Lecture 18 - Mechanics of breathing Flashcards
The three steps of respiration
1) Pulmonary ventilation
2) External pulmonary respiration
3) Internal tissue respiration
Pulmonary ventilation
The physical movement of air in and out of the lungs
It relies on two principles: Boyle’s law, air moving from high pressure to low pressure
Boyle’s law
When the temperature is constant, the pressure is inversely proportionate to the volume
Intrapulmonary pressure
The force exerted by gases in the alveoli
Will be higher than atmospheric during expiration
Will be lower than atmospheric during inspiration
Intrapleural pressure
Refers to the pressure within the pleural cavity
Its role is to keep the lungs stable so it’s lower than atmospheric and intrapulmonary pressures
Created by elastic recoil of the lungs
Resistance to breathing
Pulmonary compliance and airway tension
Pulmonary compliance: what is it and what is it affected by?
The ease with which the lungs can be expanded
The elasticity of connective tissue
Mobility of surface wall
Surface tension
The elasticity of lung tissue
The measure of elastic recoil
A measure of (lung) volume changes resulting from a given change in pressure
Mobility of surface wall
…
Surface tension
Caused by intermolecular forces between molecules in a liquid
Air-fluid interface surface of the fluid is under tension like a thin membrane being stretched
Like the thin fluid layer between the alveolar cells and the air
Laplace’s law
Describes the relationship between Pressure (P), surface tension (T) and the radius (r) of an alveolus (bubble)
At equilibrium, the tendency of increased pressure to expand the alveolus balances the tendency of surface tension to collapse it
The use of surfactant
Pulmonary surfactant greatly reduces surface tension, increasing compliance
This equalizes the pressure between small and large alveoli
Surfactant helps keep the uniform alveolar size
More concentrated in smaller alveoli (per mm s. area)
Lower surface tension helps equalise pressure among alveoli of different sizes
Easier to inflate smaller alveoli
Work needed to expand alveoli with each breath greatly reduced
T decreases as alveoli get smaller
- allows alveoli dynamically adjust their rates of inflation and deflation
NRDS
Neonatal respiratory distress syndrome - lack of surfactant secretion in premature babies (28-32 weeks gestation)
Reduced compliance
Alveoli collapse on exhalation
Difficult to inflate lungs
50% die without rapid treatment
Airway resistance
Major ‘non-elastic’ source of resistance to gas flow
Resistance high, gas flow low
Resistance determined by the radius and affected by Lung volume and Bronchial smooth muscle
Lung volume
Bronchi dilate as the lung expands
Bronchial smooth muscle
Parasympathetic nerves
- bronchoconstriction (smoke, dust, irritants, histamine)
Sympathetic nerves & adrenaline
- bronchodilation
Measuring airway resistance
Uses forcibly breathing out vital capacity (FVC) and the forced expiratory volume (FEV) (the volume of air breathed out in one second)
Dividing FEV by FVC gives a % (when you x100) and if this % is <80% then there is increased airway resistance
Spirometer
Measures lung volume and capacity
Pulmonary function tests
Measures the speed at which you are able to breathe air out
Used by chronic asthmatics on a regular basis
What do spirometers find
Inspiratory reserve volume (IRV), tidal volume (TV), expiratory reserve volume (ERV), reserve volume (RV), and total lung capacity (TLC)
Tidal volume: what is it and how much can humans do?
The volume of air moved during one quiet breathe (can be either inhalation or exhalation)
Males = 500mls; females = 500mls (0.5L)
FRC: what is it, what does it do, and how is it worked out?
Functional residual capacity - the volume of gas left in the lungs after a normal, passive exhalation
Acts as a way to stabilise the composition of the alveoli
Cannot be measured by spirometer due to it involving RV
FRC = RV + ERV
ERV: what is it and how much can humans do?
Expiratory reserve volume - the amount of air that can be forcibly exhaled after normal TV exhalation
males = 1000mls (1.0L); females = 700mls (0.7L)
IRV: what is it and how much can humans do?
Inspiratory reserve volume - the amount of air that can be forcibly inhaled after a normal tidal volume inhalation
males = 3300mls (3.3L); females = 1900mls (1.9L)
RV: what is it and how much can humans do?
Residual volume - the air remaining in the lungs after maximum expiration
males = 1200mls (1.2L) females = 1100mls (1.1L)
IC: what is it and how is it worked out?
Inspiratory capacity - the maximum amount of air that can be inhaled after a normal expiration
IC = TV + IRV
VC: what is it, how is it worked out, and how much can humans do?
The maximum amount of air that can be expired after a maximum inspiratory effort
VC = TV + IRV + ERV
Males = ~4800ml (4.8L), Females = ~4200ml (4.2L)
TLC: what is it, how is it worked out, and how much can humans do?
The maximum amount of air contained in the lungs after a maximum inspiratory effort
TLC = TV + IRV + ERV + RV
males = 6000mls (6.0L); females = 4200mls (4.2L)
Pulmonary ventilation rate
Respiratory minute volume - the amount of air moved per minute
PVR = TV x BF (breathing frequency)
How does the respiratory system adapt to changing oxygen demands?
Varying the number of breaths per minute (respiratory rate) and volume of air moved per breathe (tidal volume)
Anatomical dead space: what is it and how much is there?
The volume of air in conducting passages that does not participate in gas exchange
150ml per 500ml of TV
Alveolar ventilation
Amount of air reaching the alveoli each minute
AV (L min⁻¹) = RR x (TV - anatomical dead space)
