Notes on Lung Mechanics

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Lung Volumes-1:

lung volumes TLC VC RV ERV IRV IC FRC TV

Lung Volumes-2:

lung volumes TLC VC RV ERV IRV IC FRC TV

NOTE: VC, IC, VT, and ERV can be measured directly using spirometry.

Tidal Volume:

Amount of air that leaves/enters the lungs during a normal respiratory cycle. 500 ml.

Functional Residual Capacity:

Amount of air at end of passive expiration. 2700 ml.

Inspiratory Capacity:

Amount of air that can be inspired over FRC. 4000 ml.

Inspiratory Reserve Volume:

Amount of air that can be inhaled after normal inspiration. 3500 ml.

Expiratory Reserve Volume:

Additional air that can be expired after normal expiration. 1500 ml.

Residual Volume:

Amount of air inside lung after maximal expiration. 1200 ml.

Vital Capacity:

Amount of air that can be expired after maximal inspiration. 5500 ml.

Total Lung Capacity:

Amount of air in the lung after maximal inspiration. 6700 ml.


• Total ventilation: amount of air moved in/out of lungs per minute.

• Dead space: these regions contain air but don't participate in gas exchange; two types: anatomical and alveolar; physiologic dead space = anatomical + alveolar.

Muscles of respiration:

• Inspiration: diaphragm and muscles of chest wall.

• Expiration = passive (at rest); muscles of abdominal wall used when active.

• Inspiration = active.

Forces acting on lung:

• Lung recoil: develops as the lung expands; collapses the lung.

• Intrapleural pressure: pressure in thin film between lung and chest wall.

• Pressure lower than atmospheric intrapleural pressure (present during normal breathing): expands the lung.

• Pressure greater than atmospheric intrapleural pressure: compresses the lung.

• If intrapleural pressure is greater than lung recoil: lung expands.

• If intrapleural pressure is less than lung recoil: lung collapses.

• If intrapleural pressure = lung recoil: static lung.

Mechanics of breathing:


• Slight negative intrapleural pressure initiates inspiration.

• When intrapleural pressure becomes very negative, the recoil force increases and initiates expiration.

• Alveoli under negative alveolar pressure "want air to enter them," and make pressure 0.

• Alveoli under positive alveolar pressure "want air to out of them," and make pressure 0.

Before inspiration:

• Glottis is open.

• All muscles relaxed.

• This is the neutral or equilibrium point.

• Intrapleural and recoil force are equal and opposite; no air flow out.

During inspiration:

• Intrapleural pressure is negative.

• Airway resistance is also decreasing.

• Alveoli are under negative alveolar pressure.

• The greater the change in intrapleural pressure, the larger the force trying to expand the lung.

• Larger the lung volume, more negative the intrapleural pressure.

End of inspiration:

• When the alveolar pressure is zero again, airflow stops.


• Passive process.


• Intrapleural pressure = atmospheric pressure.

• Recoil pressure = 0 = lung collapses.

• Chest wall expands.

More compliance:

• More air-flow for a given change in pressure.

Less compliance:

• Less air-flow for a given change in pressure.

• Abnormally high compliance: emphysema.

• Abnormally low compliance: fibrosis.

(pressure-volume) Lung inflation curve:

• Steeper the curve, more the lung compliance.

• Compliance increases with age and saline-filled lung.

• Very compliant lungs are easy to inflate.

• In RDS: right-shifted and flatter.

Unstable Alveoli?

Small alveoli are more unstable.


• Lowers surface tension; lowers recoil force.

• Stabilize alveoli.

• Reduced capillary filtration forces.

• Deficiency = RDS; risk factors = premature birth and maternal diabetes.

• RDS = increased lung recoil, decreased lung compliance, atelectasis, pulmonary edema.

Resistant-volume curve:

• During inspiration, intrapleural pressure and airway resistance is decreasing.

• More negative intrapleural pressure = lower airway resistance.

Vital capacity:

• Maximum air a person can move in a single breath.

• FEV1 = maximum air moved in 1 sec = ~ 80 % of VC.

Obstructive disease:

• More time required to exhale a smaller volume of air.

• Lung volume ~ same as normal, maybe increased.

• Eg., emphysema.

Restrictive disease:

• Increased recoil.

• Less time required to exhale a really small volume of air.

• Lung volume less than normal.

• Eg., fibrosis.

Flow-volume loops:

• Used to differentiate between obstructive and restrictive diseases.

Additional Reading:

Basic Pulmonology

1. Lung Mechanics
2. Alveolar-Blood Gas Exchange
3. Gas Transport and Regulation of Respiration
4. Four Causes of Hypoxemia
5. Control of Respiration
6. Systemic vs Pulmonary Circulation FAQ
7. Principles of Gas Exchange in Lungs
8. Hypoxia, Hypoxemia & Hemoglobin-Oxygen Saturation Curve
9. FAQ on Mechanics of Breathing
10. FAQ on Control of Breathing
11. Criteria for Transudate Pleural Effusion
12. Light's Criteria for Exudate Pleural Effusion
13. Notes on Lung Sounds
14. Patient with Acute Respiratory Distress Syndrome (ARDS)
15. Management of Acute Deep Venous Thrombosis
16. Notes on Asthma Treatment

Pulmonology Videos

1. Video of Pulmonology Examination in a Clinical Setting

Related Topics

1. Histology of the Respiratory System
2. Upper and Lower Respiratory Disorders
3. Pulmonary Examination for Internal Medicine

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