Breathing and Exchange of Gases is Chapter 14 of CBSE Class 11 Biology — and one of the highest-yielding chapters for NEET, year after year. It explains how air actually moves in and out of your lungs, how oxygen reaches every cell, and how the carbon dioxide your cells make is carried back out. Master this chapter and a whole cluster of NEET questions on respiratory volumes, the oxygen dissociation curve, and gas transport become near-guaranteed marks.
By the end of these notes you will be able to label the human respiratory system, explain inspiration and expiration in terms of pressure, work out any respiratory volume or capacity, describe how O₂ and CO₂ are transported in blood, read the oxygen dissociation curve, and recall the key respiratory disorders. This is a high-weightage chapter carrying roughly 6–7% of NEET Biology and 5–6 marks in boards, and it links directly to Body Fluids and Circulation.
Table of Contents
- Key Concepts — Respiratory organs, human respiratory system, mechanism of breathing, volumes & capacities, exchange and transport of gases, regulation, disorders
- Weightage in Board & Entrance Exams
- Important Definitions
- Solved & NEET-Style Examples
- Important Questions for Board Exams
- Quick Revision Points
Key Concepts
1. Why We Breathe — Respiration vs Breathing
Breathing (or pulmonary ventilation) is the physical exchange of air between the atmosphere and the lungs. Cellular respiration is the chemical breakdown of glucose inside cells to release ATP. Breathing simply supplies the O₂ that respiration needs and removes the CO₂ it produces.
Humans take in O₂ from the air and give out CO₂. The whole pathway — from nostril to alveolus to blood to cell — exists to keep this two-way gas traffic running smoothly.
2. Respiratory Organs in Animals
Different animals breathe differently depending on their habitat and body organisation.
- Sponges, coelenterates, flatworms: simple diffusion across the body surface.
- Earthworm: moist cuticle (skin) is used for gas exchange.
- Insects (e.g. cockroach): a network of tracheal tubes carries air directly to tissues.
- Aquatic animals (fish, prawns): gills (branchial respiration).
- Terrestrial vertebrates: lungs (pulmonary respiration).
3. Human Respiratory System
The human respiratory system is a single passage that warms, filters, and humidifies air on its way to the lungs.
[DIAGRAM: Air path — external nostrils → nasal chamber → pharynx → larynx → trachea → primary bronchi → bronchioles → alveoli; two lungs enclosed in a double-layered pleura.]
The Conducting Part vs the Exchange Part
- Conducting part: external nostrils → nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles up to terminal bronchioles. It transports, clears, humidifies, and warms incoming air — no gas exchange here.
- Respiratory (exchange) part: alveoli and their ducts — the actual site of O₂–CO₂ exchange.
Key Structural Points
- The larynx is the sound box; the epiglottis prevents food entering the trachea during swallowing.
- The trachea, bronchi and initial bronchioles are supported by incomplete cartilaginous rings to prevent collapse.
- Each alveolus is thin-walled and richly supplied with capillaries — together the alveoli give a vast surface area (~70 m²) for diffusion.
- Lungs are covered by a double-layered pleura with pleural fluid in between, reducing friction.
- The right lung has 3 lobes; the left lung has 2 lobes.
4. Mechanism of Breathing
Breathing has two phases — inspiration (air in) and expiration (air out) — and both are driven by pressure gradients created by the diaphragm and intercostal muscles. Air always moves from high to low pressure.
Inspiration (active)
- The diaphragm contracts and flattens; the external intercostal muscles contract and lift the ribs and sternum.
- Thoracic volume increases → intra-pulmonary pressure falls below atmospheric pressure → air rushes in.
Expiration (normally passive)
- The diaphragm and external intercostals relax; ribs and diaphragm return to their resting position.
- Thoracic volume decreases → intra-pulmonary pressure rises above atmospheric pressure → air is pushed out.
Key idea: A healthy person breathes 12–16 times per minute. Breathing rate can be measured with a spirometer, which also gives clinical data on lung volumes.
5. Respiratory Volumes and Capacities
A capacity is simply the sum of two or more volumes. These exact values are extremely high-yield for NEET.
Respiratory Volumes
| Volume | Meaning | Value |
|---|---|---|
| Tidal Volume (TV) | Air inspired or expired in one normal breath | ~500 mL |
| Inspiratory Reserve Volume (IRV) | Extra air inspired by forcible inspiration | 2500–3000 mL |
| Expiratory Reserve Volume (ERV) | Extra air expired by forcible expiration | 1000–1100 mL |
| Residual Volume (RV) | Air remaining in lungs after forcible expiration | 1100–1200 mL |
Respiratory Capacities
| Capacity | Formula | Value |
|---|---|---|
| Inspiratory Capacity (IC) | TV + IRV | ~3500 mL |
| Expiratory Capacity (EC) | TV + ERV | ~1500 mL |
| Functional Residual Capacity (FRC) | ERV + RV | ~2300 mL |
| Vital Capacity (VC) | ERV + TV + IRV | ~3500–4500 mL |
| Total Lung Capacity (TLC) | VC + RV (= RV + ERV + TV + IRV) | ~5800–6000 mL |
Note: Vital Capacity (VC) is the maximum air a person can breathe out after a maximum inspiration — a key indicator of lung health.
6. Exchange of Gases — at the Alveoli
Gas exchange happens at the respiratory membrane (alveolar wall + capillary wall) purely by diffusion, driven by partial pressure differences. No energy is spent here.
Partial pressure (p) is the pressure exerted by an individual gas in a mixture. Each gas diffuses from where its partial pressure is high to where it is low.
| Site | pO₂ (mm Hg) | pCO₂ (mm Hg) |
|---|---|---|
| Atmospheric air | 159 | 0.3 |
| Alveoli | 104 | 40 |
| Deoxygenated blood (entering alveoli) | 40 | 45 |
| Oxygenated blood (leaving alveoli) | 95 | 40 |
| Tissues | 40 | 45 |
Direction of diffusion: At the alveoli, O₂ moves from alveolar air (104) into blood (40), and CO₂ moves from blood (45) into alveolar air (40). At the tissues, the gradients reverse.
Why CO₂ keeps up despite a small gradient: CO₂ is about 20–25 times more soluble than O₂, so even a small pressure difference moves large amounts of it.
7. Transport of Oxygen
About 97% of O₂ is carried bound to haemoglobin as oxyhaemoglobin; only about 3% is dissolved in plasma.
Hb + 4O₂ ⇌ Hb(O₂)₄ (oxyhaemoglobin)
Each haemoglobin molecule can bind a maximum of four O₂ molecules. Binding is reversible and depends mainly on the partial pressure of O₂.
Oxygen Dissociation Curve
A graph of percentage saturation of haemoglobin against pO₂ is sigmoid (S-shaped).
[DIAGRAM: Sigmoid oxygen dissociation curve — % saturation of Hb on the y-axis vs pO₂ on the x-axis; a right shift marked at high CO₂, high H⁺, high temperature.]
- In the alveoli (high pO₂, low pCO₂, lower temperature): conditions favour the formation of oxyhaemoglobin.
- In the tissues (low pO₂, high pCO₂, high H⁺, higher temperature): conditions favour the dissociation of O₂ from haemoglobin.
Bohr effect: a rise in CO₂, H⁺, or temperature shifts the curve to the right, so haemoglobin releases more O₂ exactly where active tissues need it.
8. Transport of Carbon Dioxide
CO₂ is carried in blood in three forms.
| Form | Approx. % |
|---|---|
| As bicarbonate (HCO₃⁻) in plasma | ~70% |
| Bound to haemoglobin as carbamino-haemoglobin | ~20–23% |
| Dissolved in plasma | ~7% |
The enzyme carbonic anhydrase, present in high concentration in RBCs, speeds up the reaction:
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
- At the tissues (high pCO₂): CO₂ diffuses into blood and is converted to bicarbonate.
- At the alveoli (low pCO₂): the reaction reverses, releasing CO₂ to be breathed out.
Every 100 mL of deoxygenated blood delivers about 4 mL of CO₂ to the alveoli for removal.
9. Regulation of Respiration
The body finely adjusts breathing to match its needs, mostly involuntarily.
- Respiratory rhythm centre in the medulla oblongata controls the basic rhythm of breathing.
- Pneumotaxic centre in the pons can moderate the rhythm centre and reduce the duration of inspiration.
- Chemosensitive area near the rhythm centre is sensitive to CO₂ and H⁺; a rise in these signals the centre to increase the rate and depth of breathing.
- Receptors in the aortic arch and carotid artery also detect changes in CO₂ and H⁺ and send signals to the rhythm centre.
Key idea: Oxygen plays only a minor role in the moment-to-moment regulation of breathing — CO₂ is the main stimulus.
10. Disorders of the Respiratory System
| Disorder | Cause / Description |
|---|---|
| Asthma | Difficulty in breathing due to inflammation of bronchi and bronchioles, causing wheezing. |
| Emphysema | Alveolar walls are damaged and lose elasticity, reducing the surface area for gas exchange; major cause is cigarette smoking. |
| Occupational respiratory disorders | Long-term exposure to dust in industries (e.g. grinding, stone-breaking) causes inflammation and fibrosis; silicosis and asbestosis are examples. |
Note: In occupational settings, protective masks and proper ventilation are the front-line defence against these disorders.
Weightage in Board & Entrance Exams
| Exam | Typical Weightage | Most-Tested Areas |
|---|---|---|
| CBSE Board (Class 11) | 5–6 marks | Mechanism of breathing, volumes & capacities, gas exchange |
| NEET | 1–2 questions (high-yield) | Respiratory volumes/capacities, O₂ dissociation curve, CO₂ transport, partial pressures |
| AIIMS-pattern / Olympiads | 1–2 questions | Bohr effect, regulation of respiration, disorders |
[TABLE: Question-type split — VSA (1 mark): definitions & values; SA (2–3 marks): mechanism of breathing, gas transport; LA (5 marks): respiratory volumes/capacities with calculations, oxygen dissociation curve.]
Important Definitions
| Term | Definition |
|---|---|
| Breathing | Physical exchange of air between the atmosphere and the lungs (pulmonary ventilation) |
| Tidal Volume (TV) | Volume of air inspired or expired in one normal breath (~500 mL) |
| Vital Capacity (VC) | Maximum air a person can expire after a maximum inspiration = ERV + TV + IRV |
| Residual Volume (RV) | Air remaining in the lungs after the most forcible expiration (~1100–1200 mL) |
| Partial pressure | Pressure contributed by an individual gas in a mixture of gases |
| Oxyhaemoglobin | Reversible compound formed when O₂ binds to haemoglobin: Hb + 4O₂ ⇌ Hb(O₂)₄ |
| Bohr effect | Rightward shift of the O₂ dissociation curve due to high CO₂, H⁺ or temperature, promoting O₂ release |
| Carbonic anhydrase | Enzyme in RBCs that catalyses CO₂ + H₂O ⇌ H⁺ + HCO₃⁻ |
| Emphysema | Disorder where alveolar walls are damaged, reducing gas-exchange surface; chiefly caused by smoking |
Solved & NEET-Style Examples
Example 1
Calculate the Vital Capacity of a person with TV = 500 mL, IRV = 3000 mL, ERV = 1100 mL, RV = 1200 mL.
Answer: VC = ERV + TV + IRV = 1100 + 500 + 3000 = 4600 mL.
Example 2
For the same person, find the Total Lung Capacity (TLC).
Answer: TLC = VC + RV = 4600 + 1200 = 5800 mL.
Example 3
At the alveoli, in which direction do O₂ and CO₂ diffuse, and why?
Answer: O₂ diffuses from alveolar air (pO₂ 104) into deoxygenated blood (pO₂ 40); CO₂ diffuses from blood (pCO₂ 45) into alveolar air (pCO₂ 40). Diffusion always follows the partial-pressure gradient (high → low).
Example 4
Why is the O₂ dissociation curve sigmoid, and what happens when it shifts right?
Answer: Binding of one O₂ increases haemoglobin’s affinity for the next (cooperative binding), giving the S-shape. A right shift (high CO₂, H⁺, temperature — the Bohr effect) lowers Hb’s O₂ affinity, so more O₂ is unloaded to active tissues.
Example 5
In what form is most CO₂ transported in blood, and which enzyme is responsible?
Answer: About 70% of CO₂ is transported as bicarbonate (HCO₃⁻); the enzyme carbonic anhydrase in RBCs catalyses its formation.
Example 6
A patient’s alveolar walls are damaged and have lost elasticity after years of smoking. Name the disorder and its main effect.
Answer: Emphysema. The reduced alveolar surface area impairs gas exchange, causing breathlessness.
Important Questions for Board Exams
1-Mark Questions (VSA)
- Define tidal volume and give its approximate value.
- Name the enzyme that catalyses the conversion of CO₂ into bicarbonate in RBCs.
- What is the main stimulus that regulates the rate of breathing?
- Name the structure that prevents food from entering the trachea while swallowing.
- In which form is the maximum amount of oxygen transported in blood?
2–3-Mark Questions (SA)
- Explain the mechanism of inspiration in terms of pressure changes in the thoracic cavity.
- Distinguish between Vital Capacity and Total Lung Capacity, giving the formula for each.
- Describe how carbon dioxide is transported from the tissues to the lungs.
- What is the Bohr effect? State the factors that shift the oxygen dissociation curve to the right.
5-Mark Questions (LA)
- With the help of a labelled diagram, describe the human respiratory system and distinguish the conducting part from the exchange part.
- Explain the exchange of gases at the alveoli and the tissues using partial pressure values, and state why CO₂ diffusion is efficient despite a small gradient.
- Describe the role of the medulla, pons, and chemosensitive area in the regulation of respiration.
Quick Revision Points
- Breathing = ventilation; respiration = ATP release in cells
- Air path: nostrils → nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → alveoli
- Right lung = 3 lobes, left lung = 2 lobes; lungs covered by double-layered pleura
- Inspiration: diaphragm + external intercostals contract → thoracic volume up → pressure down → air in
- Normal breathing rate = 12–16/min; measured by a spirometer
- TV ~500 mL; VC = ERV + TV + IRV; TLC = VC + RV (~5800–6000 mL)
- Gas exchange = diffusion along partial-pressure gradients; CO₂ is ~20–25× more soluble than O₂
- O₂ transport: 97% as oxyhaemoglobin (Hb binds 4 O₂), 3% dissolved; curve is sigmoid
- Bohr effect: high CO₂/H⁺/temperature → right shift → more O₂ released to tissues
- CO₂ transport: ~70% bicarbonate, ~20–23% carbamino-Hb, ~7% dissolved; enzyme = carbonic anhydrase
- Regulation: rhythm centre (medulla), pneumotaxic centre (pons); CO₂/H⁺ are main stimuli
- Disorders: asthma (bronchial inflammation), emphysema (smoking, alveolar damage), occupational (silicosis, asbestosis)
Next Chapter: Chapter 15 — Body Fluids and Circulation
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