Physiology of Respiration Flashcards
The airways are a conduit for the passage of gases. On which airway factors does the flow of gas depend
1) Pressure gradient between alveoli and atmosphere
2) Compliance and resistance of airways
What physiological changes occur to affect airway compliance/resistance
Constriction and dilatation of airways under autonomic control
How do the airways protect the lungs
1) Air filtered by nasal hair and mucociliary escalator
2) upper airway warms and humidifies the air
3) vocal folds in the larynx and cough reflex protect the lungs against aspiration
What muscles are used in inspiration
Diaphragm (main respiratory muscle) External intercostals Accessory inspiratory muscles - scalenes - sternomastoid - pectoralis - latissimus dorsi
Describe the action of inspiration (passive and forced)
Diaphragm contracts and flattens
External intercostals elevate the rib and sternum to increase the anterior-posterior dimension of thoracic cavity
Both of the above increases the intra-thoracic volume, reduces the intra-thoracic pressure and allows air to flow passively into the lungs
- forced inspiration uses accessory muscles to further increase the intra-thoracic volume and further reduce pressure
What muscles are used in expiration
Passive - elastic recoil produces positive intra-alveolar pressure
Forced - abdominal wall muscles and intercostal muscles increase the intra-alveolar pressure and drive the air out
Describe the action of expiration
Diaphragm relaxes and domes
Lungs and chest wall recoil
Thoracic volume reduces, intra-thoracic pressure rises and airflows out of the lungs
How is airway compliance calculated
compliance = change in lung volume/change in pressure
How does lung compliance affect lung volume
High/good compliance means the lungs are easily inflated
Poor/low compliance means the lungs are stiff, difficult to inflate, and therefore don’t reach normal volumes
Describe the role of surfactant in respiration
Surfactant is a substance produced by pneumocytes. It counteracts the surface tension that would otherwise make the spherical alveoli collapse. This minimises the work needed to re-inflate the lungs with each inspiration
Describe the work of breathing
During inspiration only as expiration is passive
1) work to expand lungs against elastic forces and surface tension
2) work to overcome airway resistance
3) movement of chest wall
What factors affect a persons respiratory capacity
Depends on volume of gas moved and respiratory rate at which this occurs
What is the respiratory minute volume
Tidal volume x respiratory rate
- amount of air brought into the lungs per minute
What is physiological dead space
Anatomical dead space + alveolar dead space
Anatomical dead space - e.g. nose, mouth, pharynx, larynx, trachea and bronchi
Alveolar dead space - volume of diseased lung unable to perform gas exchange
How to calculate the alveolar ventilation rate
Alveolar ventilation rate = (tidal volume - dead space) x respiratory rate
Therefore any increase in dead space causes an increase in respiratory minute volume in order to maintain the same alveolar ventilation rate
Describe the Bohr equation
Used to calculate physiological dead space
- As atmospheric pCO2 is practically zero, all expired CO2 must come from the communicating alveoli, and none from the dead space
- The Bohr equation compares to pCO2 in expired air with that of pCO2 in communicating alveoli (this is the same pCO2 as the arterial blood and so can be obtained with an ABG).
- This comparison calculates the ‘non-pCO2 containing volume’ a.k.a. the physiological dead space
Definitions of the following in spirometry
- TV - tidal volume
- IRV - inspired respiratory volume
- ERV - expiratory reserve volume
- RV - residual volume
- FRC - functional residual capacity
- VC - vital capacity
- TLC - total lung capacity
- FVC
- FEV1
TV - volume of air moved in quiet respiration (0.5L)
IRV - max volume inspirable (3L)
ERV - maximum volume expirable after tidal volume expiration (1L)
RV - residual volume in the lungs after maximum expiration (1.5L)
FRC - ERV + RV = amount left in lungs after tidal volume expiration (2L)
VC - the volume that can be expired after a maximal inspiratory effort (4.5L)
TLC - sum of vital capacity and residual volume (6L)
FVC - gives an idea of vital capacity
FEV1 - forced expiratory volume - volume expired in 1st second of a forced expiration FVC measurement
What is the function of the FEV1:FVC ratio
FVC is reduced in restrictive lung disease
FEV1 is reduced in obstructive lung disease as gas can’t be forced out quickly
FEV1/FVC ratio
- normal > 0.7
- obstructive <0.7
- restrictive >0.7 (normal ratio, but overall reduced FVC)
Describe the neurological control of breathing
Neurones controlling respiration are found in the medulla
Respiratory centres are located near the pons
- inspiratory neurones have spontaneous rhythmical activity
- expiratory neurones are inactive unless expiration is forced
Describe the action of central chemoreceptors in respiration
These detect chemical changes in the blood due to changes in partial pressure of oxygen or local hydrogen ion concentration in the blood
- central receptors detect pH changes - this is because CO2 crosses the blood brain barrier and dissolves in the CSF. Receptors detect increase in CSF H+ ion concentration
Describe the action of peripheral chemoreceptors in respiration
These detect chemical changes in the blood due to changes in partial pressure of oxygen or local hydrogen ion concentration in the blood
Location
- carotid bodies supplied by CN IX (glossopharyngeal)
- aortic bodies supplied by CN X (vagus)
Peripheral chemoreceptors detect changes to pO2 levels in the blood
Describe the action of stretch receptors in respiration
Negative feedback from lung stretch receptors as the lung inflates to cause termination of inspiration
Inhibitory impulses pass through the vagus nerve to prevent over-inflation
Physiology of gas exchange in the lungs
Simple diffusion across the alveolar-capillary interface
- driven by partial pressures and solubility of gases involved (CO2 more soluble than O2)
How to calculate the V/Q ratio
V/Q ratio = alveolar ventilation rate/pulmonary blood flow
What does a decrease in the V/Q ratio imply
Occurs when significant regions of the lungs are perfused, but not ventilated
- decrease in O2 and CO2 exchange, so there is a gradual decrease in O2 and a gradual increase in CO2 in the arterial blood
What does an increase in the V/Q ratio imply
Occurs when significant regions of the lungs are ventilated but not perfused e.g. PE
- causes an increase in physiological dead space in the lungs
How is oxygen carried in the blood
Small amount is dissolved in plasma - known as PaO2
Most oxygen is carried by haemoglobin
- Hb has four binding sites for oxygen
Describe how Hb’s affinity for oxygen changes as it circulates through the body
Hb readily binds oxygen at the capillary-alveaolar interface. As each haem binds an oxygen molecule, it increases the binding capacity of the other three sites (cooperativity)
When all sites are full, the Hb is fully saturated
When Hb reaches the capillary-tissue interface, it releases the oxygen. Dissociation of one oxygen molecule here makes it easier for the remaining molecules to dissociate
What does the oxyhaemoglobin dissociation curve represent
The relationship between the oxygen saturation of haemoglobin (SaO2) and the partial pressure of arterial oxygen (PaO2)
What is the Bohr effect
Factors that alter the position of the oxyhaemoglobin dissociation curve
In the lungs, CO2 diffuses from blood to the alveoli and the hydrogen ion concentration in the blood falls
- this pushes the curve to the left to increase the quantity of oxygen that can bind with haemoglobin
When blood reaches the tissues it absorbed the products of metabolism (CO2) and hydrogen ion concentration in the blood increases
- this pushes the curve to the right, to favour the release of oxygen molecules and delivery of oxygen to the tissues
What factors push the oxyhaemoglobin dissociation curve to the left and to the right
Pushing left
- decrease in PaCO2, decrease in hydrogen ion concentration, lower temperature, increased foetal haemoglobin, increased carboxyhaemoglobin and decreased 2,3 - DPG
Pushing right
- increase in PaCO2, increase in hydrogen ion concentration, increased temperature and increased 2,3-DPG.
How is carbon dioxide transported in the blood
1) free CO2 in plasma - roughly 10%
2) reaction with deoxyhaemoglobin to form carbamino-haemoglobin - roughly 30%
3) reaction with water in plasma to form hydrogen ions and bicarbonate
- this reaction is catalysed within red blood cells by carbonic anhydrase
- increase red blood cells, hydrogen ions bind to haemoglobin (protein buffer) and bicarbonate ions are diffused out in exchange for chloride ions
What is the Haldene effect
As partial pressure of oxygen increases, the amount of CO2 carried by the blood falls
Because venous deoxyhaemoglobin is a weaker acid than oxyhaemoglobin
