Mechanisms of Breathing Flashcards
How does the lung expand?
Muscle work, chest wall expansion + changes in pleural pressure (+ve pressure - blows air in, -ve pressure - sucks air in)
How does air move into the lung?
Air moves by bulk flow down a pressure gradient from positive atmospheric to more negative airway pressure created by lung expansion
What provides resistance to air flow?
- Diameter
2. Contents
What is the difference in effort required for inhalation and exhalation in quiet breathing?
Inhalation = active process, requires muscular work Exhalation = passive force provided by stretched elastic lung tissue
Why do you need the pleural space?
Provides lubrication
Enables transfer of expansile force to lungs
What thoracic muscles are used for breathing?
- External intercostals - elevate ribs + increase both anteroposterior + L/R dimensions (up + out)
- Diaphragm - depressed during contraction
What is the diaphragm? How is it innervated?
Principal muscle of respiration.
Innervation = phrenic nerve C3,4,5
Define the term ‘Work of Breathing’.
Energy expended during respiration
What are the 3 components for Work of Breathing?
- Compliance/elastic work - energy required to expand lungs against lung + chest elastic forces
- Tissue resistance work - energy required to overcome viscosity of lung + chest wall structures
- Airway resistance work - energy require to overcome airway resistance to move air into lungs
What happens in quiet breathing?
- Inhalation is active - lungs pulled open by muscle action against elastic tissues promoting recoil + resistance of airways to airflow (main component of Work of Breathing)
- Exhalation is passive - lungs return due to elastic recoil to functional residual capacity where opposing elastic forces of lung + chest wall are balanced
What are the muscles involved in breathing?
- Internal intercostals (extra cartilaginous portion) - move ribs down
- External intercostals - move ribs up + enlarge ribcage volume
- Internal intercostals (intra cartilaginous portions) - move ribs up
- Abdominal muscles - rectus abdomens, internal + external oblique, transverses abdomens - contract + increase intraabdominal pressure, pushing diaphragm up
- Diaphragm
- Scalenes - elevates 1st 2 ribs
- Sternocleidomastoid - raise sternum
- Pectoral girdle muscles
Which muscles are involved in inhalation and forced exhalation?
Inhalation:
- Scalenes = elevate 1st and 2nd rib
- Sternocleidomastoid = raises sternum
- External intercostals = move ribs up + enlarge ribcage volume
- Intercartilaginous internal intercostals = move ribs up + enlarge ribcage volume
- Diaphragm
Forced Exhalation:
- Abdominal muscles
- Extracartilaginous internal intercostals = move ribs down
- Pectoral girdle muscles
What is Functional Residual Capacity?
Lung volume at which opposing forces of expansile skeletal structure of chest wall (muscles at rest) and contractile lung are balanced (equilibrium volume)
Lungs and chest wall have different recoil properties. What are they? Why does the pleural space help with this?
Lungs want to collapse in
Chest wall wants to expand out
Pleural space allows 2 joined structures to move together - acts as seal that allows lungs to expand with chest
What type of pleura is associated with the lungs? Where is it attached?
Visceral pleura = attached to lung surface
Parietal pleura = attached to chest wall
What is the pleural space?
Tiny space between visceral and parietal pleura filled with thin layer of pleural fluid.
Lubricates pleural interface + provides a seal
What is the pressure within the pleura space?
2 forces opposing each other there is negative pressure (-0.5kPa)
During inhalation, chest wall is expanded and as pleural space stretches the intrapleural pressure decreases further
Because of pleural seal, suction on visceral pleura + attached lung causes lung to expand with chest wall (force greater than lung’s elastic recoil)
What lung volume is represented when the forces inflating and deflating the lungs are equal? At what phase of the respiratory cycle does this occur?
Functional residual capacity (FRC)
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?
Negative pressure within pleural space
What is the commonest mechanism allowing lung collapse away from the chest wall?
Pneumothorax - lung surface/chest wall is punctured
What happens if integrity of pleural seal is broken?
Pressure in pleural space will rise (as air fills pleural space), suction effect between 2 pleurae is removed (lose expansile force of chest wall) and lungs collapse away from chest wall under its unopposed elastic recoil force
Name 1 type of pneumothorax. What are the signs?
Tension Pneumothorax
May lead to cardiac arrest if unrecognised
Signs:
- Left lung completely compressed
- Trachea pushed to right (Xray)
- Right heart border pushed to right
- Left hemidiaphragm depressed
Define the term Compliance
Compliance = volume of change of the lung per unit of force applied
What is compliance used to measure?
- Stiffness in respiratory system
2. Disposition of lungs to expand under traction/pressure
Define the term Elastance
Elastance = measure of the disposition of the lungs to return to resting position due to their intrinsic elasticity
What are the effects of disease on compliance? Give examples.
Emphysema - increase compliance, tissue stretches more easily, requires less pressure to stretch
Fibrosis - decrease compliance, scarred lung tissue means more pressure needed to stretch lungs
Describe simple tests of lung function used in clinical practice
Look at basic lung function (FRC)
Xray
Define the term ‘Surface Tension’
Molecular interactions resulting from hydrogen bonds between water molecules in liquid but not between water + air
Explain surface tension in terms of the lungs
Surface tension of water is high and lungs would be difficult to expand in absence of agent to lower this surface tension.
Surface tension = important factor in compliance
What is Hooke’s Law?
Force needed to extend or compress a spring by some distance is proportional to that distance (but lung is not ideal spring as stiffer at high/low lung volumes)
What is the hysteresis curve?
X axis = change in intrapleural pressure
Y axis = volume change from FRC
Inhalation + exhalation plotted separately (straight line is elasticity of lung/thorax if it was the only factor)
What is Laplace’s Law?
Alveoli are like soap bubbles that are interconnected by airways.
Alveoli have only 1 fluid/air surface:
pressure = 2(surface tension)/radius
What is surfactant? What is its role in the lungs?
Surfactant - secreted by Type II epithelial cells (pneumocytes) lining alveoli
Reduces surface tension in lungs dramatically - reduces work of respiration + stabilises lung structure
Enables lungs to be expandable in early stage of inhalation - also reduces tendency for fluid transudation
What is surfactant composed of?
Mixture of compounds
- Major ingredient = phospholipid (dipalmitoyl phosphatidylcholine) - hydrophobic + projects into gas phase of alveolus
- Surfactant proteins (SP-A, B, C, D)
What are the functions of surfactant?
- Reduces surface tension - stabilises alveoli + increases compliance
- Reduces likelihood of tissue fluid transudation
- Lipid component has antioxidant activity
- SP-A and D immune activity (bind wide range of pathogens + activate macrophages and neutrophils via specific receptors)
Why is the packing of alveoli beneficial?
Interdependence - support each other and limits the tendency to collapse
What is the relationship between resistance and radius?
Resistance rises with radius to power 4
Which airways provide the greatest resistance within the lungs?
Greatest resistance = medium sized bronchi
smallest airways are many in number + collectively have lower resistance
What is the test for airway resistance?
Peak Expiratory Flow Rate (PEFR)
- resistance negligible in health, but disease e.g. asthma, flow is limited by resistance
What factors determine airway resistance?
- Autonomic NS - contraction of bronchial smooth muscle narrows airways (vagus parasympathetic NS = motor to this muscle) + relaxation of muscle by Adrenaline acting on B2 receptors (beta agonists used to treat asthma)
- Lung volume - increase volume increases airway radius (radial traction)
- Turbulent vs laminar flow - larger airways more prone to turbulent
