Breathing and Exchange of Gases deals with respiratory organs and the human airway, the mechanism of breathing, lung volumes and capacities, the diffusion of oxygen and carbon dioxide across the alveoli and tissues, the transport of these gases in blood, and the neural regulation of respiration along with common disorders. NEET commonly tests partial-pressure values, the percentages of gas transport, lung-volume definitions and the role of the medulla.
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.
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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
Class 11 Biology · Breathing and Exchange of Gases – swipe through all 9 cards to understand the whole chapter.
Why We Breathe
Big, active animals can’t soak up gases through their skin, so they evolved dedicated respiratory organs.
Earthworm skin must stay moist; gases dissolve before they diffuse.
- Sponges, Hydra, flatworms: diffusion across body surface
- Insects: tracheal tubes deliver air straight to tissues
- Fishes/prawns: gills · terrestrial vertebrates: lungs
Human Respiratory Tract
Air follows one fixed path from the nostrils down to the alveoli.
Pharynx comes BEFORE larynx; trap in many MCQs.
- Larynx = sound box; epiglottis seals airway while swallowing
- Trachea/bronchi held open by C-shaped (incomplete) cartilage rings
- Conducting part warms/cleans air; exchange ONLY in alveoli (~70 m2)
Mechanism of Breathing
Muscles change thoracic volume, which changes pressure, and air flows down the gradient.
Muscles change volume, not push air directly.
- Inspiration (active): diaphragm flattens + external intercostals lift ribs
- Normal expiration is PASSIVE (elastic recoil)
- Forced expiration uses internal intercostals + abdominal muscles
Respiratory Volumes
Four basic volumes that cannot be split further; NCERT loves their values.
Standard tidal volume is 500 mL, not 5000.
- TV = air in one normal breath
- IRV = extra forcibly inhaled · ERV = extra forcibly exhaled
- RV = air always left after the hardest exhalation
Lung Capacities
Capacities are just sums of the four volumes.
Spirometer cannot measure RV, FRC or TLC.
- IC = TV + IRV · EC = TV + ERV
- FRC = ERV + RV (air left after normal expiration)
- VC excludes RV; adding RV gives TLC
Exchange of Gases
O2 and CO2 move by passive diffusion down partial-pressure gradients, costing no energy.
Diffusion membrane = 3 thin layers, <1 mm thick.
- Tissues pO2 ≈ 40, pCO2 ≈ 45 mmHg
- O2: alveoli → blood → tissues · CO2: tissues → blood → alveoli
- CO2 diffuses ~20–25× faster than O2 (higher solubility)
Oxygen Transport
Most oxygen rides on haemoglobin rather than dissolving in plasma.
O2 dissociation curve is sigmoid (S-shaped).
- Loading in alveoli: high pO2, low pCO2, low H⁺, low temp
- Unloading at tissues: low pO2, high pCO2, high H⁺, high temp
- Bohr effect: curve shifts RIGHT under high CO2/H⁺/heat
Carbon Dioxide Transport
CO2 is mostly converted to bicarbonate so blood can carry far more than plasma alone could.
Carbonic anhydrase works inside RBCs, not plasma.
- ~70% as bicarbonate (HCO3⁻) — the main form
- ~20–25% as carbamino-haemoglobin
- ~7% dissolved directly in plasma
Regulation & Disorders
An autopilot in the brain tunes breathing chiefly to CO2 and H⁺, not to oxygen.
O2 plays an insignificant DIRECT role in routine control.
- Chemosensitive area + aortic/carotid bodies sense CO2 & H⁺
- Asthma: bronchiole inflammation + wheezing
- Emphysema: alveolar wall damage (smoking) · silicosis: dust/fibrosis
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Frequently Asked Questions
Vital capacity is the maximum volume of air a person can breathe out after a forced maximum inspiration. It equals the sum of tidal volume, inspiratory reserve volume and expiratory reserve volume (VC = TV + IRV + ERV).
About 97 percent of oxygen is carried as oxyhaemoglobin bound to haemoglobin and about 3 percent dissolved in plasma. Carbon dioxide is carried mainly as bicarbonate (about 70 percent), about 20 to 25 percent as carbamino-haemoglobin bound to haemoglobin, and about 7 percent dissolved in plasma.
The respiratory rhythm centre in the medulla oblongata sets the basic breathing rhythm. The pneumotaxic centre in the pons can reduce the duration of inspiration and alter the rate. A chemosensitive area is highly sensitive to carbon dioxide and hydrogen ions, while oxygen plays only a minor direct role in routine regulation.
Inspiration requires the diaphragm and external intercostal muscles to contract, increasing thoracic volume and lowering pulmonary pressure so air flows in, which needs energy. Normal expiration happens when these muscles simply relax, allowing the elastic lungs and chest wall to recoil and push air out, so no extra muscular effort is needed.
The Bohr effect is the rightward shift of the oxygen dissociation curve at the tissues, where high carbon dioxide, high hydrogen ion concentration (low pH) and high temperature cause haemoglobin to release oxygen more readily. This favours unloading of oxygen where the tissues need it most.