Lecture 19 - Respiration 2 - Mechanics of Breathing

7/30/2019 Lecture 19 - Respiration 2 - Mechanics of Breathing

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Dr. Christina [email protected]

PHY2021Mechanics of Breathing
http://www.google.com.sg/url?sa=i&rct=j&q=old+pictures+paintings+of+respiratory+system&source=images&cd=&cad=rja&docid=8XdAzm7byEcWJM&tbnid=7FuR29Z95rBMTM:&ved=0CAUQjRw&url=http%3A%2F%2Flapetitemort73.blogspot.com%2F2009_03_01_archive.html&ei=Fq42UYStPIGpkwXcp4GwCg&bvm=bv.43287494,d.bmk&psig=AFQjCNH7vAOSQaj7lI6QNH5dLynGrw1ygA&ust=1362623969056948


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Volume, pressure and airflow changes during respiratory cycle

Mechanical relationship between lung and chest wall:

Generation of pressure gradient between atmosphere and alveoli

Muscles of respiration

Airway resistance; Stability of alveoli; Pulmonary Surfactant

Role of lung compliance during breathing

Essential Reading: Vanders (11th ed) Chapter 13, p 44356

Key Learning Outcomes

After this lecture and with further reading you will be able to describe:

Lung Volumes and Capacities

A brief review of major respiratory disorders that change themechanics of the respiratory system


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1. Ventilation: Exchange of air between

atmosphere & alveoli by bulk flow

2. Exchange of O2 & CO2 between alveolar

air & blood in lung capillaries by diffusion

5. Cellular utilization of O2 & production

of CO2

3. Transport of O2 &CO2 through pulmonary

& systemic circulation by bulk flow

4. Exchange of O2 & CO2 between blood

in tissue capillaries & cells in tissues

by diffusion

Steps in Ventilation


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The respiratory system

Chest is a closed container.

Closed at top by muscle&

connective tissue.

Closed at sides by ribs & muscle

Sealed at bottom by diaphragm


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Boyles law: P1V1=P2V2

Volumes and pressures are related!

The pressure of a gas is inversely related to the volume of its container.

A decrease in thoracic volume leads to an increase in pressure.

As the chest is a closed container, an increase in thoracic volume will

lead to a decrease in pressure.


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Quiet Inspiration

The diaphragm and external intercostal muscles contract

External intercostal muscles elevate the rib cage the sternum

moves anteriorly

Diaphragm flattens and moves inferiorly

Volume and Pressure (within the thoracic cavity and the lungs)


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Lung expansion relies on intrapleural pressure.

How does increasing and decreasing thoracic

volume alter lung volume?

Intrapleural space

Visceral pleura

Parietal pleura

Small changes in this pressure work to couple

changes in thoracic volume with changes in

lung volume!

The lungs are only attached at the hilus!

Lung expansion following increased thoracic

volumes is dependent on intrapleural pressure.

Consequently, due to elastic recoil of the lung

during expiration, lungs collapse away from the

chest wall this draws chest wall in!

At rest, the lungs want to collapse, but chestwall wants to expand. This creates a sub-

atmospheric intrapleural pressure.


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Pressure differences involved in ventilation

Palv

Intrapleural FluidLung wall

Chest wall

Pip

Atmosphere

Patm

Palv < PatmPalv > Patm

Ptp = Palv - Pip

Pcw = Pip -Patm

Patm

Prs= Palv- Patm

Opposes inward elastic recoil of the lung

Opposes out ward elastic recoil of chest wall

Determines air flow


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Inspiration as chest wall expands, Pip

(Boyles law)

Palv

Intrapleural Fluid

Lung wall

Chest wall

Pip

Ptp Pip Lung expands

Palv Air flows inward

Ptp = Palv - Pip

Air flows inward

Greater than inward elastic recoil force


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Expiration Decrease firing of phrenic nerve and

intercostal nerve respiratory muscles

relax Chest recoils back to originalposition smaller thoracic volume

Palv

Intrapleural Fluid

Lung wall

Chest wall

Pip

Ptp Pip Lung recoils

Palv Air flows out

Ptp = Palv - Pip

Air flows outward

Lower than inward elastic recoil force


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Quiet Expiration

Expiration is due to passive recoil

During quiet breathing, expiration is passive process

(no muscle contraction)


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Stable Balance Between Breaths

Intrapleural space

Alveolus

Chest wall

Elastic recoil of lung Elastic recoil of chest wall

Patm= 0mmHg

Palv = 0mmHg

Ptp = Palv - Pip= 4mmHg

Pcw = Pip - Patm= - 4 mmHg

Pip= - 4mmHg

Prs= Palv - Patm = 0 mmHg

No

Flow


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Respiratory Cycle & Pressure Changes

Inspiration Expiration

2

- 2

- 4

- 6

0

0

0.5

4 secTime

Pressuresduringbreathing(m

mHg)

Breathvolume(L)

End of Expiration

Mid Inspiration

End Inspiration

Mid Expiration

1

2

3

4

Patm = 0

Palv = 0

Nofl

ow

Ptp = 4Pip = - 4

Palv

Pip

Ptp

Airflow

Patm = 0

Palv = -1

Palv - Patm = -1

Ptp = Patm Pip = 7

Recoil

Pip

= - 6

Pip

= - 7

Recoil

Patm = 0Noflow

Palv = 0

Palv - Patm = 0

Ptp = Patm Pip = 5

Pip = - 5

Recoil

Ptp = Patm Pip = 6

Airflow

Patm = 0

Palv = 1

Palv - Patm = 1


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Deep Breathing

Inspiration involvescontraction of extramuscles to elevateribs: scalenes,pectoralis minor, &

sternocleidomastoidmuscles

Expiration involvescontraction ofinternal intercostals& abdominal muscles


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Physical Aspects of Ventilation

Ventilation results from pressure differences induced

by changes in lung volumes

Air moves from higher to lower pressure

Compliance, elasticity, & surface tension

of lungs influence ease of ventilation


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Compliance

Magnitude of change in lung volumeproduced by a given change in Ptp

How easily lung expands with pressure

Major determinants: - Elasticity

- Surface tension

Is reduced by factors that cause resistanceto stretching


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Elasticity Tendency to return to initial size after distension

Due to high content of elastin proteins

Elastic tension increases during inspiration &is reduced by recoil during expiration

Elastic Recoil

- return to the original dimension driven by the

elastic tension generated during stretching.


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Surface tension (& elasticity) are forces that promote alveolar

collapse & resist stretching

Surface tension of H2O tends to collapse alveoli (attractive forcesbetween liquid molecules is greater than forces between liquid and

gas molecules)

Smaller alveoli tend to collapse more than larger ones

Law of Laplace relationship between pressure (P), surface tension

(T), and the radius (r) of an alveolus : P = 2T/r

Surface tension

Pressure needed to

keep a sphere shape=

2x Surface tension

radius


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Pulmonary surfactant

Produced by alveoli type II cells

Function:

- reduce surface tension during expiration

- consequently increasing the lung compliance

(decreasing the work of breathing).

- Prevents alveoli from collapse

- Preventing fluid accumulation in the alveoli

- host defense mechanism against infection

and inflammation

Surfactant

H2O molecules

Alveolar wall


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Surface tension & surfactant

Surfactant reduces surface tension

Surfactant production starts in late gestation

Secreted by type II alveolar cells

Surfactant (a mixture of phospholipids & proteins)

Diplamitoyl phosphatidyl choline (DPPC) Hydrophobic tail, hydrophillic head

R l f f t t i h t i


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Role of surfactant in hysteresis

Hysteresis

Surface tension forces

overcome by surface film

pressure (prevent collapse)

Surface tension forces

overcome by surface film

pressure

(prevent collapse)

Alveolus expands.

Not enough surfactant

molecules to totally resist

surface tension forces.- Difficult to expand

More surfactant molecules

produced.

Alveolus expands easily

Alveolus starting to decrease in

size but surfactant moleculesprevent this.

Surfactant molecules pushed

out of film allowing alveolusto decrease easily

surfactant

water

alveolus


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Resistance =

Airway resistance - effect of radius on airflow

radius 4

constant x lengthOhms and Poiseuilles Laws combined:

Halve airway radius = 16 xs increase in resistance (e.g. asthma)

Double the length = doubles the resistance

Factors affecting airway radii (resistance):

- physical - Trans pulmonary pressure (Ptp)- elastic connective fibers

- Intrapleural pressure (Pip)

- neuro-endocrine influence the airway smooth muscle- epinephrine - relaxes smooth muscles

- leukotrines - contract smooth muscles


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Airway resistance (RAW)

Raw > in medium-sized

bronchi, not small bronchioles

Example cross-sectional areas

Trachea = 2.5cm

2

Terminal bronchioles = 240cm2

The medium-sized bronchi (over

2mm) around the 7th generation

have the highest resistance

This is because the small airways

are so numerous and, existing in

parallel, have a large total cross-

sectional area

Importantly, the low resistance of the small airways makes them a

silent zone on auscultation

Early obstructive (emphysema) disease affects these vessels first,

but is hard to detect.


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Measuring lung function


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Measurement of Lung volumes and capacities

Spirometer - for measuring lung volumes & capacities

- also inspiratory and expiratory flow rates

Spirogram


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Spirogram

Measure changes in lung volumeduring breathing.

Can also be used to measure flow rates (L/sec) - inspiratory or expiratory.

Cannot measure the absolute gas volumein the lungs

- need Helium dilution or plethysmography.

Lung volumes - related to gender, height, weight, age.

- also affected by lung diseases.

Tid l V l ( i t b thi )


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Tidal Volume (quiet breathing)

Each breath is ~500 ml

- Most of this reaches the alveoli

In each breath, ~350 ml reaches

the alveoli and ~150 ml stays in

the airways (ie. dead space).Anatomical dead space is the air

in conducting airways (~150 ml).

At end of normal expiration, there

~2.5 L in the Lungs:

ie. they are never empty.

(values are for typical young adult male)


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Tidal volume

500ml

LUNG VOLUMES

Inspiratory

reserve

volume

Expiratory reservevolume

Residua

l

Volume

(1.2L)

Dead space

(airways)

Lung volumes and capacities


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Lung volumes and capacities

Expiratory Reserve Volume (ERV)

Amount of expired air during forced expiration after quiet breathingResidual Volume (RV)

Amount of air remain in lungs after maximal expiration

Tidal Volume (TV)

Amount of air that goes in/out of lungs for each inspiration/expiration

Inspiratory Reserve Volume (IRV)

Amount of inspired air more than TV during maximal inspiration

Vital Capacity (VC)

Total amount of air that is maximally expired after maximum inspiration

= IRV + TV + ERV

Lung Volumes & Capacitiescontinue


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Lung Volumes & Capacities

Inspiratory Capacity (IC)

Total amount of air that is maximally inspired after normal expiration

IC = IRV + TVFunctional Residual Capacity (FRC)

Total amount of residual air in the lungs after normal expiration

FRC = ERV + RV

Total Lung Capacity (TLC)

Total amount of air in the lungs after maximal inspiration

TLC = IRV + TV + ERV + RV

RV & FRC

cannot be measured byspirometer

RV & FRC is measurable

by N2 wash-out technique


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Pulmonary Ventilation

Minute ventilation Volume of air breathed per minute (litres/min)

MV = Tidal Volume (mL/breath) x Respiratory rate (breaths /min)

eg: 12 breaths/min x 500 ml = 6 litres/min

Alveolar minute ventilation = breath/min x (TV DeadSpace Volume)

Maximum voluntary ventilation

= maximum minute ventilation

(i.e maximum breathing effort)

Depends on muscular effort and airway resistance

Ob t ti l di


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Obstructive lung disease1- Asthma.Chronic inflammation of airways and hypersensitivity to allergens.

Increased tone of airway smooth muscle and excessive mucus secretion.

2- Chronic bronchitis.Increase in mucus secretion and inflammation of airway walls.

3- Emphysema.Destruction of alveolar walls reduce the

radial traction of airways and therefore

the airway resistance is abnormally high.

Obstructive lung diseases - airway resistance is abnormally increased.

N E

Lung volumes and work of breathing


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Lung volumes and work of breathing

Diseases/conditions

affecting themRESTRICTIVE

Pulmonary fibrosis, chest

burns, diaphragm in obesity

and pregnancy, muscleweakness

OBSTRUCTIVE

Asthma, COPD,

Bronchiectasis, bronchitis,

tumours

Work aspects of breathing

Work to expand the chest

FEV1 is severely decreased:

FEV1/FVC = < 80%

FEV1 and FVC are decreased. But:

FEV1/FVC = about 80% (normal)Airway resistance

- Work to move air into the lungs

S i t Fl l


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Spirometry: Flow-volume curves

Respiratory Physiology. John West. Chapter 10; page 160


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Reading...

Recommended reading

Vanders (chapter 13) p443-56

Silverton (chapter 17) p 568-574, p 578-589

Other very very useful information...!

Ward JPT Respiratory System at a Glance (chapters 1-3, 20)

Marieb & Hoehn Human Anatomy & Physiology7th Ed

(chapter 22)

Documents

Lecture 19 - Respiration 2 - Mechanics of Breathing