Class 11 — Breathing and Exchange of Gases

Chapter Overview & Weightage

Breathing and Exchange of Gases is a moderate-weightage chapter in Class 11 Biology and a high-weightage chapter for NEET (Human Physiology unit). In CBSE board papers, it contributes 5–7 marks each year, mostly through diagram-based questions, mechanism explanations, and respiratory volumes.

CBSE Class 11 Weightage (Year-by-Year)

Year	Marks	Topics asked
2024	6	Mechanism of breathing, oxygen-haemoglobin curve
2023	5	Pulmonary volumes, transport of $CO_2$
2022	7	Diagram of human respiratory system, gas exchange
2021	6	Respiratory disorders, regulation of respiration

For NEET, this chapter alone gives 4–5 questions every year — high-yield study.

Key Concepts You Must Know

Human respiratory system anatomy: Nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, alveoli. Diagram is asked almost every year.

Mechanism of breathing: Diaphragm and intercostal muscles; inspiration vs expiration; pressure changes in thoracic cavity.

Pulmonary volumes and capacities: Tidal volume (TV), Inspiratory Reserve Volume (IRV), Expiratory Reserve Volume (ERV), Residual Volume (RV), Vital Capacity (VC = TV + IRV + ERV), Total Lung Capacity (TLC).

Exchange of gases: Partial pressures, diffusion across alveolar membrane.

Transport of $O_2$ and $CO_2$ : Haemoglobin’s role, oxygen dissociation curve, Bohr effect, chloride shift.

Respiratory disorders: Asthma, emphysema, occupational respiratory disorders.

Important Concepts and Values

Tidal Volume (TV): 500 mL
Inspiratory Reserve Volume (IRV): 2500–3000 mL
Expiratory Reserve Volume (ERV): 1000–1100 mL
Residual Volume (RV): 1100–1200 mL
Vital Capacity (VC) = TV + IRV + ERV ≈ 4500 mL
Total Lung Capacity (TLC) = VC + RV ≈ 6000 mL

Site	$pO_2$	$pCO_2$
Atmospheric	159	0.3
Alveolar	104	40
Arterial blood	95	40
Tissues	40	45
Venous blood	40	45

The gradient drives diffusion: $O_2$ moves where $pO_2$ is lower; $CO_2$ moves where $pCO_2$ is lower.

Solved Previous Year Questions

PYQ 1 — CBSE 2024, 5 Marks

Describe the mechanism of breathing in humans. How does the air pressure change during inspiration and expiration?

During inspiration: diaphragm contracts and flattens, external intercostal muscles contract pulling ribs up and out. Thoracic cavity volume increases; intrapulmonary pressure drops below atmospheric (~−2 mmHg). Air flows in.

During expiration: diaphragm and intercostal muscles relax. Thoracic cavity volume decreases; intrapulmonary pressure rises above atmospheric (+2 mmHg). Air flows out. Normal expiration is passive (elastic recoil of lungs).

PYQ 2 — CBSE 2023, 3 Marks

Explain how $CO_2$ is transported in blood.

Three mechanisms:

Dissolved in plasma (~7%)
As bicarbonate ions ( $HCO_3^-$ ) (~70%) — formed in RBCs via carbonic anhydrase: $CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$
As carbamino-haemoglobin (~23%) — $CO_2$ binds to amino groups of Hb

The chloride shift moves $HCO_3^-$ out of RBCs in exchange for $Cl^-$ , maintaining electrical neutrality.

PYQ 3 — CBSE 2022, 5 Marks

Draw a labelled diagram of the human respiratory system and label any five parts.

Diagram should include: nostrils, nasal cavity, pharynx, larynx, trachea, primary bronchi, secondary/tertiary bronchi, bronchioles, alveoli, lungs, diaphragm, pleural membranes.

Difficulty Distribution

Difficulty	% of Marks	Sub-topics
Easy	30%	Anatomy diagrams, definitions, volumes
Medium	50%	Breathing mechanism, gas exchange, transport
Hard	20%	Oxygen dissociation curve, Bohr effect, regulation by medulla

Expert Strategy

Week 1 — Anatomy and volumes. Practise drawing the respiratory system diagram twice a day. Memorise typical volume values.

Week 2 — Mechanism and gas exchange. Master the inspiration-expiration pressure changes. Memorise partial pressure values at all four sites.

Week 3 — Transport and disorders. Three modes of $CO_2$ transport, oxygen dissociation curve (sigmoid), and the four major disorders (asthma, emphysema, occupational, fibrosis).

Diagram tip: CBSE awards 1 mark just for the diagram structure and 1 mark for correct labels in 5-mark questions. Spend 5 minutes practicing the diagram daily — it’s the easiest 2 marks.

Common Traps

Trap 1: Confusing TV and VC.

Tidal volume (TV) = volume in one normal breath ≈ 500 mL. Vital capacity (VC) = max volume one can exhale after deep inspiration ≈ 4500 mL. Different concepts.

Trap 2: Forgetting that expiration is passive.

Normal expiration uses no active muscle contraction — it’s pure elastic recoil. Forced expiration uses internal intercostals + abdominal muscles. Boards ask this distinction.

Trap 3: Wrong partial pressure values.

Memorise: alveolar $pO_2 = 104$ , $pCO_2 = 40$ . Tissues: $pO_2 = 40$ , $pCO_2 = 45$ . The gradient direction (from high to low) drives diffusion.

Trap 4: Overstating the role of plasma in $CO_2$ transport.

Only 7% of $CO_2$ is dissolved in plasma. The bulk (70%) travels as bicarbonate ions in plasma after being processed in RBCs. Don’t confuse “in plasma” with “as plasma-only mechanism”.

Trap 5: Sigmoid vs hyperbolic oxygen dissociation curve.

Haemoglobin gives a sigmoid (S-shaped) curve due to cooperative binding. Myoglobin gives a hyperbolic curve. Never confuse the two.