This document discusses key concepts in respiratory physiology, including: 1) Compliance refers to the ease with which the lungs can be stretched and is measured by the change in lung volume from a given pressure change. Total compliance includes the lungs and chest wall while pulmonary compliance considers just the lungs. 2) Work of breathing includes overcoming elastic, viscous, and airway resistance. Elastic resistance is from lung and chest wall recoil while viscous is from non-elastic tissues. 3) The alveolar ventilation-perfusion (VA/Q) ratio is the ratio of ventilation to blood flow at the alveoli and ideally is near 1. Deviations can occur from issues with ventilation or perfusion

1. Respiratory Physiology
Dr Kiran Kumar C
MBBS.,MD.,FAGE.,C DIAB
Assistant Professor

2. Compliance
• The term compliance refers to how much effort is required
to stretch or distend the lungs.
• Is analogous to how easy or hard it is to blow up a balloon.

3. • Compliance is a measure of how much change in lung
volume results from a given change in the transmural
pressure gradient, the force that stretches the lungs

4. • Compliance is expressed under two headings:
(a) Total respiratory compliance or combined compliance of lungs
and chest wall, i.e. lungs inside the thoracic cavity. Normal value
of total respiratory compliance is 0.13 L/cm H2O.
(b)Pulmonary compliance, i.e. of lungs only (lungs outside the
chest wall). Normal value of compliance for the lungs alone is
0.22 L/cm H2O.

5. Measurement of total
compliance

8. Compliance of lung

11. • It is important to remember that compliance is the inverse of
elasticity, or elastic recoil.
• Compliance denotes the ease with which something can be
stretched or distorted;
• Elasticity refers to the tendency for something to oppose
stretch or distortion, as well as to its ability to return to its
original configuration after the distorting force is removed.

14. STATIC VERSUS SPECIFIC LUNG COMPLIANCE
• Static compliance. The compliance measured as described above is
the static compliance.
• The static compliance of any system is dependent on its size.
• Thus the lung compliance depends upon the amount of functional
lung tissue.
• Thus, static compliance is not a good measure of absolute
distensibility.

15. • Specific compliance is the compliance of the lung at relaxation
volume (the point at the end of a tidal expiration), i.e. the functional
residual capacity.
• The specific compliance is expressed per litre of FRC.
• It is a measure of the absolute distensibility of a structure.

17. • FACTORS AFFECTING LUNG COMPLIANCE
• Lung compliance is inversely proportional to the lung Elastance
(elastic recoil force).
• Therefore, lung compliance is determined on the basis of the
• Elastic forces of the lung tissues
• Elastic forces caused by surface tension
• Alveolar surface tension

19. Work of Breathing

20. • RESISTANCE TO BREATHING
• Tissue resistance
• It is the resistance offered by the tissues as they expand or contract. Tissue
resistance comprises:
1. Elastic resistance. It is the sum of forces of elastic recoil exerted by the lung
and the chest wall.
• The elastic recoil of the lungs is due to the presence of elastic fibres in the
lungs and due to the alveolar surface tension.
2. Viscous resistance is the resistance offered by the nonelastic tissues in the
lungs.

21. • Airway resistance
• It is the resistance caused by the friction of gas molecules between
themselves and the walls of the airways.
• Factors affecting airway resistance are:
1. Rate of gas flow.
2. Airway radius.
3. Length of airway.
4. Type of air flow.

22. • COMPONENTS OF WORK OF BREATHING
• The total work done by the respiratory muscles during quiet
breathing may be divided into following components:
• Work done to overcome elastic resistance (65%),
• Work done to overcome viscous resistance (7%) and
• Work done to overcome airway resistance (28%).

23. Airway resistance work
Tissue resistance work
Compliance work

24. Applied

25. ALVEOLAR VENTILATION–PERFUSION RATIO
• Alveolar ventilation–perfusion ratio (VA/Q) is the ratio of
alveolar ventilation per minute to quantity of blood flow to
alveoli per minute
• The normal VA/Q is about 0.84−0.9.
• At this ratio maximum oxygenation occurs

28. • Causes of alteration in VA/Q ratio
• Obviously, the factors altering the alveolar ventilation or/ and pulmonary
perfusion will alter the VA/Q ratio.
• Causes of uneven alveolar ventilation include:
• Bronchial asthma,
• Emphysema,
• Pulmonary fibrosis,
• Pneumothorax and
• Congestive heart failure.

29. • Causes of uneven pulmonary perfusion are:
• Anatomical shunts, e.g. Fallot’s tetralogy,
• Pulmonary embolism,