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Biophysics of breathing. Spirometry
Respiratory - Structure
Lectures on Medical Biophysics
Department of Biophysics, Medical Faculty,
Masaryk University in Brno

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Lecture outline
•Mechanisms of gas exchange between organism and surroundings (respiratory movements – mechanics of
breathing, diffusion and dissolution of gases)
•Respiratory volumes and capacities
•Respiratory resistances
•Respiratory work
•Spirometry
•Some biophysical aspects of breathing

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Respiratory movements
•These movements are done mainly by intercostal muscles and diaphragm
•Thoracic breathing (predominant in women) and abdominal breathing (predominant in men)
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Dle: http://www.medem.com/MedLB/article_detaillb.
Respiratory - Structure

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Diffusion of O2 and CO2 in plasma
• Molecular weights:
MO2 = 32
MCO2 = 44
Bunsen coefficients of solubility (a) for gases in blood under normal body temperature.  The unit
of solubility is (ml of gas under normal temperature and pressure) * (ml of blood)-1 * (101.3
kPa)-1
CO2
0.52
CO
0.018
N2
(Water: 0.013;
Fat: 0.065)
O2
0.022

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Gas exchange
Medium
Way of
transport
pO2  (kPa)
pCO2  (kPa)
alveoli
streaming
13.3
5.2
alveolar-capillary wall
diffusion
-
Blood circulation: arteries                   veins
streaming
12.7
5.3
5.5
6.0
Capillary wall cellular  membrane
diffusion
-
Living cell
4.0
6.7

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 Exchange of O2 and CO2


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Changes of negative pleural pressure during respiration
pleural pressure

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spirogram
Respiratory volumes and capacities
•Air in airways – death space - 150 ml
•Residual air volume in alveoli - RV - 1 l
•Expiration reserve volume -  ERV – 1.5 l
•Resting (tidal) respiratory volume - TV - 0.5 l
•Inspiration reserve volume - IRV – 2.5 l
•
•Vital capacity
•VC = ERV + TV + IRV
•Functional residual capacity
•FRC = RV + ERV
•
•Measure of lungs ventilation: minute volume
•
•MV = Respiratory volume × breathing rate [l·min-1]
Example of a spirogram
V [l]
V [l]

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Pneumothorax
Pneumothorax_2
•http://www.pennhealth.com/health/health_info/Surgery/graphics/Pneumothorax_2.jpg
Right lung is collapsed

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Respiratory resistances
•Elastic resistance of lungs and chest is given by tension of elastic fibres in pulmonary tissue.
The surface tension of alveoli has similar effect.
•Non-elastic resistance of tissues (also tissue viscous resistance). It arises due to friction of
pulmonary tissues, chest, respiratory muscles and organs of abdominal cavity.
•Flow resistance of airways – complex of resistances caused by air flow  (effect of air viscosity,
incl. turbulences). It increases substantially when the airways are narrowed.

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Respiratory work
(picture to consider)
•This work is necessary to overcome all the respiratory resistances:
•W = pDV
•p is the difference of intrapulmonary and pleural pressures, DV is  the breathing volume
obr6_10
Respiratory work. A) – during inspiration, B) – during expiration. Area 0ACD0 – elastic work done
at the expense of body energy (during inspiration) or at the expense of potential energy of
distended elastic tissues (during expiration). Area ABCA represents active inspiration work against
the non-elastic resistance. Area ACEA represents the work against the non-elastic resistance during
expiration at the expense of body energy (after Pilawski).

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How to calculate respiratory work?
•At rest:
•minute volume MV = 7 l
•breathing rate BR = 14 min-1
•pressure p = 0.7 kPa
•respiratory volume V = 0.5 l (5·10-4 m3)
•work W = 0.35 J – for one inspiration
•294 J per 1 hour
•
•
•At great load:
•MV = 200 l
•BR 100 min-1
•p = 0,7 kPa
•V = 2 l (2·10-3 m3)
•W = 1.4 J - for one inspiration
•8400 J per 1 hour
homersleep Fightsmall

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Measurement of respiratory volumes and speeds - spirometry
spiro Výsledek obrázku pro spirometrie Výsledek obrázku pro spirography

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Spirogram
We can measure the dependence of volume on time
or
Flow rate on volume
art-asthma_fig5_2
Flow rate
[litres]

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Some biophysical aspects of breathing
•Physical properties of lungs and their manifestations in some areas of diagnostics and therapy:
ØThe lungs represent the largest contact area with ambient medium
ØMany functions of organism can be influenced by rate or depth of breathing (hyperventilation)
ØBreathing movements can disturb e.g. X-ray diagnostics
ØLungs have negative contrast in X-ray images
ØPhysical properties of alveoli are similar to bubbles – lung tissue can be impaired by cavitation
phenomena (risk in ultrasound diagnostics and lithotripsy)

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Author:
Vojtěch Mornstein
Content collaboration and language revision:
Carmel J. Caruana
Last revision and addition of soundtrack: November 2020