Breathing and Exchange of Gases

Syllabus Unit: Human Physiology

Why it matters

Chapter overview

Breathing moves air in and out of the lungs; respiration exchanges O2 and CO2 between air, blood and tissues, and transports both gases dissolved and bound to haemoglobin. NEET tests the respiratory tract's anatomy, the pressure-driven mechanics of inspiration and expiration, the diffusion gradients and partial pressures that drive gas exchange, and the disorders that disrupt each stage.

Biology Syllabus Human Physiology 20%–22% weightage

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1Nasal ChamberFilters, warms air

2Larynx & TracheaConducts air, voice box

3BronchiolesProgressive branching

4AlveoliGas exchange occurs

1Respiratory System & Organs

Air is warmed, filtered and conducted through a branching tree ending in millions of alveoli — the actual site of gas exchange.

Learning Objectives

Sequence the respiratory pathway from nostrils to alveoli.
Explain the function of the epiglottis and the role of the larynx (voice box).
Describe the alveolar structure that makes it suited for gas exchange.
Distinguish the conducting zone (no gas exchange) from the respiratory zone (gas exchange).

Key Terms

AlveoliMicroscopic, thin-walled air sacs at the end of the bronchial tree; the actual site of gas exchange, richly surrounded by capillaries.

EpiglottisA cartilaginous flap that covers the glottis (windpipe opening) during swallowing, preventing food from entering the trachea.

PleuraDouble membrane (visceral + parietal) enclosing each lung, with pleural fluid reducing friction during breathing movements.

TracheaThe windpipe; a cartilage-ringed tube carrying air from the larynx to the bronchi, held open by C-shaped rings.

The respiratory system is a branching tree: a **conducting zone** that moves and conditions air (no gas exchange) leading into a **respiratory zone** where gas exchange actually happens.

The conducting pathway

Nostrils → nasal chamber — hairs filter dust; mucosa warms and moistens air.
Pharynx — common passage for air and food; leads to the larynx.
Larynx (voice box) — contains vocal cords; guarded above by the epiglottis, which folds over the glottis during swallowing to prevent choking.
Trachea — a ~12 cm tube held open by C-shaped cartilage rings so it never collapses.
Bronchi → bronchioles — the trachea splits into two bronchi (one per lung), which branch repeatedly into progressively narrower bronchioles.

The respiratory zone: alveoli

Each terminal bronchiole ends in clusters of **alveoli** — an adult has roughly 300 million of them, giving a combined surface area of ~70-80 m² for gas exchange. Alveolar walls are a single layer of squamous epithelium, wrapped in a dense capillary network, minimising the diffusion distance between air and blood.

Key PointNEET frequently tests why alveoli are efficient exchangers: extremely thin (one-cell) walls, huge combined surface area, and rich capillary supply — memorise these three reasons together.

The lungs and pleura

Each lung is enclosed in a double-layered **pleural membrane** (visceral pleura on the lung surface, parietal pleura lining the thoracic cavity) with a thin film of pleural fluid between them, reducing friction and allowing the lungs to expand smoothly against the chest wall.