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Class 11 Biology Chapter 14 Breathing & Exchange of Gases — Formulas & Key Points

Chapter 14 of NCERT Class 11 Biology covers the physiology of breathing and the biochemical exchange of gases in the human respiratory system. While largely qualitative, the chapter includes critical numerical concepts—lung volumes, partial pressures, and transport equations—that appear regularly in CBSE board exams and competitive tests like NEET. This formula sheet organises every quantitative relationship, law, and definition into exam-ready tables and examples.

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Key takeaways

✓Tidal Volume (TV) is 500 mL per breath; Vital Capacity (VC) is approximately 3.5-4.5 L in adults.
✓Henry's Law governs gas solubility in blood: partial pressure of a gas in liquid equals its partial pressure in gas phase times solubility coefficient.
✓Boyle's Law explains inspiration and expiration: at constant temperature, pressure × volume remains constant during thoracic cavity expansion or contraction.
✓Oxygen binds to haemoglobin forming oxyhaemoglobin; the binding is influenced by PO₂, PCO₂, pH, and temperature (Bohr effect).
✓70% of CO₂ is transported as bicarbonate (HCO₃⁻) in plasma; 23% as carbamino-haemoglobin; 7% dissolved in plasma.
✓The chloride shift (Hamburger phenomenon) maintains ionic balance when HCO₃⁻ moves out of RBCs and Cl⁻ moves in.
✓Partial pressure of oxygen in alveoli (PAO₂) is ~104 mm Hg; in deoxygenated blood it is ~40 mm Hg, creating the diffusion gradient.

Respiratory Volumes and Capacities — Essential Formulas

The chapter defines six respiratory volumes and four capacities. These are measured using a spirometer and form the basis for numerical problems. Tidal Volume (TV) is the air inhaled or exhaled in one normal breath—approximately 500 mL in a healthy adult male. Inspiratory Reserve Volume (IRV) is the extra volume that can be inhaled forcefully after normal inspiration, around 2500-3000 mL. Expiratory Reserve Volume (ERV) is the extra volume that can be exhaled forcefully after normal expiration, approximately 1000-1100 mL. Residual Volume (RV) is the volume that remains in the lungs after maximum expiration, around 1100-1200 mL; it cannot be measured by spirometry alone. Vital Capacity (VC) is the maximum volume of air a person can exhale after maximum inspiration. Total Lung Capacity (TLC) is the sum of all compartments. Understanding these relationships is critical for solving any spirometry-based question in the CBSE Class 11 Biology exam.

Boyle's Law and the Mechanism of Breathing

Boyle's Law is central to understanding inspiration and expiration. It states that at constant temperature, the pressure of a gas is inversely proportional to its volume. Mathematically, P₁V₁ = P₂V₂. During inspiration, the diaphragm contracts and the thoracic cavity expands. This increases volume (V↑) and decreases intra-pulmonary pressure (P↓) below atmospheric pressure (~760 mm Hg at sea level). Air rushes in from the higher-pressure atmosphere into the lower-pressure lungs. During expiration, the diaphragm relaxes, thoracic volume decreases (V↓), intra-pulmonary pressure increases (P↑) above atmospheric, and air flows out. The numerical relationship is straightforward but must be remembered for qualitative MCQs and assertion-reason questions in CBSE exams. Always state 'at constant temperature' when writing Boyle's Law in descriptive answers to earn full marks.

Boyle's Law: P ∝ 1/V → P₁V₁ = P₂V₂ (constant temperature)
Inspiration: Thoracic volume ↑ → Pressure ↓ → Air flows in
Expiration: Thoracic volume ↓ → Pressure ↑ → Air flows out

Partial Pressures of Gases — Dalton's Law Application

Dalton's Law of Partial Pressures states that the total pressure exerted by a mixture of non-reacting gases equals the sum of the partial pressures of individual gases. Partial pressure (P_gas) = (Percentage of gas / 100) × Total pressure. In atmospheric air at sea level (760 mm Hg), oxygen makes up ~21%, so PO₂ = 0.21 × 760 ≈ 159 mm Hg. In alveolar air, water vapour dilutes the composition; alveolar PO₂ is ~104 mm Hg and alveolar PCO₂ is ~40 mm Hg. In deoxygenated (venous) blood arriving at the lungs, PO₂ ≈ 40 mm Hg and PCO₂ ≈ 45 mm Hg. Oxygenated (arterial) blood leaving the lungs has PO₂ ≈ 95 mm Hg and PCO₂ ≈ 40 mm Hg. Tissue capillaries see PO₂ ≈ 40 mm Hg and PCO₂ ≈ 45 mm Hg. These gradients drive diffusion. Memorise these six numbers for exam MCQs: atmospheric PO₂ ~159, alveolar PO₂ ~104, arterial PO₂ ~95, venous PO₂ ~40, alveolar PCO₂ ~40, venous PCO₂ ~45.

Henry's Law and Gas Solubility in Blood

Henry's Law explains how gases dissolve in liquids at the respiratory membrane. It states that the amount of gas dissolved in a liquid is directly proportional to the partial pressure of that gas in contact with the liquid, at constant temperature. Formula: C = k × P, where C is concentration (solubility), k is the solubility coefficient (unique for each gas), and P is partial pressure. Oxygen has a lower solubility coefficient than carbon dioxide, meaning CO₂ dissolves about 20 times more readily in blood plasma than O₂ at the same partial pressure. This is why CO₂ can be transported significantly in dissolved form, while O₂ relies heavily on haemoglobin binding. For CBSE exams, know that Henry's Law justifies why higher alveolar PO₂ increases oxygen uptake, and why divers get 'the bends' when nitrogen comes out of solution rapidly during ascent. Always write the law statement, formula, and one physiological application for full marks in three-mark questions.

Henry's Law: C = k × P (gas solubility proportional to partial pressure)
CO₂ is ~20× more soluble in plasma than O₂
Higher PO₂ in alveoli → more O₂ dissolves and binds to haemoglobin

Oxygen Transport — Haemoglobin Binding and Oxyhaemoglobin Formation

Approximately 97% of oxygen is transported bound to haemoglobin inside red blood cells as oxyhaemoglobin (HbO₈), while only 3% is dissolved in plasma. One haemoglobin molecule (4 subunits) can bind four O₂ molecules. The binding is cooperative: once one O₂ binds, the next binds more easily. The oxygen-haemoglobin dissociation curve is sigmoid (S-shaped). The curve shifts right (reduced affinity) under conditions of high PCO₂, low pH, high temperature, and high 2,3-BPG—collectively called the Bohr effect. The curve shifts left (increased affinity) in opposite conditions, such as in foetal haemoglobin. Although the exact curve is not a formula, exam questions ask about the factors shifting it. Remember: right shift means oxygen is released more easily to tissues; left shift means oxygen binds more tightly. For CBSE Class 11 Biology Chapter 14, focus on qualitative understanding and the four factors. Numerical integration is rare but may appear in NEET-level problems, requiring log transformations of the Hill equation, which is beyond the NCERT scope for Class 11.

97% of O₂ carried as HbO₈; 3% dissolved in plasma
Hb + 4O₂ ⇌ Hb(O₂)₄ (cooperative binding)
Bohr effect: ↑PCO₂, ↓pH, ↑temp, ↑2,3-BPG → right shift (↓ affinity)
Foetal Hb has higher O₂ affinity than adult Hb (left shift)

Carbon Dioxide Transport — Bicarbonate Formation and Chloride Shift

Carbon dioxide is transported in blood via three routes. About 7% dissolves directly in plasma. Roughly 23% binds to the amino groups of haemoglobin forming carbamino-haemoglobin (HbCO₂). The majority—70%—is converted to bicarbonate ions (HCO₃⁻) in red blood cells. The reaction is CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻, catalysed by the enzyme carbonic anhydrase. Bicarbonate ions diffuse out of RBCs into plasma, and to maintain electrical neutrality, chloride ions (Cl⁻) move into the RBCs—this is the chloride shift or Hamburger phenomenon. In the lungs, the reactions reverse: HCO₃⁻ re-enters RBCs, combines with H⁺ to form H₂CO₃, which dissociates into CO₂ and H₂O, and CO₂ is exhaled. Write the carbonic anhydrase equation and mention 'chloride shift' explicitly in exam answers for full credit. These are standard 2-3 mark CBSE board questions.

7% dissolved in plasma
23% as carbamino-haemoglobin (Hb–CO₂)
70% as HCO₃⁻ in plasma (catalysed by carbonic anhydrase in RBCs)
Chloride shift: HCO₃⁻ out, Cl⁻ in to maintain electroneutrality

Key Terms and Definitions from NCERT Chapter 14

Respiration refers to the entire process of gaseous exchange—external (breathing) and internal (cellular). Breathing (ventilation) is the mechanical process of inhaling and exhaling air. Inspiration is active (requires muscle contraction); expiration is passive at rest but can be active during exercise. The respiratory membrane is the barrier across which gases diffuse, comprising alveolar epithelium, basement membrane, and capillary endothelium, with a total thickness of ~0.5 μm. Diffusing capacity is the volume of gas diffusing per minute per mm Hg pressure difference; healthy lungs have high diffusing capacity. Hypoxia is deficiency of oxygen reaching tissues; hypoxemia is low blood oxygen. Asphyxia is the condition where gas exchange is severely impaired. Pneumothorax is air in the pleural cavity causing lung collapse. These definitions frequently appear as one-mark fill-in-the-blanks or match-the-following items in CBSE Class 11 Biology exams.

Respiration: Overall process of O₂ uptake and CO₂ release
Breathing (Ventilation): Mechanical inhalation and exhalation
Respiratory membrane: Alveolar-capillary barrier (~0.5 μm thick)
Hypoxia: Tissue oxygen deficiency; Hypoxemia: Low blood O₂
Pneumothorax: Air in pleural cavity causing lung collapse

Important Constants, Units and Notations — Avoid Common Errors

Always use mm Hg or torr for partial pressures (1 mm Hg = 1 torr ≈ 0.133 kPa). Atmospheric pressure at sea level is 760 mm Hg, not 1 atm in numerical answers unless specified. Volumes are in millilitres (mL) or litres (L); never write 'cc' in board exams—'mL' is the SI-accepted term. Temperature is assumed 37°C (body temperature) unless stated. The symbol for partial pressure is P with a subscript for the gas: PO₂, PCO₂, PN₂. Do not confuse PO₂ (partial pressure of oxygen) with %O₂ (percentage composition). Carbonic anhydrase is abbreviated CA or sometimes written in full; never abbreviate as 'carb. anh.' in answers. Haemoglobin is Hb; oxyhaemoglobin is HbO₈ or Hb(O₂)₄; carbamino-haemoglobin is HbCO₂. Red blood cells are RBCs or erythrocytes—both acceptable. Write chemical equations with double arrows (⇌) to indicate reversibility. These small notational disciplines earn precision marks in CBSE board exams and build good habits for NEET preparation.

Use mm Hg (or torr) for partial pressures; 1 atm = 760 mm Hg
Volumes in mL or L, never 'cc'
Partial pressure notation: PO₂, PCO₂ (capital P, subscript gas)
Temperature default: 37°C (body temp)
Reversible reactions: use ⇌ not →

Memory Aids and Mnemonics for Quick Recall

To remember respiratory volumes in order of size, use 'TV is a Small Remote For Total Living'. TV (Tidal Volume ~500 mL), IRV (Inspiratory Reserve ~2500-3000 mL), ERV (Expiratory Reserve ~1000-1100 mL), RV (Residual Volume ~1100-1200 mL), VC (Vital Capacity ~3500-4500 mL), TLC (Total Lung Capacity ~5000-6000 mL). For the three modes of CO₂ transport, remember 'D-C-B': Dissolved (7%), Carbamino-haemoglobin (23%), Bicarbonate (70%). To recall factors causing a right shift in the oxygen-haemoglobin dissociation curve (decreased affinity), use 'CADET, face Right!': CO₂ ↑, Acid (H⁺ ↑), 2,3-DPG ↑, Exercise (temp ↑), Temperature ↑. For the chloride shift, picture 'Bicarbonate Exits, Chloride Comes In' to maintain electrical balance in RBCs. For Boyle's Law, 'Press Volume Down, Pressure Ups' (inverse relationship). These tricks save precious seconds during board exams and reduce silly mistakes under time pressure.

Respiratory volumes: 'TV is a Small Remote For Total Living'
CO₂ transport: 'D-C-B' (Dissolved 7%, Carbamino 23%, Bicarbonate 70%)
Right shift (Bohr effect): 'CADET, face Right!' (CO₂↑, Acid↑, DPG↑, Exercise, Temp↑)
Chloride shift: 'Bicarbonate Exits, Chloride Comes In'

Solved Mini-Examples Applying the Formulas

Example 1: Calculate the Vital Capacity if TV = 500 mL, IRV = 3000 mL, ERV = 1100 mL. Solution: VC = TV + IRV + ERV = 500 + 3000 + 1100 = 4600 mL or 4.6 L. Example 2: Atmospheric pressure is 760 mm Hg. Oxygen is 21% of air. Calculate PO₂. Solution: PO₂ = (21/100) × 760 = 159.6 mm Hg ≈ 160 mm Hg. Example 3: During inspiration, thoracic volume increases from 2.5 L to 3.0 L. Initial intra-pulmonary pressure is 760 mm Hg. Find final pressure (use Boyle's Law). Solution: P₁V₁ = P₂V₂ → 760 × 2.5 = P₂ × 3.0 → P₂ = (760 × 2.5)/3.0 = 633.3 mm Hg. This drop below atmospheric (760 mm Hg) causes air to rush in. These examples cover the three main quantitative concepts in the chapter and mirror the style of CBSE board numerical questions. Practice them to gain confidence applying formulas under exam conditions.

One-Glance Last-Minute Revision Box

This single-page summary box is designed for final revision the night before the exam. Pin it to your study wall or photograph it on your phone. Respiratory volumes (mL): TV ~500, IRV ~2500-3000, ERV ~1000-1100, RV ~1100-1200. Capacities: VC = TV+IRV+ERV ~3500-4500 mL; TLC = VC+RV ~5000-6000 mL. Boyle's Law: P₁V₁=P₂V₂ (inverse pressure-volume at constant temp). Partial pressures (mm Hg): Atmospheric PO₂ ~159; Alveolar PO₂ ~104, PCO₂ ~40; Arterial PO₂ ~95, PCO₂ ~40; Venous PO₂ ~40, PCO₂ ~45. Henry's Law: C = k×P (solubility ∝ partial pressure). O₂ transport: 97% HbO₈, 3% dissolved. CO₂ transport: 70% HCO₃⁻, 23% HbCO₂, 7% dissolved. Carbonic anhydrase reaction: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. Chloride shift: HCO₃⁻ out, Cl⁻ in. Bohr effect (right shift): ↑CO₂, ↓pH, ↑temp, ↑2,3-BPG → ↓O₂ affinity. Revise this box for 5 minutes before entering the exam hall to ensure formulas are fresh in working memory.

TV ~500 mL | IRV ~2500-3000 | ERV ~1000-1100 | RV ~1100-1200
VC = TV+IRV+ERV | TLC = VC+RV
Boyle: P₁V₁=P₂V₂ | Henry: C=k×P
Alveolar: PO₂~104, PCO₂~40 | Venous: PO₂~40, PCO₂~45
O₂: 97% HbO₈, 3% dissolved | CO₂: 70% HCO₃⁻, 23% HbCO₂, 7% dissolved
Chloride shift: HCO₃⁻↓ Cl⁻↑ | Bohr effect: right shift = ↓affinity

How CBSETUTOR.ai Helps Master Chapter 14

Breathing and Exchange of Gases involves both conceptual physiology and numerical problem-solving. Many Class 11 students struggle to link Boyle's Law with the mechanical act of breathing or to remember the exact partial-pressure values under exam pressure. CBSETUTOR.ai offers a 24×7 AI tutor at a flat ₹999 per month for any class from 6 to 12, making personalised help affordable for every family. Upload a photo of any spirometry question or a tricky assertion-reason item on the Bohr effect, and receive step-by-step solutions instantly. The platform covers all NCERT Class 11 Biology chapters with solved examples, formula sheets, and topic-wise tests. Students in Kota, Delhi, or smaller towns get the same quality of on-demand doubt-clearing without expensive hourly tutors. A 3-day free trial lets your child explore interactive explanations of the oxygen-haemoglobin dissociation curve and carbonic anhydrase reactions before committing. For consistent revision and confidence-building in Chapter 14, CBSETUTOR.ai is the smart, budget-friendly companion.

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Frequently asked questions

What is the difference between Tidal Volume and Vital Capacity?+

Tidal Volume (TV) is the volume of air inhaled or exhaled in one normal, relaxed breath—approximately 500 mL. Vital Capacity (VC) is the maximum volume of air a person can exhale after a maximum inspiration, calculated as TV + IRV + ERV, typically 3500-4500 mL in healthy adults.

How does Boyle's Law explain inspiration and expiration?+

Boyle's Law states P₁V₁ = P₂V₂ at constant temperature. During inspiration, the diaphragm contracts, increasing thoracic volume (V↑), which lowers intra-pulmonary pressure (P↓) below atmospheric, drawing air in. During expiration, volume decreases (V↓), pressure rises (P↑), pushing air out.

What are the normal partial pressures of oxygen in alveolar air and arterial blood?+

In alveolar air, PO₂ is approximately 104 mm Hg and PCO₂ is about 40 mm Hg. In oxygenated arterial blood leaving the lungs, PO₂ is around 95 mm Hg and PCO₂ remains ~40 mm Hg. These values are essential for understanding diffusion gradients.

What is Henry's Law and why is it important in gas exchange?+

Henry's Law states that the amount of gas dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid: C = k × P. It explains why oxygen dissolves in blood plasma at the alveolar-capillary membrane and why CO₂, being more soluble, can be transported partly in dissolved form.

How is carbon dioxide transported in blood?+

About 7% of CO₂ dissolves directly in plasma, 23% binds to haemoglobin as carbamino-haemoglobin, and 70% is converted to bicarbonate ions (HCO₃⁻) in red blood cells via the carbonic anhydrase-catalysed reaction CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻.

What is the chloride shift (Hamburger phenomenon)?+

The chloride shift occurs in red blood cells to maintain electrical neutrality. When bicarbonate ions (HCO₃⁻) diffuse out of RBCs into plasma, chloride ions (Cl⁻) move into the RBCs. This ionic exchange prevents charge imbalance and is essential during CO₂ transport from tissues to lungs.

What factors cause a right shift in the oxygen-haemoglobin dissociation curve?+

A right shift (decreased oxygen affinity) is caused by increased CO₂ (PCO₂), decreased pH (more H⁺), increased temperature, and increased 2,3-BPG. This is collectively called the Bohr effect. It facilitates oxygen release to metabolically active tissues, such as exercising muscles.

Why does foetal haemoglobin have higher oxygen affinity than adult haemoglobin?+

Foetal haemoglobin (HbF) has a higher affinity for oxygen (left shift of the dissociation curve) compared to adult haemoglobin (HbA). This ensures efficient oxygen transfer from maternal blood across the placenta to the foetal circulation, where PO₂ is relatively low.

What is the role of carbonic anhydrase in gas transport?+

Carbonic anhydrase is an enzyme in red blood cells that catalyses the reversible reaction CO₂ + H₂O ⇌ H₂CO₃. It speeds up the conversion of CO₂ to carbonic acid, which then dissociates into H⁺ and HCO₃⁻, enabling the majority (70%) of CO₂ to be transported as bicarbonate in plasma.

How can I remember the respiratory volume values for exams?+

Use the mnemonic 'TV is a Small Remote For Total Living': TV ~500 mL, IRV ~2500-3000 mL, ERV ~1000-1100 mL, RV ~1100-1200 mL, VC ~3500-4500 mL, TLC ~5000-6000 mL. Write these down at the start of your exam to avoid memory lapses under pressure.

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