Respiratory Physiology Flashcards
Describe factors affecting airway resistance in the lung
● Some of these factors are related to Poiseuille’s (PWAH ZAY) law
○ Resistance is proportional to the length of the tube and the viscosity.
○ Resistance is inversely proportional to the radius to the 4th power
● Resistance is highest in medium sizes bronchi, low in very small airways
● It changes with different lung volumes, resistance decreases as the lung volume
increases because the airways are pulled ope by traction
● Resistance is also under the control of autonomic nervous system, where
stimulation of beta adrenergic receptors causes bronchodilation. Stimulation of
the parasympathetic system can cause bronchoconstriction. Reduced alveolar
PCO2 also causes increased resistance.
What factors determine the work of breathing?
● Elastic forces in the lung and chest wall
● Viscous resistance of the airways and tissues
What variables affect elastic workload?
● Large tidal volume increase workload
● Anything that reduces compliance of the lung
○ Lung volume
○ Increased tissue mass such as fibrosis
○ Loss of surfactant
What variables affect viscous resistance?
● Higher respiratory rates increasing flow rates
● Decreased airway radius i.e. bronchoconstriction
● Increased air density such as in scuba diving
● Increased air viscosity
What are the components of total lung volume and what are their typical volumes?
● Total lung capacity - approx 7L
● Vital capacity - the exhaled gas volume after a maximal inhale (5-6L)
● Residual volume - the amount of gas remaining after a maximal expiration
(1.5-2L)
● Functional residual capacity - the volume of gas remaining in the lung after
normal expiration (3L)
● Expiratory reserve volume 1.2L
● Inspiratory reserve volume 1.5L
● Tidal volume - the volume of gas moved in and out of the lung during normal
breathing (500ml)
Which of these can be directly measured by spirometry?
Tidal volume and vital capacity
How can the residual volume be measured?
● Helium dilution
● Nitrogen washout measurement (both gas methods only measure ventilated
residual volumes)
● Body plethysmography (measures total volume including trapped gas)
What is the anatomical dead space?
● Refers to the airway volume with ventilation but no blood flow
● The conducting airways do not participate in gas exchange
● Volume approx 150mls
How does this differ from physiological dead space?
● Anatomical dead space is determined by the morphology of the airways and the
lungs
● Physiological dead space is the volume of airways and lung that does no
eliminate CO2
● These dead space volumes are almost identical in the normal lung
● Physiological dead space is increased in many disease states due to inequality of
blood flow and ventilation in the lung i.e. VQ mismatch
How are the different dead spaces measured?
● Fowler’s method for anatomical dead space
● Bohr’s method for physiological dead space - calculated fraction of tidal volume
by measurement of mixed expired CO2 and arterial CO2
What will lead to increased physiological dead space?
● V/Q mismatch
What is pulmonary compliance?
● Compliance = volume change per unit of pressure change
● It is a measure of elastic recoil of the lungs and chest wall
● Compliance is maximal in mid inspiration and lower at extremes
● Normally 200ml/cm H20
How does compliance vary throughout the upright lung?
● At the apex the intrapleural pressure is higher to keep lung expanded against its
own weight (10cm H20, compared to 2.5cm H20 at base)
● The apex of the lung is already distended so there is less compliance
● Greater compliance at the base means it has better ability to ventilate
What are the main determinants of compliance?
● Surface tension of the alveoli (2/3rds) - which depends on alveolar pressure,
radius and surfactant (as described by the Law of Laplace where P = 2 x
Tension/Radius)
● Elastin and collagen fibres (1/3rd)
What factors increase or decrease pulmonary compliance?
● Decrease - alveolar oedema, pulmonary fibrosis, pulmonary venous
hypertension, unventilated lung
● Increase - age, emphysema, surfactant
draw a
pressure volume curve of the normal lung.
● Hysteresis (means the compliance of the
lung is different in inspiration and
expiration. Lung volume is higher for any
given pressure during deflation. You need
to show 2 curves)
● Closing volume (does not reach 0% lung
volume, lung contains air at 0 pressure)
● Lung becomes stuffer at higher lung
volumes i.e. compliance decreases at limits
of elasticity
Picture of lung volume curve
What are the physiological effects of surfactant of the lung?
● Increased lung compliance
● Reduced work of breathing
● Improved stability of alveoli
● Keeps alveoli dry via reduced transudation of fluid
What is surfactant and how does it work?
● Surfactant is a phospholipid
● Produced by type 2 pneumocytes in the alveoli
● Fast synthesis with rapid turnover
● Formed relatively late in foetal life
● With surfactant present, the surface tension changes greatly with the surface
area
● Molecules of DPPC (which make up surfactant) are hydrophobic at one end and
and hydrophilic at the other, When aligned on the surface, their repulsive forces
oppose the normal attractive forces between the liquid surface molecules.
Describe the normal distribution of pulmonary blood flow.
● Influenced by gravity
● Decreases from the base to the apex
● Under normal conditions, flow almost ceases at the apex
● Distribution is more uniform with exercise
● 3 main zones (There is also zone 4 which occurs at very low lung volumes)
What are the main determinants of flow in these 3 zones?
● Zone 1 at the apex = alveolar pressure greater than arterial and venous
pressure. Usually does not occur under normal conditions and is essentially
alveolar dead space)
● Zone 2 at the mid lung = arterial pressure greater than alveolar pressure, both
greeted than venous. (represents recruitment)
● Zone 3 at the base = Arterial pressure is the highest - allowing most blood flow
(represents distension and recruitment)
● There is also zone 4 which occurs at very low lung volumes
How is distribution of pulmonary blood flow actively controlled?
● Hypoxic pulmonary vasoconstriction - alveolar hypoxia directly constricts
pulmonary arteries, directs blood away from poorly ventilated diseased lung
areas.
● Important at birth.
● Mechanism - NO, endothelin-1, TXA2, low pH, autonomic system
What extra-pulmonary factors influence pulmonary blood flow?
● Blood volume
● Cardiac output
● Atmospheric pressure
● Temperature
● Pathology e.g anaemia, cancer, infection
● Exercise
● Posture
Please describe the relationship between pulmonary vascular resistance and
pulmonary vascular pressure.
● Respiratory circulation is a low resistance system
● There is capacity for the resistance to decrease with increased pressure
● Mechanism: vascular recruitment with low level pressure increase and vascular
distension with higher level pressure rise.
