Flashcards in Physiology Deck (72):
What are the four steps of external respiration?
Gas exchange between alveoli and blood
Gas transport in the blood
Gas exchange at tissue level
What is ventilation?
The mechanical process of moving air between the atmosphere and alveolar sacs
What does Boyle's law state?
As the volume of a gas increases the pressure exerted by the gas decreases
(At a constant temperature)
What two forces hold the thoracic wall and the lungs in close opposition?
The intrapleural fluid cohesiveness
The negative intrapleural pressure
How does the intrapleural fluid cohesiveness hold the thoracic wall and the lungs in close opposition?
The water molecules in the intrapleural fluid are attracted to each other and resist being pulled apart - this holds the pleural membranes together
How does the negative intrapleural pressure hold the thoracic wall and the lungs in close opposition?
The sub-atmospheric intrapleural pressure creates a transmural pressure gradient across the lung wall and across the chest wall
I.e. the pressure is lowest in the pleural cavity
What three pressures are important in ventilation?
What does the external intercostal muscle do during inspiration?
Contracts to lift the ribs and move out the sternum
How is the volume of the thorax increased vertically during inspiration?
Contraction of the diaphragm - this flattens out its dome shape
What does the increase in size of the lungs do to the intra-alveolar pressure?
Causes it to drop
What does the drop in intrathoracic pressure during inspiration result in?
Air entering the lungs by following a pressure gradient
Is expiration a passive or active process?
How is expiration brought about?
Relaxation of inspiratory muscles - the chest wall and stretched lungs recoil to their preinspiratory size because of their elastic properties
What does the recoiling of the lungs do to the intra-alveolar pressure?
Causes it to rise
What does the rise in intrathoracic pressure during expiration result in?
Air passes out of the lungs by following a pressure gradient
What causes the lungs to recoil during expiration?
Elastic connective tissue in the lungs
Alveolar surface tension
What is the alveolar surface tension?
The attraction between water molecules at liquid air interface i.e. the walls of the alveoli
This produces a force which resists the stretching of the lungs
What would happen to the alveoli if they were lined with water alone?
The surface tension would be too strong and the lung would collapse
What chemical is present to reduce alveolar surface tension?
What does the law of Laplace state?
The small alveoli i.e. the alveoli with smaller radius have a higher tendency to collapse
How does surfactant reduce the alveolar surface tension and which alveoli does it have more of an effect on?
Intersperses between the water molecules lining the alveoli
Lowers the surface tension of smaller alveoli more
What is alveolar interdependence?
If an alveolus starts to collapse the surrounding alveoli are stretched, and then recoil: this exerts expanding forces in the collapsing alveolus to open it
What is the tidal volume?
The volume of air entering or leaving the lungs in a normal breath (500ml)
What is the inspiratory reserve volume?
Extra volume of air which can be inspired over and above the resting tidal volume (3000ml)
What is the inspiratory capacity?
The inspiratory reseve volume and the tidal capacity (3500ml)
What is the expiratory reserve volume?
The extra volume of air that can be actively expired beyond the normal tidal volume (1000ml)
What is the residual volume?
The minimum volume of air remaining in the lungs even after a maximal expiration (this is to keep them patent) (1200ml)
What is the functional residual capacity?
The volume of air in the lungs after normal tidal expiration i.e. residual volume + expiratory reserve volume (2200ml)
What is the vital capacity?
The maximum volume of air that can be expired after a maximal inspiration i.e. ERV + TV + IRV (4500ml)
What is the total lung capacity?
The maximum volume of air that the lungs can hold (RV + VC) (5700ml)
What changes in spirometry will occur in obstructive lung disease?
FVC normal, FEV1 reduced, FEV1/FVC reduced
What changes in spirometry will occur in restrictive lung disease?
FVC reduced, FEV1 reduced, FEV1/FVC normal
What changes in spirometry will occur in a combination of obstructive and restrictive lung disease?
FVC reduced, FEV1 reduced, FEV1/FVC reduced
What is pulmonary compliance?
The measure of effort that has to go into stretching or distending the lungs
What factors decrease pulmonary compliance?
Absence of surfactant
Why does decreased pulmonary compliance result in shortness of breath?
A greater change in pressure is needed to produce a given change in volume - the lungs are stiffer
When might pulmonary compliance become abnormally increased?
If the elastic recoil of the lungs is lost e.g. in emphysema
What is anatomical dead space?
The inspired air that remains in the airways and is not available for gas exchange
How can pulmonary ventilation be calculated?
Tidal volume x respiratory rate
Why is alveolar ventilation less than pulmonary ventilation?
Because of the presence of anatomical dead space
How can alveolar ventilation be calculated?
(Tidal volume - dead space) x respiratory rate
What is the definition of pulmonary ventilation?
The volume of air breathed out and in per minute
What is the definition of alveolar ventilation?
The volume of air exchanged between the atmosphere and alveoli per minute
To increase pulmonary ventilation, what is the most useful thing to do and why?
Increase the depth of breathing because of dead space
What is the definition of perfusion?
The rate at which blood is passing through the lungs
What is the V/Q ratio at the bottom of the lung?
Blood flow > ventilation
What is the V/Q ratio at the top of the lung?
Ventilation > blood flow
What is alveolar dead space?
Ventilated alveoli which are not adequately perfused with blood
What is the physiological dead space?
The anatomical dead space + the alveolar dead space
What physiological response does an increase of CO2 in an alveoli (due to increased perfusion) trigger?
Relaxation of airway smooth muscle
Dilation of local airways
This decreases airway resistance which results in increased airflow to the affected alveoli
What physiological response does a decrease in O2 in an alveoli (due to decreased ventilation) trigger?
Contraction of local pulmonary arteriolar smooth muscle
Constriction of local blood vessels
Increased vascular resistance
Decreased blood flow
Why would an increase in O2 in an alveoli result in increased local pulmonary blood flow?
To balance up the V/Q mismatch that has occurred and to oxygenate as much blood as possible
What is the diffusion coefficient of a gas?
The solubility of the gas in membranes
How is most oxygen transported in the blood?
Bound to haemoglobin
In what two forms is oxygen present in blood?
Dissolved in blood
Bound to haemoglobin
What might shift the oxygen-haemoglobin dissociation curve to the right?
An increase in PCO
An increase in [H+]
An increase in temperature
An increase in 2,3-bisphosphoglycerate
What shape is the oxygen-haemoglobin dissociation curve?
What shape is the oxygen-myoglobin dissociation curve?
How is CO2 transported in the blood?
As bicarbonate - main form of transport
As carbamino compounds
Where does the formation of bicarbonate in the blood take place?
Red blood cells
What enzyme facilitates the formation of bicarbonate in blood?
How are carbamino compounds formed?
Combination of CO2 with terminal amine groups in blood proteins
Where is the breathing rhythm generated?
A network of neurones called the pre-botzinger complex
Where is the pre-botzinger complex generated?
The upper end of the medullary respiratory centre
Where is the impulse for inspiration generated?
The dorsal group of neurones in the medulla
What happens during hyperventilation when there is increased firing from the dorsal group of neurones?
The ventral respiratory group neurones are activated which excite internal intercostal muscles, abdominal muscles etc.
What is the pneumotaxic centre?
An area in the pons which can modify the respiratory rhythm generated in the medulla
What does stimulation of the pneumotaxic centre do?
What is the apneustic centre?
An area in the pons which can modify the respiratory rhythm generated by the medulla
What does stimulation of the apneustic centre do??
When are pulmonary stretch receptors activated and what is this reflex called?