6.8 - Ventilation and Gas Exchange Flashcards
What is minute ventilation (L/min)?
- the volume of air expired in one minute or per minute
- minute ventilation (L/min) = tidal volume (L) x breathing frequency (breaths/min)
- typical range for 70kg healthy male is 0.5L x 12 breaths/min = 6L/min
- gas entering and leaving the LUNGS
What is respiratory rate (Rf)?
The frequency of breathing per minute
What is alveolar ventilation (Valv) (L/min)?
- volume of air reaching the respiratory zone/alveoli per minute
- alveolar ventilation (L/min) = (tidal volume (L) - dead space (L)) x breathing frequency (breaths/min)
- typical for 70kg healthy male is (0.5L - 0.15L) x 12 breaths/min = 4.2 L/min
- gas entering and leaving the ALVEOLI
What is respiration?
The process of generating ATP either with an excess of oxygen (aerobic) or a shortfall (anaerobic)
What is anatomical dead space?
The capacity of the airways incapable of undertaking gas exchange
What is alveolar dead space?
Capacity of the airways that should be able to undertake gas exchange but cannot (e.g. hypoperfused alveoli)
What is physiological dead space?
Equivalent to the sum of anatomical and alveolar dead space (conducting zone + non-perfused parenchyma)
What is hypoventilation?
Deficient ventilation of the lungs; unable to meet metabolic demand (leads to increased PO2 - acidosis)
What is hyperventilation?
Excessive ventilation of the lungs atop of metabolic demand (results in reduced PO2 - alkalosis)
What is hyperpnoea?
Increased depth of breathing (to meet metabolic demand)
What is hypopnea?
Decreased depth of breathing (inadequate to meet metabolic demand)
What is apnoea?
Cessation of breathing (no air movement)
What is dyspnoea?
Difficulty in breathing
What is bradypnoea?
Abnormally slow breathing rate
What is tachypnoea?
Abnormally fast breathing rate
What is orthopnoea?
Positional difficulty in breathing (when lying down)
Why does the max inspiratory and expiratory effort plateau?
It takes a lot of effort from muscles of airways to hold in/squeeze out the last bit of air
What is tidal volume?
Volume of air going in and out with each breath - normally 0.5L
What is inspiratory reserve volume?
Extra volume of air that you can get into lung on top of tidal volume
What is expiratory reserve volume?
The volume of air that you can empty past your tidal volume
What is residual volume?
- the volume of air left in the lungs
- you cannot fully empty your lungs of air due to lungs holding their structure to prevent collapse via surfactants etc
What is vital capacity?
Difference between max air you can get into lungs and min air (IRV + TV + ERV)
What is functional residual capacity?
- everything below default position of lung capacity (bottom of tidal volume) e.g. if you take in a deep breath and die, your lungs won’t empty all the way to bottom since that takes muscle effort, but to a baseline level due to elastic fibres of lung recoiling
- ERV + residual volume
What is inspiratory capacity?
- everything above baseline value (bottom of tidal volume)
- IRV + tidal volume
What do volumes not do?
Volumes are discrete sections of the graph and do not overlap
What are capacities?
Capacities are the sum of two or more volumes
What factors affect lung volumes and capacities?
- body size - height, shape
- sex - male, female (average male has larger lung volume and total lung capacity)
- disease - pulmonary, neurological
- age - chronological, physical
- fitness - innate, training (if you have athletic parents you tend to have larger lungs)
What is the conducting zone?
- 16 generations
- no gas exchange
- consists of the structures that provide passageways for air to travel into and out of the lungs e.g. nasal cavity, pharynx, trachea, bronchi, most bronchioles
- typically 150 mL in adults at FRC
- equivalent to anatomical dead space
What is the respiratory zone?
- 7 generations
- gas exchange
- corresponds to lung parenchyma and includes some bronchioles, alveolar ducts and alveoli
- air reaching here is equivalent to alveolar ventilation
What are non-perfused parenchyma?
- alveoli without a blood supply
- no gas exchange
- typically 0 mL in adults
- called alveolar dead space
Name two procedures that can decrease the volume of someone’s dead space?
- tracheostomy
- cricothyrotomy
Name two procedures that can increase the volume of someone’s dead space?
- anaesthetic circuit
- snorkelling
What does the chest wall have a tendency to do?
- chest wall has a tendency to spring outwards, and the lung has a tendency to recoil inwards
- these forces are in equilibrium at end-tidal expiration (functional residual capacity, FRC) which is the neutral position of the intact chest
- changes in external forces are needed to change the equilibrium
What changes in these forces would result in inspiration?
inspiratory muscle effort + chest recoil > lung recoil
What changes in these forces would result in expiration?
chest recoil < lung recoil + expiratory muscle effort
Describe the anatomy of the chest wall.
- lungs surrounded by a visceral pleural membrane
- inner surface of the chest wall is covered by a parietal pleural membrane
- the pleural cavity (gap between pleural membranes) is a fixed volume and contains protein-rich pleural fluid - lubricates surface
- chest wall and lungs have their own physical properties that in combination dictate position, characteristics and behaviour of intact chest wall
What drives the flow of air in and out of the lungs?
High pressure –> low pressure
What is negative pressure breathing and give an example?
- alveolar pressure is reduced below atmospheric pressure
- this is normal breathing
What is positive pressure breathing and give examples?
- atmospheric pressure is increased above alveolar pressure
- e.g. mechanical ventilation, CPR, fighter pilots
What is the effect of the diaphragm when breathing?
- diaphragm is like a syringe
- pulling force in one direction
- when diaphragm pulls down = increases volume and decreases alveolar pressure = negative pressure = inspiration
What is the effect of the other respiratory muscles when breathing?
- other muscles e.g. external intercostal muscles are like a bucket handle
- upwards and outwards swinging force
Key terminology - prefix symbols
- P - partial pressure (kPa or mmHg)
- F - fraction (% or decimal)
- S - Hb saturation (%)
- C - content (mL)
- Hb - volume bound to Hb (ML)
Key terminology - middle (subscripts) symbols
- I - inspired
- E - expired
- A - alveolar
- a - arterial
- v(line) - mixed venous
- P - peripheral
- D - dissolved
Key terminology - suffix symbols
- O2 - oxygen
- CO2 - carbon dioxide
- N2 - nitrogen
- Ar - argon
- CO - carbon monoxide
- H2O - water vapour
What is Dalton’s Law?
Pressure of a gas mixture is equal to the sum of the partial pressures (P) of gases in that mixture
What is Fick’s Law?
Molecules diffuse from regions of high to low concentration at a rate proportional to the concentration gradient (P1-P2), the exchange surface area (A) and the diffusion capacity (D) of the gas, and inversely proportional to the thickness of the exchange surface (T)
What is Henry’s Law?
At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid
What is Boyle’s Law?
At a constant temperature, the volume of a gas is inversely proportional to the pressure of that gas
What is Charles’ Law?
At a constant pressure, the volume of a gas is proportional to the temperature of that gas
What gases make up the air and at what % at sea level?
- N2 - 78.09%
- O2 - 20.95%
- Ar - 0.93%
- CO2 - 0.04%
- Ne, He, H2, Kr etc - <0.01%
How is high-altitude air different to sea level air?
It has the same % of gases as sea level but lower partial pressures of each
How is inspired gas modified in the airways?
- the air is warmed, humidified, slowed (so alveoli do not burst) and mixed to help protect respiratory exchange surface as air passes down the respiratory tree
- dry air at sea level –> conducting airways –> respiratory airways
What is total O2 delivery at rest?
- 16 mL/min
- at rest we need approx. 250 mL/min, so relying on diffused O2 alone is not conducive with life
What are haemoglobin monomers made of?
A ferrous iron ion (Fe2+, haem-) at the centre of a tetrapyrrole porphyrin ring connected to a protein chain (-globin); covalently bonded at the proximal histamine residue
What is the positive cooperative effect?
- as O2 binds to Hb, this leads to a conformational change that increases affinity of Hb for O2 which allows it to bind more easily - allosteric behaviour of Hb
- this change in the protein also creates a binding site for 2,3-DPG which facilitates unloading of O2 at tissues where it is needed
- the amount of 2,3-DPG increases proportionally to metabolic demand as you need to release more O2 at tissues
What causes left shift?
- left shift = increased affinity (loading)
- decreased temperature
- alkalosis
- hypocapnia (low CO2)
- reduced 2,3-DPG
What causes right shift?
- exercise
- increased temperature
- acidosis
- hypercapnia (high CO2)
- increased 2,3-DPG
- Bohr effect
What causes upwards shift?
- upwards shift - increased oxygen-carrying capacity
- polycythaemia
What causes downwards shift?
- downwards shift - impaired oxygen-carrying capacity
- anaemia
How does carbon monoxide change the oxygen dissociation curve?
- downwards and leftwards shift
- decreased capacity
- increased affinity
What is the oxygen dissociation curve for foetal haemoglobin?
Greater affinity than adult HbA to ‘extract’ oxygen from mothers’ blood in placenta
What is the oxygen dissociation curve for myoglobin?
Much greater affinity than adult HbA to ‘extract’ oxygen from circulating blood and store it
How is oxygen loading in lungs?
- RBCs entering from left are not deoxygenated - should be called mixed venous blood - is actually at 75% saturation
- from lungs –> RBC
How is oxygen unloaded at tissues?
- partial pressure of O2 has decreased as venous supply dumps blood with less O2 into blood at tissues which mixes and dilutes it, but amount of Hb same
- 5mL/dL of oxygen flux means 250mL/min is transferred to tissues - which is how much the body needs
How is CO2 loaded in tissues?
- CO2 moves into blood where it binds very slowly (as non-enzymatic) with H2O –> carbonic acid
- carbonic acid is weak so dissociates into HCO3- and H+
How is CO2 loaded into RBC?
- most CO2 leaving tissues goes into RBCs where it reacts with water in presence of carbonic anhydrase to produce carbonic acid
- H2CO3 –> H+ + HCO3-
- HCO3- moves out into blood and Cl- moves in (chloride shift) through AE1 transporter to maintain resting membrane potential
- Cl- binds to Hb allosterically causing right shift
- CO2 is mainly transported as bicarbonate in blood via RBCs but also binds to Hb at amine end to produce carbaminohaemoglobin (HbCO2)
- Hb is a good buffer so can bind protons that H2CO3 dissociates into to maintain pH inside RBC so carbonic anhydrase can work optimally
What changes happen in pH and other measures of CO2 levels during CO2 loading in tissues?
- slight pH decrease
- for the amount of O2 we are using, we do not produce a proportional amount of CO2 due to formation of water in respiration
