Breathing Mechanism Volumes

The trap that costs marks on this topic: confusing which volumes are included in a capacity and which are excluded. Specifically, students write VC = TV + IRV and forget ERV — or worse, include RV in vital capacity.

The mechanism of breathing involves pressure changes driven by the diaphragm and external intercostal muscles. During inspiration, the diaphragm contracts (flattens), thoracic volume increases, intra-pulmonary pressure drops below atmospheric pressure, and air rushes in. Expiration at rest is largely passive — the diaphragm relaxes, elastic recoil of lungs raises intra-pulmonary pressure above atmospheric, and air is expelled (NCERT Class 11 Biology Chapter 17, page 320).

Respiratory volumes (measured by spirometry):

• Tidal Volume (TV): ~500 mL — air in a normal breath.
• Inspiratory Reserve Volume (IRV): ~3000 mL — extra air inhaled after normal inspiration.
• Expiratory Reserve Volume (ERV): ~1100 mL — extra air exhaled after normal expiration.
• Residual Volume (RV): ~1200 mL — air remaining after maximum expiration (cannot be measured by simple spirometry).

Capacities are sums of volumes:

• Vital Capacity (VC) = TV + IRV + ERV ≈ 4600 mL. Excludes RV.
• Total Lung Capacity (TLC) = VC + RV ≈ 5800 mL.
• Functional Residual Capacity (FRC) = ERV + RV ≈ 2300 mL.
• Inspiratory Capacity (IC) = TV + IRV ≈ 3500 mL.

Watch-out: VC is the maximum volume you can move — it never includes residual volume. If a question asks "which capacity includes RV?" — only TLC and FRC qualify.

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