Respiratory: Physiology - Mechanisms of breathing Flashcards
Two chief muscles of inspiration
- Diaphragm
- External intercostals
Three accessory muscles of inspiration
- Scalenes
- Sternocleidomastoid
- Alae nasi
Five muscles of active expiration
- Internal intercostals
- Rectus abdominis
- External obliques
- Internal obliques
- Transversus abdominis
Nerve innervating diaphragm
Phrenic nerve (C3-5)
What dimensions of the chest are increased by the action of the external intercostals?
Anterior-posterior (predominantly) and lateral via “bucket-handle” motion
What is hysteresis with respect to the elastic properties of the lung?
Describes the phenomenon whereby lunge volume at any given pressure during deflation is greater than during inflation
What is the transpulmonary pressure?
Pressure difference between inside and outside lung
Compliance formula
Compliance = ΔV/ΔP
What is specific compliance?
Compliance per unit volume of lung
What is normal compliance?
200ml/cm H2O
Five factors which reduce compliance
Pulmonary fibrosis
Alveolar oedema
Atelectasis
Increased surface tension (decreased surfactant)
Increased pulmonary venous pressure (engorged lung)
Two factors which increase compliance
Age
Emphysema (loss of elastic recoil)
What two factors determine lung elasticity and therefore affect compliance?
- Elastic tissue (elastin, collagen)
- Surface tension
What is surface tension? What produces surface tension?
Force acting across an imaginary line 1cm long in surface of liquid
Arises because attraction is greater between liquid molecules than between liquid and gas molecules
What is the relationship between surface tension and pressure? What law describes this?
Laplace’s law:
P = 4T/r where P = pressure, T = surface tension, r = radius
How is the surface tension of alveolar lining fluid reduced to prevent collapse in normal lung function?
By surfactant (phospholipid) secreted by type II pneumocytes
Three physiological advantages offered by surfactant
- Reduced surface tension causes increased compliance (less recoil)
- Improves alveolar stability: less collapse means less physiologic dead space
- Keeps alveoli dry: decreased hydrostatic pressure outside capillaries decreases transudation
What is interdependence?
Tendency of expansion of surrounding lung parenchyma to prevent alveolar collapse
Three changes seen with absence of surfactant
- Reduced compliance
- Atelectasis
- Alveolar oedema
Why do basal regions of the lung ventilate more effectively than apical? How does this change at full expiration?
To counteract effects of gravity on weight of lung, intrapleural pressure is higher (less negative) at base
This means it has a smaller resting volume and expands more easily on inspiration
At residual volume (i.e. after full expiration), intrapleural pressure exceeds atmospheric pressure
Basal lung will therefore not ventilate until intrapleural pressure exceeds atmospheric, which may not occur if small inspiration taken after full expiration
What is the Hagen-Poiseuille equation?
V = (Pπr^4)/8nl where V = flow rate, P = ΔP, r = radius, n = viscosity, l = length
Therefore R (resistance) = 8nl/πr^4
What is the impact of halving the radius of a tube on the resistance?
Resistance increases 16x
Describe the difference between laminar and turbulent flow
Laminar: flow in which stream lines are parallel to sides of a tube, and gas in centre of tube moves at twice the average velocity
Turbulent flow: fluid motion with changes in direction and velocity
What is pressure proportional to in the setting of turbulent flow? What is more important in turbulent flow: viscosity or density?
Square of flow rate
Density more important in turbulent flow (increased density = increased turbulence)
What is Reynold’s number? How is it calculated and how is it interpreted?
Measure of whether flow will be laminar or turbulent
Re = 2rvd/n where r = radius, v = velocity, d = density and n = viscosity
Re > 2000 suggests turbulent flow
Where in the bronchial tree does laminar flow occur?
Very small airways (terminal bronchioles)
Describe the pattern of flow in most of the bronchial tree
Transitional
Where in the bronchial tree may true turbulence occur?
In the trachea, particularly with increased velocity during exercise
Describe the two mechanisms of measuring airway resistance
May be measured as the pressure difference between mouth and alveoli divided by the flow rate
Can be measured either by body plethysmography using Boyle’s law, or via measurement of intra-pleural pressure using oesophageal balloon
Three reasons intra-pleural pressures falls during inspiration
- Elastic recoil as lung expands
- Reduction in alveolar pressure further reduces intra-pleural pressure
- Tissue resistance
Does alveolar pressure curve correspond with flow or volume?
Flow: would be identical if airway resistance remained constant
Does intra-pleural pressure curve correspond with flow or volume?
Volume: would be identical if compliance remained constant
What is the major site of airway resistance in the bronchial tree? What is the significance of this clinically?
Medium-sized bronchi, up to 7th generation
Small airways disease may be present but not detectable on tests of pulmonary function
What happens to airway resistance as lung volume falls and why?
Resistance increases as volume falls due to reduction in radial tension on bronchi (+/- small airway collapse as a result)
What is the effect of the sympathetic and parasympathetic NS on bronchial smooth muscle tone and how is this mediated?
Sympathetic: bronchodilation via B2 receptors
Parasympathetic: bronchoconstriction via muscarinic receptors
What is the effect of decreased PCO2 on airway resistance?
Increases
What is the effect of histamine on bronchial smooth muscle?
Acts as bronchconstrictor
What is the effect of gas density on airway resistance? Give a clinical example
Increased density causes increased resistance
As with diving (increased atmospheric pressure causes increased gas density)
Describe dynamic airway collapse
Collapse of airways that occurs when intrapleural pressure exceeds alveolar pressure
Occurs in normal lung: at certain point, increasing effort does not increase flow rate
What is the effect of lung disease on effort-independent flow?
May occur at lower volumes
Due to loss of elastic recoil and/or loss of radial traction
Three factors which exaggerate dynamic airway collapse
- Increased resistance (e.g. in obstructive airways disease)
- Reduced elastic recoil
- Decreased lung volume
What is FEV1? What is the normal value? What change in FEV1 is seen in obstructive vs restrictive lung disease?
Forced expiratory volume: volume exhaled in first second of maximal expiration following maximal inspiration
Normal value 4L
Decreased in obstructive and restrictive lung disease but to greater extent in obstructive
What is FVC? How does it compare to VC? What is the normal value? What change in FVC is seen in obstructive vs restrictive lung disease?
Forced vital capacity
Slightly less than VC of slow expiration
Normal value 5L
Decreased in obstructive and restrictive lung disease
What is FEV1/FVC? What is the normal value? What change in FEV1/FVC is seen in obstructive vs restrictive lung disease?
Ratio of FEV1 to FVC
Normal value 80%
Normal or increased in restrictive lung disease, reduced in obstructive lung disease
What is PEF(25-75%)?
Forced expiratory flow rate over middle half of expiration
What might cause inequality of ventilation in conducting vs respiratory zones?
Conducting: local differences in distensibility and resistance (influenced by RR: if there is a long time constant, inspiration may not be complete before expiration begins)
Respiratory: diffusion limitation (e.g. if dilation increases distance over which diffusion must occur)
What % of total resistance is represented by tissue resistance in healthy young adults?
~20%
What is tissue resistance?
Pressure required to overcome viscous force of tissues sliding over one another during respiration
What equation can be used to define work of breathing?
Work = pressure x volume
What parameters increase viscous vs elastic work?
Increased RR causes increased viscous work (so those with obstructive lung disease tend to decrease their RR)
Increased TV causes increased elastic work (so those with restrictive lung disease tend to decrease their TV)
How is efficiency defined in terms of work of breathing? What is the normal efficiency % during (including during quiet breathing) and what can this be increased to with voluntary hyperventilation?
Efficiency % = work required to ventilate lung / total O2 cost x 100
Normal 5-10%, <5% quiet breathing, can be increased up to 30% with voluntary hyperventilation
What is the difference in terms of the generation of driving pressure in positive pressure ventilation vs normal respiration?
Positive pressure ventilation generates driving pressure by increasing pressure at mouth rather than decreasing alveolar pressure
