Lecture 3- Mechanics underlying ventilation of the lungs Flashcards
ventilation is the process of
inspiration and expiration/ physical action of breathing and moving air into and out of the lung/ movement of a volume of gas into and out of the lungs
respiration is the process of
oxygen and CO2 exchange across a membrane in the lungs at a cellular layer
normal tidal volume in quiet inspiration and expir
400-500ml
Quite inspiration and expiration is a
rhythmic and involuntary process - neurones in resp centre of brain automatically generate impulses to inspiratory muscles
what underlies why we are able to breath
due to Boyles law
boyles law shows
Inverse relationship between the volume that a gas occupies and the pressure exerted on that gas
Gas contained in a smaller volume=
higher pressure
Gas contained in a larger volume=
lower pressure
lung mechanics in quiet inspiration
- Requires active expansion of the thoracic cavity - which in turn expands the lungs – increase in volume causes decrease in pressure relative to atmosphere - Air is drawn into the airways
outline the lung mechanics in quiet expiration
- Air expelled from the airways passively, by relaxing muscles used in inspiration reduces volume of thoracic cavity - reduces volume of lungs - this increases pressure inside lungs relative to atmosphere and air expelled
Lungs have a natural
elastic recoil - Tendency to want to collapse into o Especially with increased stretch o Yet lungs are kept against chest wall within physical attachment during inspiration and expiration
What keeps the lungs against the chest
- PLEURAL FLUID is found between visceral and parietal pleura (in intrapleural spcace) forms seal between lungs and thoracic wall because of surface tension o So lungs expand with the thoracic cavity
the pleural seal
Surface tension between the pleural surfaces created by the presence of thin film of pleural fluid holds outer surface of lung (base of my cup/bottle) to inner surface of chest
Summary of pressure changes and air movement Inspiration
- expansion of thoracic cavity and lungs – ↑volume and ↓pressure (relative to atmospheric pressure) – Moves air into airways (lung) – Active requiring contraction of external intercostals and diaphragm (80%)
Summary of pressure changes and air movement expiration
– reducing size of thoracic cavity and lungs – ↓ volume and ↑ pressure (relative to atmospheric pressure) – Moves air out of airways (lung)
intrapleural sapce
space between pleuras
pressure in the intrapleural space
negative (relative to atmospheric pressure)
why is the intrapleural space negative
due to recoil of lung pulling visceral pleura inwards and the chest wall pulling parietal pleura outwards - Intrapleural pressure is negative throughout expiration and inspiration (becomes more negative up until end of inspiration
what holds the visceral and parietal pleura together
the pleural seal
What would happen if the pleural seal were broken?
- The lungs would collapse- pneumothorax
What do we call the state when we have just expired and before we start inspiring?
- Resting expiratory level (during quite breathing)- represents a physiological state
Resting expiratory level represents
o Represents equilibrium (balance) between elastic recoil chest outwards and elastic recoil inwards
resting expiratory level is not a
volume
the volume of air during resting expiraotry level (REL)=
functional residual capacity (FRC)
during REL (the state of equilibirum) what is happening to the lungs and the chest wall
- Lungs collapse inwards (lung elastic recoil) - Chest wall pulls out (chest elastic recoil) o Forces are equal and opposite so balance out (no movement of chest wall) - Tendency to always want to return to this resting state
ventilation disturbs REL… inspiration
- Inspiration is active a. Muscles contract to allow the chest wall and diaphragm to overcome inward pull of the lung recoil
inspiration is an
active process
Expiration is
passive
ventilation disturbs this equilibrium… expiration
a. Muscles stop contracting b. Chest wall and diaphragm no longer overcome inward pull of lung recoil c. Return to REL
forced ins and ex uses
lung reserve volumes but requires more effort
difference b/w quiet and forced ventilation
- Going beyond quiet inspiration and expiration - Used during exercise but also when diseases affect the lungs - Required involvement of more muscles
name four accessory muscles of inspiration
Sternocleidomastoid Scalene muscles Serratus anterior Pectoralis major
name two accessory muscles of expiration
internal intercostals abdominal wall muscles
forced expiration is an……. process compared to being an ….. process during quiet ventilation
active passive
tidal volume
represents the volume of air entering and leaving the lungs in a single breath (during quiet inspiration and expiration) (400-500ml)
Inspiratory reserve volume-
air breathed in above a quiet inspiration during exercise
Expiratory reserve volume-
extra air that can be forced out of lungs after normal breath
Residual volume-
the air left in the lungs after the expiratory reserve volume (to keep alveoli open) (cannot be measured by spirometer)
Functional residual capacity-
amount of air left in the lungs after a quiet expiration (at resting respiratory level- functional residual capacity)
Inspiratory capacity-
the amount you take in on a normal breath + the amount above and beyond you can take in
Vital capacity-
the amount of air that we can expire after a maximal inspiration
Forced vital capacity-
the amount of air that can be expired when forced
why is the alveoli surface lined by fluid
- Creates surface tension that limits their expansion
- Decreases compliance making it difficult for alveolar (and therefore lungs) to expand
what counters the surface tension created by fluid which lines alveoli
surfactant
surfactant produced by
type II pneumocytes in lungs
surfactant has what properties
detergent- o Acts to disrupt interaction between fluid molecules on alveolus surface.. reducing surface tension
to summarise surfactant..
o Acts to disrupt interaction between fluid molecules on alveolus surface.. reducing surface tension
surfactant is more effective at disrupting surface tension when its
molecules are closer together i.e. around smaller alveoli
smaller alveoli have surfactant molecules which are
close together More effective at disrupting surface tension of fluid Reduced surface tension Reducing surface tension in smaller alveoli prevents pressure rising (within the alveolus) as a result of the smaller volume
reducing surface tension (by having surfactant which is closer together) in smaller alveoli…
prevents pressure rising (within alveolus) as a result of the smaller violume
larger alveoli surfactant molecules spread
further apart - less effective at disturbing the surface tension
surface tension increase as
alveoli increase in size i.e. as lungs expand (hence forced inspiration harder than quiet inspiration)
Due to surfactant, surface tension in larger alveoli
>>smaller alveoli – effectively surfactant ‘diluted’ in larger alveoli
Due to surfactant, surface tension in larger alveoli >>smaller alveoli – effectively surfactant ‘diluted’ in larger alveoli thus….
pressure inside the bigger alveoli stays high despite it being bigger
surfactant ensures pressure sin alveoli does not drop despite increase in size
• i.e. pressure does not drop despite increased ‘volume’ of the alveolus, as the increased pull ‘inwards’ from the surface tension counters this • If pressure remains high in bigger alveoli, keeps pressures equal to smaller alveoli • Prevents collapsing of small alveoli into big alveoli
Ventilate the lungs to allow for gaseous exchange at the alveoli
- Air moves through a series of airways to reach alveoli - Need to overcome resistance to flow - Tubes of smaller diameter have higher resistance to flow (Poiseuilles law) o Many airways in the lungs are small o Individual resistance is high
what compensates for the small airways with higher resistance to flow
Parallel arrangement of small airways compensates for increase in their individual resistance Over whole expanse of airways • Numerous airways running in parallel • Compensates for increase in their ‘individual’ resistance
Summary (to read)
• Ventilation is the movement of air into and out of our lungs • Governed by lung and thoracic cavity mechanical properties and forces • (Quiet) Inspiration active process – Boyle’s Law- increase volume, decrease pressure, intrapulmonary pressure < atmospheric and air travels inwards • (Quiet) Expiration passive process – Boyle’s law - decrease volume, increase pressure, intrapulmonary pressure >atmospheric and air travels outwards • Pleural seal connects visceral and parietal pleura so that when chest cavity expands so do lungs • Surfactant lowers alveolar surface tension – diluted in larger alveoli so that even though they are bigger Boyle’s law partially countered- keeps smaller alveoli inflated • Ventilation has to overcome airways resistance – numerous small airways running in parallel creates effective massive diameter and hence low resistance
