Lecture 2: Ventilation and lung mechanics Flashcards
What is ventilation?
Process of inspiration and expiration
What is responsible for the normal involuntary rhythmic breathing pattern?
Neurons in the respiratory centres of the brain generate automatic rhythmic impulses.
They travel via the spinal cord and peripheral nerves to the inspiratory muscles
How is air drawn into the lungs?
By expanding the volume of the thoracic cavity, decreasing intrapleural pressure. This draws air in because the air pressure within the lung falls below atmospheric pressure
What is the tidal volume?
The volume of air that enters and leaves the lungs with each breath
What is the inspiratory reserve volume (IRV)?
During normal respiration the increase in lung volume is not maximal, it can be increased to the extent of the IRV
What is the expiratory reserve volume (ERV)?
We can breath out more than at rest using the ERV
What is the residual volume (RV)?
We can’t empty our lungs completely, so even after forced expiration a RV of air will remain
Do lung capacities change?
No. Unlike lung volumes, which change with changes in tidal volume, lung capacities do not change as they are defined relative to fixed points in the breathing cycle
- maximum inspiration
- maximum expiration
- end of quiet expiration
What is inspiration capacity?
From the end of quiet expiration to maximum inspiration
IRV + TV
What is functional residual capacity?
Volume of air in the lungs at the end of quiet expiration
ERV + RV
What is vital capacity?
Inspiration capacity + expiratory reserve volume
What is total lung capacity?
Vital capacity + RV
What is the anatomical dead space?
The volume of the conducting airways
(only part of the tidal volume is available for gas exchange, the rest fills the conducting airways: nostrils->terminal bronchioles)
What is the alveolar dead space?
Alveoli which aren’t perfused or are damaged do not take part in gas exchange so the ventilation of these is wasted
What is the physiological dead space?
Anatomical dead space + alveolar dead space
no gas exchange occurs in dead space
What is the total pulmonary ventilation?
Tidal volume x respiratory rate
also known as minute volume
What is alveolar ventilation?
(Tidal volume - dead space) x respiratory rate
What forces are involved with the resting expiratory level?
At rest, at the end of quiet expiration, the respiratory muscles are relaxed.
The lung is subject to 2 equal and opposing forces
Inward: lungs elasticity and surface tension
Outward: muscles and connective tissues associate with the rib cage have elasticity, at rest they favour the outward movement
These 2 opposing forces at rest balance each other, creating a negative pressure in the intrapleural space relative to the atmospheric pressure
What does the pleural space contain?
- 10ml of pleural fluid as a film over the parietal and visceral surfaces
- surface tension between the molecules of this fluid forms a seal, holding the outer surface of the lungs to the inner chest wall
What ensures the chest wall and lungs move together?
Pleural seal
What happens during inspiration?
- contraction of the diaphragm and external intercostal muscles expands the thoracic cavity outwards from its equilibrium position
- the pleural seal ensures the lungs expand along with the thorax
- as lung volume increases, the air pressure in the lungs falls below atmospheric pressure and air flows into the lungs
What happens during quiet expiration?
- muscle contraction ceases
- elastic recoil of the lungs results in the thoracic cavity and lung returning to the equilibrium position
- passive process
What happens to the intrapleural pressure during the respiratory cycle?
- intrapleural pressure is negative at rest (relative to atmospheric pressure)
- intrapleural pressure becomes more negative during the inspiratory phase due to expansion of the thorax
- it then returns to resting (negative pressure) at the end of quiet expiration
What are the muscles of quiet inspiration/expiration?
Quiet inspiration: diaphragm (>70%) and external intercostal muscles
Quiet expiration: passive and due to elastic recoil so no muscles are needed
What are the accessory muscles of respiration?
Forced inspiration:
-sternocleidomastoid muscle and scalene muscles of the neck, serratus anterior and pectoralis major muscles of the chest wall
Forced expiration:
-internal intercostal muscles, abdominal wall muscles (external and internal oblique, rectus abdominus)
Why is energy required during inspiration?
- stretch the lungs
- overcome airways resistance
What is compliance of the lungs?
The distensibility (stretchiness) of the lungs -volume change per unit pressure change
Where do the elastic recoil preperties of the lung come from?
In order to stretch the lungs, elastic recoil must be overcome
Elastic recoil properties come from:
-elastic tissue in the lungs
-surface tension of the fluid lining the alveoli
When the lung expands, what opposes this?
Airways and alveoli of the lungs are lined with a film of fluid which is stretched as the lung expands
- increased in alveolar surface area is opposed by surface tension of the lining fluid
- this is because a gas-liquid interface always tends towards achieving the minimum SA
What is surfactant?
Secreted by type 2 pneumocytes in the lung
-mixture of phospholipids and proteins with detergent properties
-hydrophilic ends lie in the alveolar fluid
-hydrophobic ends project into the alveolar gas
(therefore they float on the surface of the lining fluid)
What is the function of surfactant?
They are interspersed between the fluid molecules, disrupting the interaction between fluid molecules on the surface
= therefore reducing the surface tension
What happens to the surface tension as an alveolus expands?
As an alveolus expands, surfactant molecules are spread further apart, making them less efficient.
Therefore as the alveolus expands, the surface tension increases
What happens to the surface tension as an alveolus shrinks?
Surfactant molecules come closer together, increasing their concentration on the surface, so they act more efficiently.
Therefore as the alveolus SA decreases, the surfactant helps to reduce surface tension
Why is it easier to expand smaller alveoli compared to larger ones?
Because smaller alveoli have less surface area, so the surfactant is more concentrated and therefore allows for more expansion due to the decrease in surface tension
How do you work out the pressure inside a bubble (alveoli)?
Law of Laplace
Pressure = (2 x surface tension)/ radius
P= 2T/r
How does surfactant stabilise the lungs?
stabilises the lungs by preventing small alveoli from collapsing into big ones
Alveoli vary in size, so if surface tension was constant in all alveoli, smaller alveoli (smaller radius) would have higher pressures within it.
Therefore if two unequal sized alveoli where connected by an airway, the smaller alveolus will empty into the larger alveolus which has a lower pressure
= this would mean that the smaller alveoli would collapse into the larger alveoli forming huge air filled spaces called BULLAE
= this would drastically reduce the surface area for gas exchange
Surfactant prevents this from happening because as alveolus expands the ‘r’ and ‘T’ both increase, and visa versa, meaning different sized alveoli can have the same pressure within them
What are the 3 main roles of surfactant?
- increases lung compliance by reducing the surface tension of alveolar fluid
- stabilises the lungs
- prevents the surface tension in the alveoli creating a suction force, which can cause transudation fluid from pulmonary capillaries to enter the alveoli
What is respiratory distress syndrome (RDS) in newborns?
Surfactant is absent from fetuses younger than 25 weeks
-this condition is seen in premature babies (<30 weeks) due to the lack of surfactant
What is airway resistance?
Energy must be expended to force air through the airways
What is Poiseuille’s law?
Works out the resistance of an airway to flow, when flow is laminar
Resistance of tube= (pressure/rate of flow)
How does resistance change in tubes?
Resistance of a single tube increases sharply with a falling radius
BUT
The combined resistance of small airways connected in parallel is normally low because their effective diameter is huge
-therefore the total resistance to flow in downstream branches is less than the resistance of the upstream branches
Where does most of the resistance to breathing reside?
Upper respiratory tract, except when small airways are compressed during forced expiration
What is laminar flow?
Flow of gas when each gas particle follows a smooth path and the paths do not interfere with each other
-the velocity of the fluid is constant at any point in the fluid
What contributes most to the total work load of breathing during inspiration?
-overcoming the elastic recoil of lung tissue and surface tension
(resistance to flow through airways is of little significance to total work load in healthy subjects, however it can be affected by disease)
Is breathing efficient?
Yes, the work of breathing consumes only 0.1% of total oxygen consumption
