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NDA Biology · Human Physiology

The Respiratory System

Air travels down a branching airway to the alveoli, where gas exchange happens; the volumes of air moved are named, with tidal volume the smallest.

All Human Physiology notesNDA Biology strategy

Why this matters

5 PYQs. Two ideas carry them: gas exchange happens ONLY in the alveoli (the bronchi just conduct air), and the lung-volume names — especially that tidal volume is the air of a normal quiet breath, and the lowest of the volumes.

Concept 1 of 2: The airway and gas exchange in the alveoli

Air flows in through a branching pipe system — nostrils, pharynx, larynx, trachea, bronchi, then ever-smaller bronchioles — ending in tiny balloons called alveoli. Only the alveoli actually exchange gas with the blood; everything before them is just plumbing. The alveoli are built for the job: ultra-thin walls, elastic fibres, and a dense capillary net.

Definition

The pathway and the exchange site:

  • Air path: nostrils → pharynx → larynx → trachea → bronchi → bronchioles → alveoli.
  • Gas exchange happens ONLY in the alveoli — the bronchi and bronchioles are conducting (air-transport) tubes only.
  • Alveoli are efficient because (i) their epithelium is very thin (short diffusion distance), (ii) elastic fibres let them expand and recoil, and (iii) they are wrapped in many blood capillaries (steep concentration gradient).
Alveolus(thin wall, air)BloodcapillaryO₂CO₂Gas exchange happens ONLY here — not in the bronchi.

Worked example

Three features of an alveolus are given: thin epithelium, elastic fibres in the wall, and a surrounding capillary network. Which of these help gas exchange?
Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

NDA · Apr 2026 · Q91Moderate

Example 1 · Human Physiology · Respiratory System

Which properties of alveoli are helpful in gaseous exchange? I. The epithelial lining of alveoli is very thin. II. Elastic fibres are present in alveolar wall. III. Wall of alveoli is surrounded by many blood capillaries.

Bowman's capsule is NOT a breathing structure

A 'which part does not take part in breathing' question slips in Bowman's capsule — that is part of the nephron in the kidney, not the respiratory system. Bronchi, trachea and diaphragm all do take part.
Drill 2 more on the airway and gas exchange in the alveoli

Concept 2 of 2: Lung volumes and capacities

The lungs move different amounts of air depending on how hard you breathe. A quiet resting breath moves only the tidal volume — the smallest of the named volumes. Forced breaths add the reserve volumes, and some air (residual volume) can never be expelled.

Definition

The named volumes, smallest to largest:

  • Tidal volume (TV) — air in a normal quiet breath (~500 mL). The lowest of the volumes.
  • Expiratory reserve volume (ERV) — extra air forced out after a normal breath (~1100 mL).
  • Residual volume (RV) — air that always remains, cannot be exhaled (~1200 mL).
  • Inspiratory reserve volume (IRV) — extra air forced in (~3000 mL).
  • Vital capacity = TV + IRV + ERV (the maximum you can move).
Lung volumes (approx.)IRV3000 mLTidal volume500 mLERV1100 mLResidual1200 mLTidal volume = a normal quiet breath, the smallest band.
VolumeMeaningApprox.
Tidal volumeNormal quiet breath~500 mL (lowest)
NDA 2025/2026 — tidal volume is the air of a normal breath AND the smallest named volume.
Expiratory reserveExtra forced out~1100 mL
Residual volumeAlways remains~1200 mL
Inspiratory reserveExtra forced in~3000 mL
Vital capacityTV + IRV + ERV~4600 mL
Practice this concept3 quick reps

The same idea in a real exam question:

NDA · Sep 2025 · Q134Easy

Example 2 · Human Physiology · Respiratory System

The amount of air breathed in and out during a normal respiration by human lungs is referred to as

Tidal volume is the smallest — not residual

Among tidal, residual, IRV and ERV, tidal volume (~500 mL) is the lowest. Residual volume (~1200 mL) is larger and is the air you can never breathe out, so don't confuse 'cannot be exhaled' with 'smallest'.
Drill 1 more on lung volumes and capacities

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (1)

Lung volumes and capacities5 rows
VolumeMeaningApprox.
Tidal volumeNormal quiet breath~500 mL (lowest)
NDA 2025/2026 — tidal volume is the air of a normal breath AND the smallest named volume.
Expiratory reserveExtra forced out~1100 mL
Residual volumeAlways remains~1200 mL
Inspiratory reserveExtra forced in~3000 mL
Vital capacityTV + IRV + ERV~4600 mL

Watch out for (2)

Test yourself on Human Physiology

15 past NDA questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

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