mechanics of breathing

block 2 week 1 ctb

Boyle’s Law

at a given temperature, the pressure and volume of an ideal gas are inversely proportional 

air flow

• air moves by bulk flow down a pressure gradient: high pressure → low pressure

• airflow achieved by creating a pressure gradient with the air around us

• an INCREASE in lung volume DECREASES pressure, air flows in

• a DECREASE in lung volume INCREASES pressure, air flows out

tidal breathing

• quiet breathing at rest

tidal breathing: inspiration

• contraction/flattening of the diaphragm INCREASES thoracic volume

• contraction of external intercostals → rib cage moves upwards and outwards

tidal breathing: expiration

• passive

• muscle relaxation and elastic recoil of the lungs

forced inspiration

• involves accessory muscles of respiration

• accessory muscles are not primarily involved in respiration but act to enlarge the ribcage in any way possible, to increase the amount of air breathed in

• greater contraction of external intercostals: they raise ribs to a greater extent than in quiet inspiration

• forced breathing occurs when demand for oxygen is high (eg exercise, diseased states, singing)

• lateral and anteroposterior diameter of thorax is increased

• joints between posterior ends of the ribs and the transverse processes of the vertebrase enable lower ribs to swivel upwards and outwards to increase lateral diameter of chest (bucket handle effect)

• sternocleidomastoid muscle and neck scalene contraction: raise ribs

• tripod sign: arms planted, shoulder girdle fixed and pectoralis major and latissimus dorsi pulls chest outwards (sign of resp distress when displayed at rest)

forced expiration

• active process

• abdominal muscles contract, increasing intrabdominal pressure

• forced abdo organs up against diaphragm (helps to decrease volume of the thoracic cavity)

• contraction of internal and innermost intercostal muscles, displacing ribs down and back to decrease volume of the thorax

pleura

• serous membrane that covers lungs and thoracic cavity

• similar to peritoneal coverings in abdomen (in lungs: referred to as pleura)

• visceral pleura = layer that covers lungs:

• visceral pleura adheres tightly to lung tissue at reflects back on itself at hilum to become parietal pleura

• parietal pleura = lines the mediastinum, diphragm and ribcage

• between visceral and parietal pleura = pleural cavity/pleural space

pleural space/pleural cavity

• contains few mm of pleural fluid (which acts as lubricant)

• pleural layers can only be separated by considerable force but they can slide over each other easily

• pleural space is held at NEGATIVE PRESSURE

• -ve pressure is key in preventing pneumothorax

pressures

• atmospheric pressure (P_B⁾

• alveolar (intrapulomary or intralaveolar pressure) (P_A)

• intrapleural pressure (P_IP)

atmospheric pressure (P_B⁾

• pressure exerted by weight of the atmosphere

• at sea level: 101.3 kilopascals

• relatively constant so referred to as being ‘zero cm of water’

• pressures > atmospheric pressures = positive

• pressures < atmospheric pressures = negative

alveolar (intrapulomary or intralaveolar pressure) (P_A)

• prsssure inside the alveoli

• must be equal to atomospheric pressure at the end of inspiration and expiration

intrapleural pressure (P_IP) definition

• pressure within the pleural space (between parietal and visceral pleural layers)

intrapleural pressure

• opposing forces of outward recoil and inward recoil of the lungs pull the pleural layers apart 

• the 2 opposing forces pull the visceral and parietal layers apart and mean that the pleural space has a -ve pressure

• intrapleural space is subatmospheric: a vacuum is created 

• during breathing, intrapleural pressure changes by becoming more or less -ve, which affects volume of the lung tissue

transpulmonary pressure (TPP)

• alveolar pressure - pleural pressure = TPP

• it is a transmural pressure (pressure across a wall)

• if TPP is +ve, it acts as an expanding pressure on the lungs

• after expiration, the alverolar pressure will be zero (same as atomospheric pressure) and intrapleural pressure will be -5cm of water

  • therefore TPP will be +5cm of water

  • helps hold lungs partially expanded

• TPP is the force resisting the inward elastic recoil of the lungs

• the greater the TPP, the greater the lung volume up

• as TPP increases, lungs will expand further

tidal inspiration

↑ thoracic cavity volume

↑ pleural space volume

↓ intrapleural pressure (becomes more -ve)

↑ transpulmonary pressure

↑ lung volume

↓ alveolar pressure

airflow in, down the pressure gradient from atmosphere into the lungs

will continue until alveolar pressure once again equals atmospheric pressure

tidal expiration

↓ thoracic cavity

↓ pleural space volume

↑ intrapleural pressure

↓ transpulmonary pressure

↓ lung volume

↑ alveolar pressure

airflow out, air will be expelled out of lungs until alveolar pressure = atmospheric pressure

compliance

• how easily the lungs can be distended/stretched when external force is applied on them

• compliance = ΔV / ΔP

• a measure of how much the volume changes with a change in the transpulmonary pressure

• volume is directly proportional to compliance

• the greater the change in volume per unit change in pressure, the greater the compliance

elasticity

• resistance to stretch  and tendency to return to their previous configuration when distorting force is remove

• opposite of compliance

respiratory compliance

• distensibility of chest wall and lungs

• chest wall tendency: to spring outwards and lungs to spring inwards

• to change the thorax volume, resp muscles must overcome lung and chest wall’s mechanical properties: most importantly the lungs’ tendency to recoil

elastic properties of the lung

• caused by lung tissue itself: elastin and collagen fibres

• elasticity: caused by elastin and surface tension

• emphysema: loss of elastic lung tissue (elastic properties decreased)

• pulmonary fibrosis (replacement of normal lung tissue with scar tissue)

static compliance

• static lung mechanics because the volume is measured at each change in pressure but doesn’t take into account airflow (which changes with time)

• lung doesn’t expand linearly with increasing pressure, so compliance is not the same throughout lung expansion

• at first, a higher pressure is required for a small change in volume (compliance = low)

• the lungs then become easier to distend → slope steepens

  • small change in transpulmonary pressure results in a large change in lung volume

• curve flattens out again: any further increase in pressure will not lead to a change in volume

• alveoli = well inflated and close to their elastic limit so compliance is low again

• steepest part of the curve= normal tidal breathing (this reduces trhe work of breathing)

• slope of the curve = compliance

compliance during tidal breathing

• residual volume = air left in the lungs when you have breathed out as much air as possible

  • lungs never fully deflate (always some air left inside them)

• FRC (functional residual capacity)= air left in lungs at the end of a normal tidal expiration

• TLC (total lung capacity)= how mucb air in lungs when you’ve taken the biggest breath you can

• hysteresis= a phenomenon when the pressure vol curve is different for inspiration and expiration

lung volume and compliance

• compliance highest= low lung volumes

• compliance lowest= high lung volumes

  • a larger increase in transpulmonary pressure is needed to produce only a small change in volume (because alveoli are maximally stretched)

compliance throughout lung

• base of lung → compressed by lung tissue above it and has greater intrapleural pressure compared to apex

  • weight of fluid in the pleural cavity increases intrapleural pressure at base (so it’s less -ve than at the apex)

• based of lung also has greater initial compliance compared to the apex

• magnitude of pressure changes the same throughout, base of lung will expand more (greater ventilation)

alveoli at the base are less distended than at the apex at the end of expiration

• as alveolar pressure is same throughout the lung, the difference between the alveolar pressure and the intrapleural pressure (transpulmonary pressure) is therefore less at the base at the end of expiration

• therefore alveoli at the base are less distended than at the apex at the end of expiration

• base of lung has a greater initial compliance than the apex as compliance is greater at lower lung volumes

ventilation diagram

alveolar fluid

functions:

• protects alveolar epithelium

• immune role: solvent for antimicrobial peptides and cytokines, environment for alveolar macrophages

• mediates gas transfer

• forms an air-fluid interface

alveolus diagram

surface tension

• cohesive forces via hydrogen bonds between surface molecules are stronger, allowing the surface of a liquid to resist external force

  • they don’t have any molecules above them, so they attract more strongly to the molecules on either side and below them

  • allows surface of the liquid to resist external forces applied to it

• surface of water contracts to minimise contact with air, leading to formation of bubbles

surface tension in alveoli

• alveolar can be thought of as spherical structures, considered similar to tiny interconnecting bubbles

• surface tension at the air-fluid interface creates an inward collapsing pressure in the alveoli

• alveolar surface is covered by a thin layer of fluid and the air-fluid interface has surface tension that’s trying to reduce the area of the interface and so is trying to collapse the alveoli 

Laplace’s Law

• pressure within the alveoli can be predicted using Laplace’s Law

• states that the pressure within the bubble = twice the surface tension / radius

• P= 2T/ r

• pressure and radius are inversely proportional

• the smaller the bubble/alveoli, the greater the inward collapsing pressure and a greater pressure is needed to keep it inflated

surface tension and lung elastic recoil

• elastic recoil in the saline-filled lung is a result of the lung tissue alone

• elastic recoil in air-filled lung is the lung tissue and the surface tension at the air-fluid interface in the alveoli

• surface tension contributes more to elastic recoil than elastic lung tissue, affecting compliance

problems with surface tension

• smaller alveoli are harder to inflate as they require a greater transpulomary pressure to overcome the inwards collapsing pressure

  • significantly increases the work of breathing

• if alveoli of different sizes connected to a common airway, the smaller alveoli may collapse into the bigger alveoli

  • if surface tension is constant, the smaller alveolus would have a greater pressure compared to the larger alveolus, so air would move down the pressure gradient, causing the smaller alveolus to empty into larger alveolus

  • collapsed alveoli would then require a higher distending pressure to inflate again

• inwards force of the surface tension would tend to suck fluid would suck fluid from the interstitium into the alveolus

forces which stabilise alveoli

mechanisms which act to reduce surface tension in the lungs

• structural interdependence of alveoli 

  • other than the alveoli on the pleural surface, alveoli are surrounded by other alveoli which exerts traction which opposes collapse 

• pores of Kohn and canals of Lambert 

  • connect adjacent alveoli and provide collateral ventilation and help equalise pressure 

• surfactant

  • decreases surface tension in smaller alveoli more than larger alveoli 

  • results in stabilisation of the alveoli 

surfactant

• secreted by exocytosis from cuboidal type II pneumocytes

• a mix of phospholipids, neutral lipids, fatty acids, proteins

• disrupts hydrogen bonds between surface water molecules and reduces surface tension

• forms a thin film that lines the alveoli and acts as a barrier at the air-liquid interface

• stabilises inflation of alveoli as it differentially reduces surface tension more at lower volumes (where surfactant molecules are closer together) and higher volumes (where surfactant molecules are further apart)

• instead of staying constant, the surface tension increases as the alveoli become larger and decreases as alveoli become smaller

  • Laplace’s Law: it will reduce the pressure needed to keep the alveoli inflated as the lung volume decreases

  • makes it easier for lungs to inflate during inspiration and therefore increases compliance/reduces effort needed for expansion  

work of breathing

• energy consumed by respiratory muscles during resp cycle

• consists of:

  • elastic work

  • resistive work

elastic work

• done on inspiration to overcome elastic properties of the respiratory system

1. outward recoil of the chest wall

2. inwards recoil of the lung tissue

3. inward recoil of the alveolar surface tension

• some of the energy used in elastic work is stored as potential energy in elastic structures of the lung/chest and used as a driving force for normal expiration

resistive work

• to overcome friction

  • tissue resistance: result of tissues moving against each other during breathing

  • airway resistance: frictional forces on gas molecules during air flow

    • majority of resistive work

    • due to frictional forces on gas molecules as they interact with each other and the walls of the airways

    • the energy used in resistive work is wasted as it dissipates as heat and sound

work of tidal breathing

• quiet tidal breathing is very efficient

• doesn’t require that much work and energy required is less than 2% of the basal metabolic rate 

• lung pathology can increase the work of breathing substantially (up to 30% of BMR in some cases) and can lead to respiratory muscle fatigue and resp failure

airway resistance

• factors affecting airway resistance

  1. turbulent air flow

  2. change in airway radius 

• when airway resistance is increased, pressure gradient must be increased to maintain flow (increased resp effort)

• flow = pressure gradient / resistance 

turbulent air flow

• gas flow is only turbulent in the trachea

• increased veolcity (due to increased resp rate can cause turbulent flow in the large bronchi for a greater propoertion of the respiratory cycle

• upper airway obstructuin can increase velocity and turbulence

reduced airway radius

• bronchoconstriction

• low lung volume- airway reduces on expiration due to reduced radial traction from deflating lung

• dynamic airway compression: during forced expiration, intrapleural pressure can become positive, causing collapse of aurwats without cartilage in their walls 

  • can be worsened in diseases which cause airway narrowing or loss of elastic tissue