Mechanics of respiration Dr Kalpana Specific Learning objectives

Mechanics of respiration Dr. Kalpana

Specific Learning objectives • At the end of class students should be able to: 1. Explain the muscles involved in breathing 2. Explain pressure changes during ventilation 3. Explain the role of lung surfactant

The Respiratory System The Mechanics of Breathing GCSE PE Mechanics of Breathing

VENTILATION o Breathing in – inspiration / inhalation o Breathing out – expiration / exhalation

Ventilation • Occurs because thoracic cavity changes volume • Insipiration uses external intercostals and diaphragm • Expiration is passive at rest, but uses internal intercostals and abdominals during severe respiratory load • Breathing rate is 10 -20 breaths / minute at rest, 40 - 45 at maximum exercise in adults 6

MUSCLES OF INSPIRATION § Diaphragm - increases the vertical diameter of chest wall (75% of inspiration) § External intercostals - increase the transverse and A-P diameters of chest wall (25% of inspiration)

• Diaphragm is the main muscle of inhalation – When the diaphragm contracts, it pulls from the ribcage, flattening out • Inhalation is also assisted by the external intercostals muscles – Connect higher rib to lower – When the muscle contracts, it causes expansion in the ribcage in all directions

External intercostal muscle

2 nd -6 th ribs

7 th -10 th ribs

ACCESSORY MUSCLES OF INSPIRATION ü Sternocleidomastoid ü Scaleni ü Pectoralis minor ( Work during forced inspiration, exercise, airway obstruction)

EXPIRATION v Passive process v Results from elastic recoil of chest wall and lungs

MUSCLES OF FORCED EXPIRATION Ø Abdominal recti, transversus abdominis, obliques. Ø Internal intercostals

Internal intercostal muscle Abdominals

Movement of the Diaphragm Figure 17 -9 b

Movement of the Rib Cage during Inspiration Figure 17 -10 a

Movement of the Rib Cage during Inspiration Figure 17 -10 b

Inspiration • Inspiration-Active process • Contraction of these muscles increase the size of the thoracic cavity. • Both the pleura will expand accordingly. • Diaphragm & Intercostal muscles • Diaphragm contracts increased thoracic volume vertically. • External Intercostals contract, expanding rib cage • They elevate the ribs upward and outward through pump handle and bucket handle movement. Lateral and anteroposterior dimension of the thorax. • More volume -> lowered pressure -> air in. • (Negative pressure breathing. ) • Intrapulmonary pressure decreases (758 mm Hg

Inspiration 1) Our diaphragm pulls down 4) Air is sucked through the tubes into the lungs 2) Our intercostal muscles contract GCSE PE 3) Our chest expands Mechanics of Breathing

Inspiration Figure 22. 13. 1

Expiration • Expiration – Due to recoil of elastic lungs. – Passive. – Less volume -> pressure within alveoli is above atmospheric pressure -> air leaves lungs. – Note: Residual volume of air is always left behind, so alveoli do not collapse.

Expiration Figure 22. 13. 2

Pressures That Cause the Movement of Air In and Out of the Lungs (Pressure changes during ventilation)

Mechanisms of Breathing: How do we change the volume of the rib cage ? To Inhale is an ACTIVE process • External Intercostal Muscles • Diaphragm Rib Cage Spine Intercostals Rib Contract to Lift Contract Ribs Diaphragm Volume 27

Modes of Breathing • Quiet Breathing – Inhalation requires muscles • Contraction of diaphragm (75%), external intercostals (25%) – Exhalation passive • Lungs recoil due to elasticity • Forced Breathing – Inhalation • Accessory muscles include sternocleidomastoid and scalenes (muscles of the neck) – Exhalation • Internal intercostals, abdominal muscles

Pleural Cavity Very small space Maintained at negative pressure Transmits pressure changes Allows lung and ribs to slide Lungs Gas Exchange Chest Wall (muscle, ribs) Pleural Cavity Diaphragm (muscle) Imaginary Space between Lungs and chest wall 29

• Pleural fluid produced by pleural membranes –Acts as lubricant –Helps hold parietal and visceral pleural membranes together 30

Pressure Relationships in the Thoracic Cavity • Respiratory pressure is always described relative to atmospheric pressure • Atmospheric pressure (Patm) - pressure exerted by the air surrounding the body • Intrapulmonary pressure (Ppul) – pressure within the alveoli • Intrapleural pressure (Pip) – pressure within the pleural cavity

Intra pulmonary pressure changes in the alveoli normal value -2 to -3 mm. Hg to +2 to +3 mm. Hg Intrapleural pressure in the pleural space normal value -6 mm. Hg to -3 mm. Hg Significance –prevent collapse of lung and facilitates in venous return. Transmural pressure difference across the lung

Intrapulmonary Pressure • Also called intra-alveolar pressure • Is relative to Patm • In relaxed breathing, the difference between Patm and intrapulmonary pressure is small: – about — 2 mm Hg on inhalation or +2 mm Hg on expiration

The Respiratory Pump • Cyclical changes in intrapleural pressure operate the respiratory pump: – which aids in venous return to heart

REST PRESSURES Atmosphere Airways Pleural Sac 760 MM Hg Intrapleural pressure 756 mm. Hg Thoracic Wall Lungs

AIR IS A COMPRESSABLE GAS WHICH OBEYS BOYLE’S LAW • P 1 V 1 = P 2 V 2 • If Volume increases, Pressure must decrease • As lungs expand, pressure inside falls

INSPIRATION • Elevation of ribs expands lungs • Lowering of diaphragm by contraction also expands lungs • Expansion of lungs causes pressure inside to drop below atmospheric pressure • Air rushes in to fill the expanded lungs

INSPIRATION Atmosphere Airways 760 mm Hg 758 mm Hg Pleural Sac Intrapleural pressure 754 mm. Hg Thoracic Wall Lungs

• The lung is an elastic structure that collapses like a balloon and expels all its air through the trachea whenever there is no force to keep it inflated. • There are no attachments between the lung and the walls of the chest cage, except where it is suspended at its hilum from the mediastinum.

EXPIRATION • Return of ribs to rest position causes diminishing of lung volume • Return of diaphragm to rest position also causes diminishing of lung volume • Diminishing of lung volume causes pressure in lung to raise to a higher value than atmospheric pressure • Air flows out of the lungs

EXPIRATION Atmosphere 760 mm Hg Airways 762 mm Hg Pleural Sac Intrapleural pressure 756 mm. Hg Thoracic Wall Lungs

• Lung "floats" in the thoracic cavity, surrounded by a thin layer of pleural fluid that lubricates movement of the lungs within the cavity. • There are two layers that contribute to the pleural cavity: • Parietal pleura • Visceral pleura.

Intrapulmonary and Intrapleural Pressures • During inspiration, intrapulmonary pressure is about -3 mm Hg pressure; during expiration is about +3 mm Hg • Positive transmural pressure (intrapulmonary minus intrapleural pressure) keeps lungs inflated

Pressure in the Pleural Cavity Pneumothorax results in collapsed lung that can not function normally Figure 17 -12 b

• The pleural fluid (∼ 15– 20 ml) forms a thin layer between the pleural membranes and prevents friction between surfaces during inspiration and expiration. • Continual suction of excess fluid into lymphatic channels maintains a slight suction between the visceral and the parietal pleural surface.

Pleural Pressure and Its Changes During Respiration • Pleural pressure is the pressure of the fluid in the thin space between the lung pleura and the chest wall pleura. • The normal pleural pressure at the beginning of inspiration is about -5 centimeters of water. • During normal inspiration, expansion of the chest cage creates more negative pressure -7. 5 centimeters of water.

Injury to the Chest Wall • Pneumothorax: – allows air into pleural cavity • Atelectasis: – also called a collapsed lung – result of pneumothorax

Alveolar Pressure • Alveolar pressure is the pressure of the air inside the lung alveoli. • When the glottis is open and no air is flowing into or out of the lungs, the pressures in all parts of the respiratory tree, all the way to the alveoli, are equal to atmospheric pressure (0 cm water pressure).

• To cause inward flow of air into the alveoli during inspiration, the pressure in the alveoli must fall to a value slightly below atmospheric pressure (-1 centimeters of water). • During expiration, alveolar pressure rises to about +1 centimeter of water, and 0. 5 liter of inspired air is expelled out of the lungs.

Transpulmonary Pressure • The difference between the alveolar pressure and the pleural pressure. • It is the pressure difference between that in the alveoli and that on the outer surfaces of the lungs. • It is a measure of the elastic forces in the lungs that tend to collapse the lungs at each instant of respiration (recoil pressure).

Ventilation

Alveolar surface tension

Type II

SURFACE TENSION AIR LIQUID

WHAT PREVENTS ALVEOLI FROM COLLAPSING?

Type I cell Type II cell

Surfactant ü Dipalmitoyl phosphatidylcholine ü Secreted by type II pneumocytes ü Present in the alveolar fluid ü Reduces the surface tension ü Prevents the alveoli from collapsing

• Surfactant keeps the alveoli dry. Pulmonary capillary hydrostatic pressure is 8 -10 mm Hg Oncotic pressure is 25 mm. Hg. Inwardly directed pressure gradient. • Helps prevent pulmonary edema

Respiratory distress syndrome • Occurs in premature infants(born before 7 completed months) • Deficiency of surfactant • Alveoli collapse after the first breath • Baby goes to severe respiratory distress

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