mechanics of respiration

Dr. Niranjan Murthy H.L

Assistant Professor of Physiology

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

INTRODUCTION

• Components of respiratory system-

(i) Respiratory tract

(ii) Alveolo-capillary membrane

(iii) Blood

(iv) Peripheral cells

Components of respiratory tract-

Nose

Pharynx

Larynx

Bronchi

Bronchioles

Alveoli

Alveolo-capillary membrane

• 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

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

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

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

Structure-function relationship

Weibel’s model-

• Swiss anatomist

• 23 generations

• Conducting zone- 16 generations

• Respiratory zone- 7 generations

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

Alveoli

• 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

MECHANICS OF BREATHING

• It includes forces that support and move the chest wall & the lung, together with resistances they overcome and the resulting flows

Muscles of respiration

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

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

• Muscles of expiration

1) Internal intercostals- run downwards & backwards

2) Abdominal muscles

-external oblique -internal oblique -rectus

abdominis -transversus

abdominis

Abdominal muscles

INSPIRATION

• 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

EXPIRATION

Pressure changes during respiration

• Intrapleural pressure

• Intra-alveolar pressure

• Transpulmonary pressure

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

inspiration expiration

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

Measurement of intrapleural pressure

• Direct measurement by inserting a needle into the pleural space

• Intra-esophageal pressure measurement

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

Factors affecting intrapulmonary pressure

• Valsalva manoeuvre- forced expiration against closed glottis.

• Muller’s manoeuvre- forced inspiration against closed glottis

Transpulmonary pressure

• Distending pressure

• Difference between intrapleural and intra-alveolar pressures

Inspiration

Contraction of diaphragm/ external intercostal muscles

Expansion of thoracic cage

intrapleural pressure decreases

Intrapulmonary pressure decreases

Air flows into the lungs

Expiration

Relaxation of diaphragm / intercostal muscles

Elastic recoil of thoracic cage

Intrapulmonary pressure increases

Air flows out of the lungs

Elastic properties of the lung

• Elastic behaviour of lung is due to the presence of

(i) elastin fibers

(ii) collagen fibers

(iii) surfactant

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

COMPLIANCE

• Volume changes per unit change in pressure

• Measure of stiffness

• Ltr/cm of H2O

• Hysteresis

• Compliance of lung and compliance of chest wall

Compliance of lung

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

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

• 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

P1

P2

T

T

r1

r2

Surfactant

• Von neergard’s experiment, 1929

• Pattle, 1955

• Clements, 1962

Clements experiment

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

Surfactant

• Physiological advantages-

1. Increases compliance

2. Promotes stability of alveoli

3. Keeps alveoli dry

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

Elastic properties of chest wall

• Elastic recoil of chest wall is outwards

• Outward recoil of chest wall balances inward recoil of the lung

Factors affecting compliance

1. Lung volume- directly proportional

2. Respiratory phase- more during deflation

3. Surfactant levels4. Gravity 5. Age

Regional alveolar distension

Clinical significance

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

• 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

• Reynolds number- Re=Vdρ/η• Laminar flow• Turbulent flow- Re > 2000

• Trachea and bigger airways upto 7th generation-80% of Raw

• Small airways represent silent zone

Factors affecting airway resistance

• Lung volume

• 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

TISSUE RESISTANCE

• Viscous forces of tissue

• 20% of total resistance in young

• Increased in certain diseases

• Tissue resistance + airway resistance= pulmonary resistance

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

• 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

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

Flow limitation in emphysema

normal emphysema

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

Work of breathing

• Compliance or elastic work 65%

• Tissue resistance work 7%

• Airway resistance work 28%

• 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

Calculation of work done

Significance of understanding mechanics of respiration

• Acute Respiratory Distress Syndrome of Infancy

• Assisted ventilation

• Obstructive sleep apnoea

• COPD & Asthma

• Lung volume reduction surgery