Mechanics Of Breathing Flashcards
What is the mechanics of breathing?
How air is moved in + out of lungs
Physics/physiology of chest wall, airways + lungs
What happens to the lungs in inhalation and exhalation?
Lungs expand in response to muscle work, chest wall expansion + changes in pleural pressure
Air moves by bulk flow down a pressure gradient from more positive atmospheric to more negative peripheral airways pressure creating by lung expansion
Exhalation is passive + does not require muscular work as lungs are elastic + constrict on own
What provides resistance to air flow?
Airway diameter + contents
What is lung compliance and what does it depend on?
Lung stretchability
Depends on size + physical state
What occurs during quiet respiration? What changes if tidal volume increases?
Inhalation: lungs pulled open by muscle action against elastic tissue promoting recoil + resistance of airways to air flow (active)
Exhalation: lungs return due to elastic recoil to FRC where the opposing elastic forces of lung + chest wall are in balance
If tidal volume increases, exhalation must use elastic recoil AND muscular work to complete exhalation of larger volume in same 2/3rds of respiratory cycle of unchanged length
What are the mechanical components of respiration? What are their functions?
Chest wall tissues + ribcage: respiratory muscles/diaphragm act here
Pleural space: provides lubrication + transfer of expansile force to lung
Respiratory muscles:
- External intercostals: elevate ribs + increase both anteroposterior + left (right dimensions) - bucket handle effect in inspiration
- Diaphragm: dome becomes depressed in contraction increasing thorax height
Diaphragm is principle muscle of respiration. What does it do? What is its innervation?
Diaphragm moves down ~ 1 cm during quiet respiration in inspiration as muscles will shorten, dome of diaphragm goes flat, pulling base of lung down increasing thorax height + volume expanding the lungs
Innervated by phrenic nerve C3-5
The position of the first rib is stabilised by __ ___ so the rib is not very ___. This impacts upon movement of ___ in breathing.
Scalene muscles
Mobile
Sternum
What is the work of breathing and its 3 components?
Energy expended during respiration, 3 components:
- That required to expand lungs against lung/chest elastic forces (compliance/elastic work)
- That required to overcome viscosity of lung/chest wall structures (tissue resistance work)
- That required to overcome airway resistance to movement of air into lungs (airway resistance work)
Constitutes 5% of total energy expenditure at rest but can increase up to 50x during strenuous exercise
What is Functional Residual Capacity?
Lung volume where opposing forces of expansile skeletal structure of chest wall (with muscles at rest) + contractile lung are in balance with each other -> combined forces reach equilibrium volume
OR
The amount of air left in the lungs after passive exhalation
What muscle work is done in forced exhalation?
- Extracartilaginous portion of internal intercostals moves rib downwards
- Abdominal wall muscles
contract + increase pressure in abdomen pushing diaphragm up beyond FRC position - Pectoral girdle muscles
-> depress ribs + decrease ribcage volume
What muscles are used in inhalation? What do aim to do?
- Mainly external intercostal muscles
- Intercartilaginous portions of internal intercostals
- Diaphragm
- > elevate ribs + enlarge ribcage volume
In extremes of respiratory effect, accessory muscles become important in inhalation. What 2 muscles are these and what do they do?
- Scalenes: elevate first 2 ribs
2. Sternocleidomastoid muscles: raise sternum
Describe the layout + features of the lungs pleural membranes.
Visceral pleura: pleural membrane covering lung surface
Parietal pleura: pleural membrane attached to inside of thoracic cage/chest wall
Thin layer of pleural fluid between the 2 lubricates the pleural interface/space creating a seal effect
Why do movements of the diaphragm + chest wall permit expansion of the lung?
2 opposing forces of chest wall + lung -> negative pressure of ~-0.5kPa in pleural space
During inhalation, chest wall expands stretching pleural space decreasing the intrapleural space further
Pleural seal have suction effect on visceral pleura + attached lung causing them to expand with chest wall as force of lungs elastic recoil is exceeded
What lung volume is represented when the forces inflating + deflating the lungs are equal? At what phase of the respiratory cycle does this occur?
FRC
At end of exhalation of quiet breathing
The lungs are not adherent to the inside of the thoracic cavity. What stops them from collapsing away from the chest wall?
The negative pressure within the pleural space i.e. pleural seal
What is the commonest mechanism allowing lung collapse away from the chest wall?
Pneumothorax = puncture of lung surface or chest wall letting air into the pleural space (may lead to cardiac arrest + death if not recognised quickly)
Why do the lungs collapse in a pneumothorax?
Integrity of pleural seal broken raising the intrapleural pressure -> suction effect between 2 pleurae removed -> disconnection of lung from expansile force of chest wall -> lungs collapse away from chest wall under unopposed force of its elastic recoil
What will a pneumothorax on one side of the lungs look like on a scan?
- Hemithorax black due to air in pleural cavity
- Wont see any spidery capillaries as lung is completely collapsed
- Hemidiaphragm depressed
- Trachea may be pushed to one side (deviated)
- Heart/border of heart may be pushed to one side
What are the 2 important factors in the energy ‘cost’ of breathing?
- Resistance to flow
2. Compliance (how stretchy/stiff the combination of elastic lungs + semi-rigid chest wall are)
What is compliance of the lungs?
Volume change of lung (or length change of a substance) per unit of force (or pressure) applied
A measure of disposition of lungs to expand under traction or pressure i.e. stiffness of respiratory system
What is elastance of lungs?
Measure of disposition of lungs to return to resting position due to their intrinsic elasticity
What is Hooke’s law?
The force needed to extend or compress a spring by some distance, is proportional to that distance (if elasticity of lung + thorax were only factor)BUT the lung is stiffer at high + low lung volumes
Can be presented by a lung inflation curve where a hysteresis curve of inhalation + exhalation would be shown (slope of curve is compliance of whole system at that moment)
What is the effect of disease on lung compliance?
Increased in emphysema as there is less tissue to stretch due to tissue destruction in alveoli (essentially holes) so reduced elastance of lungs so expansial forces of thoracic wall are dominant (hyperinflated lungs)
Reduced in restrictive lung disease e.g. fibrotic lungs as lungs are scarred + scars are difficult to stretch so lungs are smaller + need more pressure to stretch them
What will lung disease do to the FRC?
Emphysema: decreased FRC as lungs are hyperinflated (barrel chest of patients)
Restrictive: increased FRC as lungs are smaller with increased resistance to chest expansion
What can give you a false positive appearance in a chest X-ray when scanning a patient for emphysema?
If a patient takes a big inhalation because the diaphragm will be flatter, the gaps between ribs will be bigger + lungs will look hyperinflated as in emphysema
What will you see on a chest X-ray of restrictive/fibrotic lung disease?
Dense scarring that is white in lungs where it should not be present
What is surface tension?
At a surface attractive forces between molecules of a liquid are stronger than those between liquid + gas due to hydrogen bonds between H2O in liquid -> surface becomes as small as possible so force must be applied to change the SA
Measured in terms of force per unit length as N/m or dynes/cm (1 dyne = 10^-5N)
What would happen to the lungs if water was the liquid that surrounded them?
Surface tension of water is high due to hydrogen bonds so would be difficult to expand in absence of agent to lower this surface tension i.e. lung structure would be unstable
What is Laplace’s law?
Alveoli are interconnected by airways + only have 1 fluid/air surface so pressure is proportional to surface tension, whilst it is inversely proportional to radius so:
P = 2T/r
P = pressure T = surface tension r = radius)
What is surfactant secreted by?
Type II epithelial cells called pneumocytes lining the alveoli
High turnover of surfactant so area of lungs denied bloody supply may be depleted of surfactant making it harder to breath
What is surfactant composed of?
- Major ingredient = phospholipid (principally dipalmitoyl phosphatidylcholine)
- Surfactant proteins (SP) A, B, C + D
What are the 6 functions of surfactant?
- Reduce surface tension stabilising alveoli + increasing compliance especially at low lung volumes
- Reduces likelihood of tissue fluid transudation
- Lipid component of surfactant has antioxidant activity
- Phospholipid is hydrophobic + projects into gas phase of alveolus
- SP-A + D bind a wide range of pathogens
- SP-A + D also activate macrophages + neutrophils via specific receptors
Surfactant proteins can bind pathogens in a process called ____ making them more ___ to the immune system.
Optimisation
Edible
What is another factor of the lungs that limits any tendency to collapse?/
The packing of alveoli result in their supporting each other so this interdependence is an additional factor that prevents lung collapse
What is Poiseuille’s law in terms of flow of air through airways?
Flow of air is laminar so:
R = 8nl/πr^4
R = resistance to flow n = viscosity l = length of tubing r = radius
So for individual airways, resistance rises with fourth power of radius e.g. if radius halved, resistance increases by 16x
Where does the greatest resistance in the airway tubes lie?
Might expect it to be greater for smallest individual bronchi(oles) but smallest airways are many in number + collectively have lowest resistance at physiological lung volumes unless narrowed by pathology
Thus, greatest resistance lies in medium sized bronchi (those < 2mm in diameter contribute < 20% resistance)
Where does the main part of airway resistance lie in health?
Max inspiratory + expiratory flows are limited by muscle strength but resistance is negligible
Where does the main part of airway resistance lie in disease?
Airway diameter reduced e.g. asthma so air flow is limited by resistance
What is the bedside test of airways resistance?
Peak expiratory flow rate (PEFR)
What are the 3 factors that determine airways resistance?
- Autonomic NS
- Lung volume
- Turbulent vs laminar flow
How does the autonomic NS determine airway resistance?
PS vagus nerve supplies motor innervation to bronchial SM so contraction narrows airways
Relaxation of muscle generated by catelcholamines e.g. adrenaline acting on B2 receptors (why B-agonists are used to treat asthma)
How does lung volume determine airway resistance?
Increased lung volume increases airway radius (radial traction) decreasing resistance
How does turbulent vs laminar flow determine airway resistance?
Larger airways more prone to turbulent flow than smaller airways where turbulent flow produces more resistance than laminar flow
What can happen at low lung volumes to the air flow resistance?
To achieve maximal exhalation to residual volume, alveoli + small airways must become smaller + low lung volumes makes small airways collapse i.e. closing volume
How does closing volume vary with age?
Can be at 10% of vital capacity in young
Rises to 40% or > in elderly
In elderly, it may even be > FRC
So in elderly, airways are collapsing at higher volumes when they should be open; this is problematic
