Respiratory Volumes and Capacities (Spirometry)

Respiratory volumes are the actual measurable amounts of air moved in and out of lungs during different breathing conditions. By adding these volumes, we get respiratory capacities.

Respiratory Volumes

1. Tidal Volume (TV)

  • Meaning: Air inhaled or exhaled in one normal quiet breath.
  • Value: ~ 500 mL
  • Minute ventilation (extra info): A healthy adult breathes ~12–16 times/min → total air moved per minute can be ~6–8 L/min.

Key point: TV is the “normal breath” volume.

2. Inspiratory Reserve Volume (IRV)

  • Meaning: Extra air you can inhale after a normal inspiration (deep breath in).
  • Value: 2500–3000 mL

Key point: IRV = “how much more you can pull in after a normal breath in”.

3. Expiratory Reserve Volume (ERV)

  • Meaning: Extra air you can exhale after a normal expiration (forceful breath out).
  • Value: 1000–1100 mL

Key point: ERV = “how much more you can push out after a normal breath out”.

4. Residual Volume (RV)

  • Meaning: Air left in lungs even after the strongest forced expiration.
  • Value: 1100–1200 mL
  • RV cannot be measured directly by spirometer.

Why RV matters:

  • Prevents lung collapse
  • Keeps alveoli partly inflated so gas exchange can continue between breaths

Respiratory Capacities

1. Inspiratory Capacity (IC)

  • Meaning: Maximum air you can inhale after a normal expiration.
  • Formula: IC = TV + IRV

Logic: After you breathe out normally, you can inhale a normal breath (TV) + extra (IRV).

2. Expiratory Capacity (EC)

  • Meaning: Maximum air you can exhale after a normal inspiration.
  • Formula: EC = TV + ERV

Logic: After you breathe in normally, you can breathe out a normal breath (TV) + extra (ERV).

3. Functional Residual Capacity (FRC)

  • Meaning: Air remaining in lungs after a normal expiration.
  • Formula: FRC = ERV + RV

Most Confused Concept:

  • Normal expiration leaves FRC in lungs.
  • That leftover is forced extra air still possible (ERV) + always-stuck air (RV).

4. Vital Capacity (VC)

  • Meaning: Maximum air you can expel after a maximum (deep) inspiration OR maximum air you can inhale after a forced expiration.
  • Formula: VC = IRV + TV + ERV
  • Value: 4000–4600 mL

Key point: VC does NOT include RV.

5. Total Lung Capacity (TLC)

  • Meaning: Total air present in lungs at the end of a forced inspiration.
  • Formula: TLC = VC + RV = IRV + TV + ERV + RV
  • Value: 5100–5800 mL

Key point: TLC is the maximum lung fill (includes RV).

Summary Table

Term Meaning Approx Value
TV Normal breath ~500 mL
IRV Extra inhale after normal inhale 2500–3000 mL
ERV Extra exhale after normal exhale 1000–1100 mL
RV Air left after forced exhale 1100–1200 mL
VC IRV + TV + ERV 4000–4600 mL
TLC VC + RV 5100–5800 mL

Must-Remember Rule:

  • Volumes = TV, IRV, ERV, RV
  • Capacities = IC, EC, FRC, VC, TLC

Most asked NCERT one-liner:

  • RV cannot be measured directly by spirometer.

Respiratory Volumes and Capacities

Volumes (4): TV, IRV, ERV, RV

  • TV: normal breath (~500 mL)
  • IRV: extra IN after normal IN (2500–3000)
  • ERV: extra OUT after normal OUT (1000–1100)
  • RV: air left after forced OUT (1100–1200) → not measured by spirometer

Capacities (5):

  • IC = TV + IRV
  • EC = TV + ERV
  • FRC = ERV + RV (air left after normal expiration)
  • VC = IRV + TV + ERV ⭐ (max usable air; no RV)
  • TLC = VC + RV (total air in lungs at max inspiration)

Super-mnemonic:

  • VC = (All except RV)
  • TLC = (Everything = VC + RV)

💡 Questions and Answers

Q1. Define Tidal Volume (TV).

A1: Tidal volume is the amount of air I breathe in or out during a normal, quiet breath. In adults it is around 500 mL.

Key points:

  • Normal breathing
  • ~500 mL

Q2. Which volume cannot be measured directly by a spirometer? Why?

A2: Residual Volume (RV) cannot be measured directly because this air never comes out of the lungs, even after forced exhalation.

Key points:

  • RV not measured by spirometer
  • RV always remains inside lungs

Q3. Calculate Vital Capacity (VC) if TV = 500 mL, IRV = 3000 mL, ERV = 1100 mL.

A3: Vital capacity is the sum of IRV + TV + ERV. VC=3000+500+1100=4600 mLVC = 3000 + 500 + 1100 = 4600\, mL

Key points:

  • VC = IRV + TV + ERV
  • Here, VC = 4600 mL

Q4. Differentiate between Functional Residual Capacity (FRC) and Residual Volume (RV).

A4:

  • RV is the air left after I try to breathe out as much as possible.
  • FRC is the air left after a normal quiet exhalation. So, FRC is bigger because it includes ERV + RV.

Key points:

  • RV: after forced expiration
  • FRC: after normal expiration
  • FRC = ERV + RV

Q5. What is Total Lung Capacity (TLC)?

A5: TLC is the total air my lungs can hold when I take the deepest breath possible. It includes all volumes, so: TLC = VC + RV.

Key points:

  • Max lung fill
  • TLC = VC + RV

Q6. Why is Residual Volume (RV) important?

A6: RV is important because it prevents the lungs from collapsing and keeps some air in alveoli so oxygen exchange can continue even between breaths.

Key points:

  • Prevents lung collapse
  • Helps continuous gas exchange