2/s2
5.6·10-17 kg*m2/s2
0.56·10-15 kg*m2/s2
5.6·10-20 kg*m/s2

24. Elementary Units (SI)
Dimension
Unit
Abbreviation
Length
meter m
Mass
kilogram kg
Time
second s
Amount of substance
mole
mol
Current
ampere A
Temperature
kelvin K
Luminous intensity
candela cd
SI units refer to Système international d'unités”.
Follows “meter-kilogram-seconds” system (MKS).
kg is special case as it has a “multiple” in front (see next).

25. Some Derived Dimensions
Unit
Abbrev.
Frequency
Hertz / …
Hz / bpm
s-1 / min-1
Force
Newton N
kg·m·s-2
Pressure
Pascal / torr Pa
N·m-2 = kg·m-1·s-2
Heat / Energy / Work
Joule J
N·m = kg·m2·s-2
Power
Watt W
J·s-1 = kg·m2·s-3
Electric potential
Volt V
W·A-1 = kg·m2·s-3·A-1
Electric resistance
Ohm Ω
V·A-1 = kg·m2·s-3·A-2
Conductivity
Siemens S
Ω-1 = kg-1·m-2·s3·A2 …
List incomplete (some physiologically relevant ones…)
It is possible to describe all measurements using the 7 elementary SI units.
Dependency often established via the energy associated.

26. Multiples and Fractions of Units
Factor
Prefix
Symbol
101
deca- da
10-1
deci- d
102
hecto- h
10-2
centi- c
103
kilo- k
10-3
milli- m
106
mega- M
10-6
micro-
109
giga- G
10-9
nano- n
1012
tera- T
10-12
pico- p
1015
peta- P
10-15
femto- f
1018
exa- E
10-18
atto- a
Multiples are typically upper, fractions lower case.
Because of inability to print Greek character µ early on, sometimes people use ‘mu’ or ‘u’ for it.