Mechanics of respiration
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Transcript of Mechanics of respiration
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Dr. Niranjan Murthy H.L
Assistant Professor of Physiology
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Learning objectives
• To learn physiological anatomy of the lung• To learn the muscles involved in
respiration• To learn various pressure changes during
respiration• To learn in detail, the mechanics of
respiration• To appreciate the clinical correlation of
mechanics of respiration
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INTRODUCTION
• Components of respiratory system-
(i) Respiratory tract
(ii) Alveolo-capillary membrane
(iii) Blood
(iv) Peripheral cells
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Components of respiratory tract-
Nose
Pharynx
Larynx
Bronchi
Bronchioles
Alveoli
Alveolo-capillary membrane
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• Pulmonary membrane is involuted deep inside thorax
• Fragile but protected
• Respiratory movements for oxygen intake and CO2 removal
• More particular for CO2 homeostasis
• Inefficient system
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Development of the lung
• Begins as a groove in ventral wall of gut in <1 month
• 60gm at birth and 700gm in adult
• Filled with lung fluid in fetus
• Respiratory movements as early as 5months
• Highly resistant circulatory system in fetus
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Links in processes involved in gas exchange-
1) Ventilation
2) Diffusion
3) Matching of ventilation & perfusion
4) Pulmonary blood flow
5) Blood gas transport
6) Transfer of gases between capillaries & cells
7) Utilization of O2 in cells
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8) Structure-function relationships of lung
9) Lung mechanics
10) Control of ventilation
11) Metabolic functions of lung
12) Respiration in unusual environments
13) Tests of lung function
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Structure-function relationship
Weibel’s model-
• Swiss anatomist
• 23 generations
• Conducting zone- 16 generations
• Respiratory zone- 7 generations
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Histology trachea Initial
bronchiTerminal bronchiole
Respbronchiole
alveoli
Cartilage
Rings20 no def post
present absent absent absent
Smoothmuscles
little Little Largest More absent
Lining Epithelium
Columnar
Columnar
Cuboidal Cuboidal SimpleSquamous
(1) Cilia Present Present Present Present Absent
(2) GlandsMucous membrane
present Present absent Absent Absent
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Alveoli
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• Smallest airway of conducting zone is terminal bronchiole
• Respiratory zone begins with respiratory bronchiole
• Alveoli made of collagen and elastin
• Gas exchange barrier is 50-100m2
• Alveoli is held expanded by intrapleural pressure
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MECHANICS OF BREATHING
• It includes forces that support and move the chest wall & the lung, together with resistances they overcome and the resulting flows
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Muscles of respiration
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Muscles of respiration cont..• Muscles of inspiration-
1) Diaphragm
- attached to lower ribs, sternum & vertebral column
- dome shaped
- moves down on contraction
- supplied by phrenic nerve
- increase vertical dimension of thorax
- cause ribs to move outward & upward
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2) External intercostals-
- between adjacent ribs
- runs downwards & forwards
- increase in AP & lateral diameter
3) Accessory muscles of inspiration
(i) scalenei- elevate first two ribs
(ii) sternocleidomastoids- elevate sternum
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• Muscles of expiration
1) Internal intercostals- run downwards & backwards
2) Abdominal muscles
-external oblique -internal oblique -rectus
abdominis -transversus
abdominis
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Abdominal muscles
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INSPIRATION
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• Bucket handle movement- lower ribs(7-10) move out increasing transverse diameter
• Pump handle movement- upper ribs(2-6) move forwards and upwards increasing AP diameter
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EXPIRATION
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Pressure changes during respiration
• Intrapleural pressure
• Intra-alveolar pressure
• Transpulmonary pressure
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Intrapleural pressure
• Lungs tend to collapse and chest wall tend to expand
• Pleurae are held together by a thin layer of fluid
• Intrapleural space is continuously drained by lymphatics
• -2mm of Hg at the end of expiration to -6mm of Hg at the end of inspiration
• It is sub-atmospheric throughout respiratory cycle
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inspiration expiration
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Factors affecting intra-pleural pressure
I. Physiological factors(i) deep inspiration(ii) sudden forceful expiratory movements(iii) gravity
II. Pathological factors(i) emphysema(ii) injury to thoracic wall
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Measurement of intrapleural pressure
• Direct measurement by inserting a needle into the pleural space
• Intra-esophageal pressure measurement
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Intra-alveolar pressure
• Reduces from 0 to -1mm of Hg during inspiration and comes back to 0 at the end of inspiration
• Increases to +1mm of Hg and comes back to 0 at the end of expiration
inspiration expiration
0
+1
-1
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Factors affecting intrapulmonary pressure
• Valsalva manoeuvre- forced expiration against closed glottis.
• Muller’s manoeuvre- forced inspiration against closed glottis
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Transpulmonary pressure
• Distending pressure
• Difference between intrapleural and intra-alveolar pressures
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Inspiration
Contraction of diaphragm/ external intercostal muscles
Expansion of thoracic cage
intrapleural pressure decreases
Intrapulmonary pressure decreases
Air flows into the lungs
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Expiration
Relaxation of diaphragm / intercostal muscles
Elastic recoil of thoracic cage
Intrapulmonary pressure increases
Air flows out of the lungs
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Elastic properties of the lung
• Elastic behaviour of lung is due to the presence of
(i) elastin fibers
(ii) collagen fibers
(iii) surfactant
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Pressure-volume relationship
Hooke’s law- length is directly proportion to force till elastic limits
It can be applied to the lung and chest wall
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COMPLIANCE
• Volume changes per unit change in pressure
• Measure of stiffness
• Ltr/cm of H2O
• Hysteresis
• Compliance of lung and compliance of chest wall
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Compliance of lung
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Compliance of lung
Inspiratory & expiratory compliance curveNormal value- 200ml/cm of H2OSpecific compliance- compliance per unit
volume (expressed as a function of FRC)Characteristics of compliance diagram is
due to- (i) elastin fibers- nylon stocking
arrangement (ii) surface tension
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Surface tension
• Force acting across an imaginary line 1cm long on liquid surface
• Develops because of cohesive force between water molecules
• Inner surface of alveoli are lined by a thin layer of fluid
• Lining fluid tend to collapse and push the air out
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• Laplace law- P=T(1/r1+1/r2) where P is distending pressure, T is tension in the vessel wall and r is radius
• In alveoli- P=2T/r• Small bubbles tend to blow up larger
bubble• This doesn’t occur in the lung because of-
(i) surfactant(ii) interdependence of alveoli
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P1
P2
T
T
r1
r2
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Surfactant
• Von neergard’s experiment, 1929
• Pattle, 1955
• Clements, 1962
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Clements experiment
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Surfactant
• Secreted by type II alveolar cells
• Dipalmitoyl phosphatidyl choline+lipids+proteins
• Lipid surface lowering agent
• Hyaline membrane disease/IRDS
• Smoking, 100% O2- reduce surfactant
• Glucorticoid receptors in lung
• Atelectasis following surgery
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Surfactant
• Physiological advantages-
1. Increases compliance
2. Promotes stability of alveoli
3. Keeps alveoli dry
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Surface tension of-
(i) Pure water- 72 dynes/cm
(ii) Alveolar fluid- 50 dynes/cm
(iii) Alveolar fluid with surfactant- 5 to 30 dyne/cm
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Elastic properties of chest wall
• Elastic recoil of chest wall is outwards
• Outward recoil of chest wall balances inward recoil of the lung
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Factors affecting compliance
1. Lung volume- directly proportional
2. Respiratory phase- more during deflation
3. Surfactant levels4. Gravity 5. Age
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Regional alveolar distension
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Clinical significance
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Airway resistance
• Ohm’s law- I=E/R
so, R=E/I
• When applied to airflow- Raw= ΔP/V where Raw is airway resistance,
ΔP is pressure difference, and
V is volume of airflow
• ΔP= Pmouth-Palveoli
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• Poiseuille-Hagen formula: V= ΔPπr4/8ηl where r is radius of tube,
η is viscosity, and
l is length of the tube
• R=8ηl/πr4
• radius of the tube has critical importance
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• Reynolds number- Re=Vdρ/η• Laminar flow• Turbulent flow- Re > 2000
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• Trachea and bigger airways upto 7th generation-80% of Raw
• Small airways represent silent zone
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Factors affecting airway resistance
• Lung volume
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• Density and viscosity of the gas
• Tone of the bronchial smooth muscle-
(i) autonomic nerves
(ii) hormones
(iii) drugs
(iv) environmental factors
• Type of flow
• Phase of respiration
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TISSUE RESISTANCE
• Viscous forces of tissue
• 20% of total resistance in young
• Increased in certain diseases
• Tissue resistance + airway resistance= pulmonary resistance
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Dynamic lung compression
• Subject expire hard from TLC to RV and flow rate is plotted against volume
• Flow rate is independent of effort over most part
volume
flo
w
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• Reasons for independence of flow rate-
(i) driving pressure remains constant
(ii) elastic recoil forces reduce with reducing volume
(iii) resistance of peripheral airways increase with decreasing volume
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Clinical significance
• In emphysema, there is reduction in the traction on airways as well as driving pressure
• In fibrosis, maximal flow rate for given lung volume is higher
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Flow limitation in emphysema
normal emphysema
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Airway closure
• Occurs at low lung volumes in young adults
• In elderly, it may be as high as FRC
• It occurs at high lung volumes in chronic lung diseases leading to defective air exchange
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Work of breathing
• Compliance or elastic work 65%
• Tissue resistance work 7%
• Airway resistance work 28%
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• Work done by respiratory muscles
• Work required by lung-thorax system is twice that of lung alone
• In normal breathing, most energy is used to expand lungs
• During heavy breathing, most energy is used to overcome airway resistance
• In restrictive diseases, compliance and tissue resistance works are increased
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Calculation of work done
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Significance of understanding mechanics of respiration
• Acute Respiratory Distress Syndrome of Infancy
• Assisted ventilation
• Obstructive sleep apnoea
• COPD & Asthma
• Lung volume reduction surgery