Resp Physiology Flashcards
Surfactant
Phosopholipid-rich detergant
Produced by type II alveolar cells
Coats luminal surface of alveoli
Reduces surface tension of water which is opposing expansion
Water molecules are attracted more to each other than they are to gas molecules.
When any liquid surrounds a gas, i.e. in the alveolus, this produces an inward pressure.
Functions of surfactant
Lowers surface tension
- -> increases compliance of lungs
- -> reduces work of breathing
Prevents fluid accumulating in the alveoli
Reduces the tendency of alveoli to collapse (alveolar instability).
Conditions that decrease lung compliance
Fibrosis
Pulmonary oedema
Deficiency of surfactant e.g. premature babies
Decreased lung expansion e.g. respiratory muscle paralysis
Supine position
Breathing 100 O2
Mechanical ventilation due to reduced pulmonary blood flow
Age
Conditions that increase compliance
COPD
due to destruction of elastic parenchyma
Components of work of breathing
Two major compenents:
Work required to over-come elastic recoil of lungs
Non-elastic forces:
- Air resistance (most significant)
- Frictional forces
- Inertia of air and tissues
One-third of airway resistance occurs in the upper
airways – nose, pharynx and larynx. This can be
greatly reduced by breathing through the mouth
Ventilation of upper vs lower zones
Lower zones better ventilated because:
-the weight of the lungs
-the compliance curve is sigmoid, and the upper
and lower parts of the lung lie on different parts
of this curve.
Definition of tidal volume
Volume of air inspired and expired during quiet breathing
Definition of functional residual capacity
Functional residual capacity is the volume of air in the the lungs at the end of passive expiration
FRC = Residual volume + expiratory reserve volume
At FRC, the opposing elastic recoil forces of the lungs and chest wall are in equilibrium and there is no exertion by the diaphragm or other respiratory muscles.
Measuring functional residual capacity
As it consists partly of residual volume, it cannot be measured by spirometry
Nitrogen wash-out test, helium dilution or body plethysmography
Definition of vital capacity
Volume of air that is expelled from maximal inspiration to maximal expiration
Fowler’s method
Measures anatomical deadspace using single breath of 100% O2 and a nitrogen analyser
As subject starts to expire, the nitrogen content of
alveolar air is measured.
Increasing anatomical deadspace
Increasing size of person
Standing position
Increased lung volume
Bronchodilatation
Increasing physiological deadspace
Hypotension
Hypoventilation
Emphysema
PE
Positive pressure ventilation
Nitrogen expiration curve
Phase 1
-Pure O2 = dead space
Phase 2
- Mixture of dead space and alveolar gas
- Increasing nitrogen / volume
- Mid-point here represents calculation of anatomical deadspace
Phase 3
-Plateu of nitrogen as pure alveolar air is expired
Phase 4
- Abrupt increase in nitrogen concentration as airways at the base of the lung close
- Expired air at this point is from the apex, which has received less O2, and thus the nitrogen is less dilute.
Factors affecting the closing capacity
Closing capacity is volume at which airways begin to close
–> phase 4 of single breath nitrogen test
Factors:
Age: increases with age
Posture: in a supine position in a 40-year-old
subject, the closing capacity is equal to the FRC
Anaesthesia: decrease in lung volumes results in closing capacity exceeding FRC, even in the youngest patients.
