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(ii) Respiratory tract (iii) Alveolo-capillary membrane (iv) Blood (v) Peripheral cells

Components of respiratory tractNose 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• Ventilation • Diffusion • Matching of ventilation & perfusion • Pulmonary blood flow • Blood gas transport • Transfer of gases between capillaries & cells • 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 Cartilag e Smooth muscles

trachea

Initial bronchi present

Terminal bronchiol e absent

Resp bronchiol e absent

Rings20 no def post little

Little

Largest

More

Lining Columna Columna Cuboidal r Epitheliu r m (1) Cilia Present Present Present

Cuboidal

(2) Glands Mucous membra ne

Absent

present

Present

absent

Present

alveol i absen t absen t Simpl e Squa Absen mous t Absen t

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 inspiration2) 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) sternocleidomastoidselevate sternum

• •

•

Muscles of expiration Internal intercostals- run downwards & backwards 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.

V.

Physiological factors (i) deep inspiration (ii) sudden forceful expiratory movements (iii) gravity 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

+1

0

-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 intraalveolar 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 curve Normal value- 200ml/cm of H2O  Specific 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

T

P1

r1

T

P2 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 advantages2. Increases compliance 3. Promotes stability of alveoli 4. Keeps alveoli dry

Surface tension of(ii) Pure water- 72 dynes/cm (iii) Alveolar fluid- 50 dynes/cm (iv) 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 volumedirectly proportional 2. Respiratory phase- more during deflation 3. Surfactant levels 4. 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

• Subject expire hard from TLC to RV and flow rate is plotted against volume • Flow rate is independent of effort over most part

flow

Dynamic lung compression

volume

• 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 R espir ato ry D istr ess Syndrome of In fa ncy • Assist ed v entila tion • Obstr uctive sle ep a pnoea • COPD & Asthma • Lung vo lume re ductio n su rg ery