Breathing and Exchange of Gases ISC Class 11: Mechanism, Transport and Volumes

What Breathing and Exchange of Gases is about

Weightage: Breathing and Exchange of Gases belongs to the unit Human Physiology, which CISCE weights at 18 of the 70 marks in the ISC Class XI Biology Paper I (Theory). Class XI is examined by the school, so your school sets the paper within this weightage.Source: CISCE ISC Biology syllabus, examination year 2028, Class XI unit weightage

Key concepts

Respiratory organs in animals (recall) and the human respiratory system

Recall: lower invertebrates such as sponges, cnidarians and flatworms exchange gases by simple diffusion over the whole body surface; earthworms use their moist cuticle; insects use a network of tracheal tubes; most aquatic arthropods and molluscs use gills (branchial respiration), as do fishes; amphibians use moist skin and lungs; reptiles, birds and mammals use lungs (pulmonary respiration). Human respiratory system: a pair of external nostrils opens into the nasal chamber, which leads to the pharynx, a common passage for food and air. The pharynx opens through the larynx, a cartilaginous sound box, into the trachea. During swallowing the glottis is covered by a thin elastic flap, the epiglottis, which stops food entering the larynx. The trachea runs down to the middle of the thorax and divides (at about the level of the 5th thoracic vertebra) into right and left primary bronchi; these branch repeatedly into secondary and tertiary bronchi and bronchioles, ending in very thin terminal bronchioles. The trachea, bronchi and early bronchioles are kept open by incomplete (C-shaped) cartilaginous rings. Each terminal bronchiole ends in many thin-walled, highly vascular bag-like alveoli. The branching tubes and alveoli form the two lungs, covered by a double-layered pleura with pleural fluid between the layers, which reduces friction. The part from nostrils to terminal bronchioles is the conducting part (it warms, humidifies and cleans the air); the alveoli and their ducts form the respiratory or exchange part. The lungs lie in the air-tight thoracic chamber, bounded by the vertebral column (dorsally), sternum (ventrally), ribs (laterally) and the dome-shaped diaphragm (below).

Worked example

Why are the trachea and bronchi supported by cartilage rings, and why are the rings incomplete?

Try it first, then show the solutionSolution

1. Cartilage keeps the airway open so it does not collapse when pressure inside falls during inspiration
2. The rings are open at the back where the trachea lies against the oesophagus, so swallowed food can expand the oesophagus

Answer: To prevent collapse of the airway while allowing the oesophagus to expand

Mechanism of breathing: role of diaphragm and intercostal muscles

Breathing has two phases: inspiration (air in) and expiration (air out). Air moves because of a pressure gradient between the lungs and the atmosphere, created by changing the volume of the thoracic cavity; since the lungs are attached to it through the pleura, lung volume changes with it. Inspiration (active): (1) the diaphragm contracts and flattens, increasing the volume of the thorax in the antero-posterior (top-to-bottom) axis; (2) the external intercostal muscles contract, lifting the ribs and sternum upwards and outwards, increasing the volume in the dorso-ventral axis; (3) the larger volume lowers intra-pulmonary pressure below atmospheric pressure; (4) air flows into the lungs. Expiration (normally passive): the diaphragm and external intercostals relax; the diaphragm domes up and the ribs and sternum return; thoracic and lung volume fall; intra-pulmonary pressure rises above atmospheric, and air is expelled. During forced breathing (exercise), internal intercostal muscles and abdominal muscles contract to push air out more strongly, and extra muscles help inspiration. A healthy adult at rest breathes about 12–16 times per minute. Breathing volumes can be measured with a spirometer.

Worked example

If the pleural membrane is punctured so that air enters between the lung and the chest wall, the lung on that side collapses. Explain.

Try it first, then show the solutionSolution

1. Normally the pressure between the pleural layers is below atmospheric, which keeps the elastic lungs stretched against the chest wall
2. When air enters, this pressure equals atmospheric, the lung is no longer held out and its elasticity makes it recoil and collapse

Answer: Loss of the pressure difference that keeps the lung expanded

Pulmonary gas exchange

Alveoli are the main sites of exchange; exchange also takes place between blood and tissues. Gases move by simple diffusion, which depends on the partial pressure gradient, the solubility of the gas and the thickness of the membrane. Partial pressures (mm Hg, typical values): atmospheric air: pO₂ 159, pCO₂ 0.3; alveoli: pO₂ 104, pCO₂ 40; blood arriving at lungs (deoxygenated): pO₂ 40, pCO₂ 45; blood leaving lungs (oxygenated): pO₂ 95, pCO₂ 40; tissues: pO₂ 40, pCO₂ 45. So O₂ diffuses from alveoli (104) into blood (40) and from blood (95) into tissues (40); CO₂ diffuses from tissues (45) into blood (40) and from blood (45) into alveoli (40). Although the CO₂ gradient is small, CO₂ is about 20–25 times more soluble than O₂, so the amount of CO₂ that diffuses per unit difference is much higher. The diffusion membrane has three layers: the thin squamous epithelium of the alveolus, the endothelium of the capillary, and a basement substance between them; its total thickness is far less than a millimetre, which favours diffusion. The very large total alveolar area and the rich capillary network also help.

Worked example

Using typical partial pressures, give the O₂ gradient from alveolus to blood and the CO₂ gradient from blood to alveolus. Why does CO₂ still diffuse efficiently?

Try it first, then show the solutionSolution

1. O₂: 104 − 40 = 64 mm Hg; CO₂: 45 − 40 = 5 mm Hg
2. CO₂ is 20–25 times more soluble than O₂, so a small gradient moves a large amount

Answer: 64 mm Hg and 5 mm Hg; CO₂'s high solubility compensates

Transport of oxygen and the oxygen–haemoglobin dissociation curve

About 97% of O₂ is carried by haemoglobin in red blood cells, and only about 3% dissolved in plasma. Haemoglobin is a red, iron-containing pigment; each molecule can bind up to four O₂ molecules reversibly, forming oxyhaemoglobin. Binding depends mainly on pO₂, and also on pCO₂, H⁺ concentration and temperature. The oxygen–haemoglobin dissociation curve plots percentage saturation of haemoglobin against pO₂. It is sigmoid (S-shaped): at low pO₂ saturation rises slowly, then steeply over the range found in tissues, and flattens near 100% at the pO₂ of the lungs. In the alveoli (high pO₂, low pCO₂, fewer H⁺, lower temperature) haemoglobin loads O₂ almost fully. In the tissues (low pO₂, high pCO₂, more H⁺, higher temperature) these conditions shift the curve to the right, lowering haemoglobin's affinity so that it unloads more O₂ (the Bohr effect). Under normal resting conditions, every 100 mL of oxygenated blood delivers about 5 mL of O₂ to the tissues.

Worked example

During exercise, muscle temperature and CO₂ rise. What happens to the dissociation curve and why is this useful?

Try it first, then show the solutionSolution

1. Higher pCO₂, H⁺ and temperature shift the curve to the right
2. At the same pO₂, haemoglobin is less saturated, so it releases more O₂ to the working muscle

Answer: The curve shifts right, increasing O₂ unloading where it is needed

Transport of carbon dioxide and the chloride shift

CO₂ is carried in three ways: (1) about 7% dissolved in plasma; (2) about 20–25% combined with haemoglobin as carbaminohaemoglobin (CO₂ binds to amino groups of the globin, not to iron); binding is favoured by high pCO₂ and low pO₂ (tissues) and reversed in the lungs; (3) about 70% as bicarbonate ions. In the tissues, CO₂ diffuses into red blood cells, where the enzyme carbonic anhydrase rapidly converts it: CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺. The H⁺ is buffered by haemoglobin. HCO₃⁻ diffuses out of the red cell into the plasma, and to keep the cell electrically neutral chloride ions (Cl⁻) move from the plasma into the red cell: this exchange is the chloride shift (Hamburger phenomenon). In the lungs, where pCO₂ is low, the reaction runs in reverse: HCO₃⁻ re-enters the red cell (Cl⁻ moves out, a reverse chloride shift), is converted back to CO₂ and water, and CO₂ diffuses into the alveoli. Every 100 mL of deoxygenated blood delivers about 4 mL of CO₂ to the alveoli.

Worked example

Why does the chloride ion move into red blood cells as blood passes through tissues?

Try it first, then show the solutionSolution

1. Carbonic anhydrase in the RBC forms H₂CO₃, which gives HCO₃⁻ and H⁺; HCO₃⁻ leaves the cell into plasma
2. Loss of negative ions would upset the cell's charge balance, so Cl⁻ enters from plasma in exchange

Answer: To maintain electrical neutrality as bicarbonate leaves (chloride shift)

Regulation of breathing

Humans can adjust the rhythm and depth of breathing to match the body's demands. This is controlled by the neural system. (1) A respiratory rhythm centre in the medulla oblongata is mainly responsible for setting the basic rhythm. (2) A pneumotaxic centre in the pons can moderate the rhythm centre; its signals shorten inspiration and so change the breathing rate. (3) A chemosensitive area next to the rhythm centre is highly sensitive to CO₂ and H⁺; when these rise, it activates the rhythm centre to increase breathing so they are removed. (4) Receptors in the aortic arch and carotid arteries also detect changes in CO₂ and H⁺ and send signals to the rhythm centre. The role of O₂ in regulating normal breathing is minor; it becomes important only when O₂ falls very low (e.g. at high altitude).

Worked example

A person who holds their breath for a minute feels a strong urge to breathe. What triggers this?

Try it first, then show the solutionSolution

1. CO₂ accumulates in the blood, raising pCO₂ and H⁺
2. The chemosensitive area and aortic and carotid receptors stimulate the rhythm centre, forcing breathing

Answer: The rise in CO₂ and H⁺, not mainly the fall in O₂

Respiratory volumes and capacities

Volumes (healthy adult, approximate): Tidal volume (TV): air breathed in or out in a normal breath, about 500 mL (about 6000–8000 mL per minute at rest). Inspiratory reserve volume (IRV): extra air that can be inspired forcibly after a normal inspiration, about 2500–3000 mL. Expiratory reserve volume (ERV): extra air that can be expired forcibly after a normal expiration, about 1000–1100 mL. Residual volume (RV): air that remains in the lungs even after forcible expiration, about 1100–1200 mL. Capacities (sums): Inspiratory capacity (IC) = TV + IRV. Expiratory capacity (EC) = TV + ERV. Functional residual capacity (FRC) = ERV + RV (air left after a normal expiration). Vital capacity (VC) = ERV + TV + IRV: the maximum air a person can breathe out after a forced inspiration (or in after a forced expiration). Total lung capacity (TLC) = VC + RV. Not all inspired air reaches the alveoli: about 150 mL of each breath stays in the conducting passages (anatomical dead space). Vital capacity is higher in athletes, wind-instrument players and mountain dwellers, and lower in smokers and people with lung disease.

Worked example

A student has TV 500 mL, IRV 2800 mL, ERV 1100 mL and RV 1200 mL. Find VC, TLC and FRC.

Try it first, then show the solutionSolution

1. VC = TV + IRV + ERV = 500 + 2800 + 1100 = 4400 mL
2. TLC = VC + RV = 5600 mL; FRC = ERV + RV = 2300 mL

Answer: VC 4400 mL; TLC 5600 mL; FRC 2300 mL

Disorders of the respiratory system

Asthma: difficulty in breathing with wheezing, caused by inflammation and narrowing of the bronchi and bronchioles, often an allergic reaction to dust, pollen, smoke or other triggers; spasm of smooth muscle and extra mucus narrow the airways further. Emphysema: a chronic disorder in which the walls of the alveoli are damaged and many alveoli merge into larger sacs, so the respiratory surface area decreases and exchange becomes inefficient; the lungs also lose elasticity, making expiration difficult. Cigarette smoking is a major cause. Occupational respiratory disorders: in industries such as stone-grinding, quarrying, mining, construction and asbestos work, workers inhale large amounts of dust that the body's defences cannot fully clear. Long exposure causes inflammation leading to fibrosis (growth of fibrous tissue), which stiffens the lungs and seriously damages them, e.g. silicosis (silica dust) and asbestosis (asbestos fibres). Prevention: protective masks, dust control and regular health checks.

Worked example

Why do people with emphysema become breathless even at rest?

Try it first, then show the solutionSolution

1. Damaged alveolar walls merge into fewer, larger sacs, so the total surface for gas exchange falls
2. Less O₂ can diffuse into the blood per breath, and the stiff, less elastic lungs trap air, so breathing must be faster and harder

Answer: Loss of alveolar surface area and elasticity reduces gas exchange

Breathing and Exchange of Gases in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Gas Exchange, Transport of Gases and Regulation
• Mechanism of Breathing and Respiratory Volumes

Breathing and Exchange of Gases: key facts

Lung capacities

$VC=TV+IRV+ERV;TLC=VC+RV;IC=TV+IRV;FRC=ERV+RV;EC=TV+ERV$

Typical: TV 500, IRV 2500–3000, ERV 1000–1100, RV 1100–1200 mL.

Ventilation

$\text{pulmonary ventilation}=TV\times f;\text{alveolar ventilation}=(TV-\text{dead space})\times f$

f = breaths per minute; dead space about 150 mL.

Carbonic anhydrase reaction

$C{O}_2+H_{2}O\rightleftharpoons H_{2}C{O}_3\rightleftharpoons HC{O}_3^{-}+H^{+}$

Forward in tissues, reverse in lungs.

Oxygen loading and unloading

$Hb+4O_2\rightleftharpoons Hb(O_2{)}_4$

High pO₂ favours loading (lungs); low pO₂, high pCO₂, H⁺ and temperature favour unloading (tissues).

Partial pressures (mm Hg)

$\text{alveoli: }p{O}_{2}104,pC{O}_{2}40;\text{tissues: }p{O}_{2}40,pC{O}_{2}45$

Oxygenated blood pO₂ 95; deoxygenated blood pO₂ 40, pCO₂ 45.

Breathing and Exchange of Gases: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Breathing and Exchange of Gases

The question patterns to expect, and the method that works for each.

Tricks to remember Breathing and Exchange of Gases

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Diaphragm Down = Draw in

  Diaphragm contracts and moves down; external intercostals lift ribs: volume up, pressure down, air in.

  Use it when: Inspiration.

• Tired Insects Eat Rice

  TV, IRV, ERV, RV: VC is the first three; TLC adds RV.

  Use it when: Lung volumes.

• 97-3 and 70-23-7

  O₂: 97% Hb, 3% plasma. CO₂: 70% bicarbonate, 20–25% carbamino-Hb, 7% dissolved.

  Use it when: Transport shares.

• CADET, face right

  CO₂, Acid (H⁺), 2,3-BPG, Exercise, Temperature shift the curve to the right.

  Use it when: Dissociation curve shifts.

• Bicarb out, chloride in

  In tissues HCO₃⁻ leaves the RBC and Cl⁻ enters (chloride shift).

  Use it when: CO₂ transport.

Breathing and Exchange of Gases: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section D style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 5-mark question split into 1 + 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Breathing and Exchange of Gases

The slips examiners see most often, and what to write instead.

• ✗Mistake: Saying expiration is caused by muscle contraction at rest

  Both phases are assumed to be active.

  Wrong working

  Internal intercostals contract in every expiration.

  Right working

  Normal expiration is passive (relaxation and elastic recoil); internal intercostals and abdominal muscles act only in forced expiration.

  ✓Do this: Specify 'normal' or 'forced'.

  Usually costs: 1 mark.

• ✗Mistake: Adding the wrong volumes

  Capacities are confused.

  Wrong working

  VC = TV + IRV + ERV + RV.

  Right working

  VC excludes RV; TLC = VC + RV.

  ✓Do this: VC is what you can blow out.

  Usually costs: 1-2 marks.

• ✗Mistake: Saying CO₂ binds to the iron of haemoglobin

  CO₂ and O₂ are assumed to bind the same site.

  Wrong working

  Carbaminohaemoglobin forms when CO₂ binds to haem iron.

  Right working

  CO₂ binds to amino groups of the globin protein; O₂ binds to iron.

  ✓Do this: Carbamino = amino group.

  Usually costs: 1 mark.

• ✗Mistake: Saying O₂ is the main regulator of breathing

  We breathe to get oxygen.

  Wrong working

  Low O₂ is the main stimulus for breathing.

  Right working

  Rising CO₂ and H⁺ are the main stimuli; O₂ plays a minor role normally.

  ✓Do this: CO₂ drives the urge to breathe.

  Usually costs: 1 mark.

• ✗Mistake: Wrong direction of chloride shift

  Ion movements are reversed.

  Wrong working

  In tissues Cl⁻ leaves the RBC.

  Right working

  In tissues HCO₃⁻ leaves and Cl⁻ enters the RBC; the reverse occurs in the lungs.

  ✓Do this: Bicarb out, chloride in (at tissues).

  Usually costs: 1 mark.

Practice questions on Breathing and Exchange of Gases

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   The respiratory rhythm centre is located in the:
   (a) cerebrum
   (b) pons
   (c) medulla oblongata
   (d) cerebellum

   Hint

   Hindbrain.

   Answer

   (c)

2. Q2 · 1 mark · MCQExam level

   Most CO₂ is transported in blood as:
   (a) dissolved CO₂
   (b) carbaminohaemoglobin
   (c) bicarbonate
   (d) carbonic acid

   Hint

   About 70%.

   Answer

   (c)

3. Q3 · 1 mark · Very short answerExam level

   Name the flap that prevents food from entering the larynx.

   Hint

   Covers the glottis.

   Answer

   Epiglottis

4. Q4 · 1 mark · Very short answerExam level

   Name the enzyme in RBCs that speeds up the formation of carbonic acid.

   Hint

   Contains zinc.

   Answer

   Carbonic anhydrase

5. Q5 · 2 marks · Short answerExam level

   Name the respiratory organs of an earthworm, a cockroach and a fish.

   Hint

   Recall.

   Answer

   Earthworm: moist skin (cuticle); cockroach: tracheal tubes; fish: gills.

6. Q6 · 2 marks · Short answerHarder exam level

   Distinguish between the conducting part and the respiratory part of the respiratory system.

   Hint

   Exchange happens where?

   Answer

   Conducting part: nostrils to terminal bronchioles; transports air, warms, humidifies and filters it; no exchange. Respiratory part: alveoli and alveolar ducts; site of actual diffusion of O₂ and CO₂.

7. Q7 · 2 marks · Short answerHarder exam level

   What is the significance of the sigmoid shape of the dissociation curve?

   Hint

   Flat top, steep middle.

   Answer

   The flat top ensures near-full loading even if alveolar pO₂ falls a little; the steep middle means that a small fall in pO₂ in tissues releases a large amount of O₂.

8. Q8 · 3 marks · Short answerHarder exam level

   Define tidal volume, inspiratory reserve volume and expiratory reserve volume, giving approximate values.

   Hint

   500; 2500–3000; 1000–1100.

   Answer

   TV: air moved in a normal breath, about 500 mL. IRV: extra air inspired by forcible inspiration after a normal one, about 2500–3000 mL. ERV: extra air expired by forcible expiration after a normal one, about 1000–1100 mL.

9. Q9 · 3 marks · Short answerHarder exam level

   Describe asthma and emphysema and give one cause of each.

   Hint

   Bronchi vs alveoli.

   Answer

   Asthma: inflammation and narrowing of bronchi and bronchioles causing wheezing and breathlessness; often allergic (dust, pollen). Emphysema: chronic damage to alveolar walls, reducing respiratory surface and lung elasticity; mainly due to cigarette smoking.

10. Q10 · 3 marks · Short answerHarder exam level

    Give the typical partial pressures of O₂ and CO₂ in alveoli, deoxygenated blood and tissues, and use them to explain the direction of diffusion.

    Hint

    104/40; 40/45; 40/45.

    Answer

    Alveoli pO₂ 104, pCO₂ 40; deoxygenated blood pO₂ 40, pCO₂ 45; tissues pO₂ 40, pCO₂ 45 (oxygenated blood pO₂ 95, pCO₂ 40). O₂ moves alveoli → blood → tissues; CO₂ moves tissues → blood → alveoli, each from higher to lower partial pressure.

11. Q11 · 5 marks · Long answerHarder exam level

    (a) Draw a labelled diagram of the human respiratory system. [3]
    (b) Explain the role of the pleura and of cartilage rings. [2]

    Hint

    Nostrils to alveoli.

    Answer

    (a) Diagram with nasal chamber, pharynx, epiglottis, larynx, trachea with rings, bronchi, bronchioles, alveoli, lungs, pleura, diaphragm. (b) Pleura: double membrane with pleural fluid that reduces friction as lungs expand and keeps them attached to the chest wall. Cartilage rings keep the trachea and bronchi open against pressure changes.

12. Q12 · 5 marks · Long answerHarder exam level

    (a) Explain the transport of oxygen in blood. [3]
    (b) What are occupational respiratory disorders? Give two examples and one preventive measure. [2]

    Hint

    97% Hb.

    Answer

    (a) About 97% bound to Hb (up to 4 O₂ per molecule) as oxyhaemoglobin; 3% dissolved in plasma. Loading at high pO₂ in lungs; unloading in tissues at low pO₂, high pCO₂, H⁺ and temperature; about 5 mL O₂ per 100 mL blood delivered at rest. (b) Lung diseases due to long exposure to dust at work, causing inflammation and fibrosis, e.g. silicosis and asbestosis; workers should wear protective masks.

13. Q13 · 2 marks · Short answerHarder exam level

    A person breathes 16 times per minute with a tidal volume of 450 mL. Calculate the pulmonary ventilation and the alveolar ventilation (dead space 150 mL).

    Hint

    TV × f.

    Answer

    Pulmonary ventilation = 450 × 16 = 7200 mL/min; alveolar ventilation = (450 − 150) × 16 = 4800 mL/min.

14. Q14 · 3 marks · Short answerHOTS / competency

    Foetal haemoglobin has a dissociation curve to the left of adult haemoglobin. Explain why this is an advantage to the foetus.

    Hint

    Higher affinity.

    Answer

    A curve to the left means higher affinity for O₂ at the same pO₂. In the placenta, the mother's haemoglobin (lower affinity) releases O₂ while the foetal haemoglobin, with higher affinity, picks it up even at the low pO₂ there, so O₂ moves from mother to foetus.

15. Q15 · 3 marks · Short answerHOTS / competency

    A patient's lung disease thickens the diffusion membrane but does not change lung volumes. Predict the effect on blood O₂ and CO₂ levels and explain which gas is affected more.

    Hint

    Solubility.

    Answer

    A thicker membrane slows diffusion of both gases, so blood leaving the lungs carries less O₂ and more CO₂ than normal. O₂ is affected more because it is far less soluble than CO₂; CO₂, being 20–25 times more soluble, still diffuses relatively well, so low blood O₂ appears first.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Sound box?Show answer

   Larynx.

2. 2.Membrane covering the lungs?Show answer

   Pleura (double-layered).

3. 3.Muscles contracting in normal inspiration?Show answer

   Diaphragm and external intercostals.

4. 4.Alveolar pO₂?Show answer

   About 104 mm Hg.

5. 5.O₂ carried by haemoglobin?Show answer

   About 97%.

6. 6.Chloride shift is also called?Show answer

   Hamburger phenomenon.

7. 7.Typical tidal volume?Show answer

   About 500 mL.

8. 8.Centre that moderates the rhythm?Show answer

   Pneumotaxic centre (pons).

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you trace the path of air from nostrils to alveoli?
• ☐Can you explain inspiration and expiration?
• ☐Can you quote partial pressures of O₂ and CO₂ in alveoli, blood and tissues?
• ☐Can you draw the dissociation curve and explain right shift?
• ☐Can you describe the three forms of CO₂ transport?
• ☐Can you explain chloride shift?
• ☐Can you define all lung volumes and capacities?
• ☐Can you describe asthma, emphysema and silicosis?

Breathing and Exchange of Gases: frequently asked questions