Breathing and Exchange of Gases Class 11 Biology: Lungs, Respiratory Volumes and Gas Transport

What Breathing and Exchange of Gases is about

Weightage: This chapter is part of Unit V, Human Physiology, which carries 18 of the 70 theory marks in the school annual exam together with Body Fluids and Circulation, Excretory Products and their Elimination, Locomotion and Movement, Neural Control and Coordination and Chemical Coordination and Integration.Source: CBSE Biology (044) syllabus 2026-27, Class XI course structure, Unit V Human Physiology

Key concepts

Respiratory organs and the human respiratory system

NCERT section 14.1 Respiratory Organs (14.1.1 Human Respiratory System)

Respiratory organs in animals (recall only): the mechanism depends on habitat and level of organisation.

• Simple diffusion over the whole body surface: sponges, coelenterates, flatworms.
• Moist cuticle: earthworms.
• Network of tubes (tracheal tubes): insects.
• Gills (special vascularised structures): most aquatic arthropods and molluscs; fishes (branchial respiration).
• Lungs (vascularised bags): terrestrial forms (pulmonary respiration). Amphibians such as frogs also use their moist skin (cutaneous respiration).
• Lungs in reptiles, birds and mammals.

Human respiratory system (follow the path of air):
1. External nostrils open above the upper lip into the nasal passage and nasal chamber.
2. Pharynx: the common passage for food and air.
3. Larynx (sound box): a cartilaginous box that helps in sound production. During swallowing, the epiglottis, a thin elastic cartilaginous flap, covers the glottis and stops food entering the larynx.
4. Trachea (windpipe): a straight tube to the mid-thoracic cavity, where it divides at the level of the 5th thoracic vertebra into right and left primary bronchi.
5. Each bronchus divides repeatedly: secondary and tertiary bronchi and bronchioles, ending in very thin terminal bronchioles.
6. Each terminal bronchiole gives rise to many very thin, irregular-walled, vascularised bag-like structures called alveoli.

• Trachea, primary, secondary, tertiary bronchi and initial bronchioles are supported by incomplete cartilaginous rings (keeping them open).
• The branching network of bronchi, bronchioles and alveoli makes up the lungs.

Lungs and thoracic chamber:

• Two lungs covered by a double-layered pleura with pleural fluid between the layers, which reduces friction on the lung surface.
• Outer pleural membrane: in contact with the thoracic lining; inner pleural membrane: in contact with the lung surface.
• Conducting part: external nostrils up to terminal bronchioles; transports air, removes foreign particles, humidifies it and brings it to body temperature.
• Respiratory (exchange) part: alveoli and their ducts, the site of actual diffusion of O₂ and CO₂ between blood and air.
• Thoracic chamber: an anatomically air-tight chamber bounded by the backbone behind (dorsal), the breastbone or sternum in front (ventral), the ribs at the sides and the dome of the diaphragm at the floor. Any change in its volume is reflected in the lung (pulmonary) cavity, which is essential for breathing because we cannot directly change lung volume.

Worked example

Distinguish between the conducting part and the respiratory part of the human respiratory system, and name the structure that prevents food from entering the larynx. (3 marks)

Try it first, then show the solutionSolution

1. Conducting part: from external nostrils to terminal bronchioles; transports, cleans, humidifies and warms air. (1)
2. Respiratory or exchange part: alveoli and their ducts, where O₂ and CO₂ diffuse between blood and air. (1)
3. The epiglottis covers the glottis during swallowing. (1)

Answer: Conducting = air passages; respiratory = alveoli; epiglottis.

Mechanism of breathing

NCERT section 14.2 Mechanism of Breathing

Breathing has two stages: inspiration (air drawn in) and expiration (alveolar air released). Air moves because of a pressure gradient between the lungs and the atmosphere.

Inspiration (lung pressure must fall below atmospheric pressure, a negative pressure):
1. The diaphragm contracts, flattening and increasing the thoracic volume in the antero-posterior axis.
2. The external intercostal muscles contract, lifting the ribs and sternum, increasing volume in the dorso-ventral axis.
3. The increased thoracic volume increases pulmonary volume, so intra-pulmonary pressure drops below atmospheric pressure.
4. Air rushes into the lungs.

Expiration (lung pressure becomes slightly above atmospheric):
1. The diaphragm and intercostal muscles relax; the diaphragm and sternum return to normal.
2. Thoracic and pulmonary volume decrease, so intra-pulmonary pressure rises above atmospheric.
3. Air is expelled.

• Normal quiet expiration is largely passive (relaxation).

Forceful breathing: additional muscles in the abdomen help increase the strength of inspiration and expiration.

Rate: a healthy human breathes 12-16 times per minute. Volumes of air in breathing are measured with a spirometer, which helps in clinical assessment of lung function.

Flow chart:
Diaphragm + external intercostals contract → thoracic volume ↑ → lung volume ↑ → lung pressure ↓ (below atmosphere) → air in.
Muscles relax → thoracic volume ↓ → lung pressure ↑ (above atmosphere) → air out.

Worked example

A patient's diaphragm is paralysed, but the intercostal muscles work normally. (a) Can the patient still inhale? Along which axis will the thoracic volume fail to increase? (b) Why does a normal quiet exhalation need no muscular effort at all? (3 marks)

Try it first, then show the solutionSolution

1. (a) Yes, partly: the external intercostal muscles can still lift the ribs and sternum, enlarging the thorax in the dorso-ventral axis and lowering lung pressure below atmospheric. (1)
2. But the paralysed diaphragm cannot contract and flatten, so the antero-posterior increase in volume is lost and each breath is shallower. (1)
3. (b) In quiet breathing the diaphragm and intercostals simply relax back to their resting positions; thoracic and lung volume fall, intra-pulmonary pressure rises slightly above atmospheric and air flows out. (1)

Answer: Shallow breathing by the intercostals only (no antero-posterior increase); quiet expiration is passive relaxation.

Respiratory volumes and capacities

NCERT section 14.2.1 Respiratory Volumes and Capacities

Volumes (approximate values for a healthy adult):

• Tidal volume (TV): air inspired or expired in a normal breath: about 500 mL. So a healthy person moves about 6000-8000 mL of air per minute (TV × breathing rate).
• Inspiratory reserve volume (IRV): extra air that can be inspired by a forcible inspiration: about 2500-3000 mL.
• Expiratory reserve volume (ERV): extra air that can be expired by a forcible expiration: about 1000-1100 mL.
• Residual volume (RV): air that remains in the lungs even after a forcible expiration: about 1100-1200 mL.

Capacities (sums of volumes):

• Inspiratory capacity (IC) = TV + IRV: total air a person can inspire after a normal expiration.
• Expiratory capacity (EC) = TV + ERV: total air a person can expire after a normal inspiration.
• Functional residual capacity (FRC) = ERV + RV: air left in the lungs after a normal expiration.
• Vital capacity (VC) = ERV + TV + IRV: the maximum air a person can breathe in after a forced expiration, or breathe out after a forced inspiration.
• Total lung capacity (TLC) = VC + RV = RV + ERV + TV + IRV: total air the lungs can hold at the end of a forced inspiration.

Why it matters: residual volume keeps the alveoli from collapsing and allows continuous gas exchange even between breaths; vital capacity is higher in athletes and lower in lung diseases.

Worked example

A person has TV = 500 mL, IRV = 2800 mL, ERV = 1000 mL and RV = 1200 mL. Calculate vital capacity and total lung capacity. (2 marks)

Try it first, then show the solutionSolution

1. VC = IRV + TV + ERV = 2800 + 500 + 1000 = 4300 mL. (1)
2. TLC = VC + RV = 4300 + 1200 = 5500 mL. Check: RV + ERV + TV + IRV = 1200 + 1000 + 500 + 2800 = 5500 mL. (1)

Answer: VC = 4300 mL; TLC = 5500 mL.

Exchange of gases

NCERT section 14.3 Exchange of Gases

Sites: alveoli (primary site) and tissues. Exchange occurs by simple diffusion, mainly based on the pressure/concentration gradient.

Factors affecting the rate of diffusion:
1. Partial pressure gradient: the pressure a gas contributes in a mixture (pO₂ for oxygen, pCO₂ for carbon dioxide).
2. Solubility of the gases: CO₂ is 20-25 times more soluble than O₂, so much more CO₂ diffuses per unit difference in partial pressure.
3. Thickness of the membrane: the diffusion membrane is less than a millimetre thick.

Partial pressures (mm Hg):

• Atmospheric air: pO₂ 159; pCO₂ 0.3.
• Alveoli: pO₂ 104; pCO₂ 40.
• Deoxygenated blood: pO₂ 40; pCO₂ 45.
• Oxygenated blood: pO₂ 95; pCO₂ 40.
• Tissues: pO₂ 40; pCO₂ 45.

So O₂ moves alveoli (104) → blood (40) and blood (95) → tissues (40); CO₂ moves tissues (45) → blood (40) and blood (45) → alveoli (40). The gradient for CO₂ is smaller, but its high solubility compensates.

Diffusion membrane (three layers):
1. Thin squamous epithelium of the alveoli.
2. Endothelium of alveolar capillaries.
3. Basement substance between them.
Total thickness is much less than a millimetre, so all the factors favour diffusion of O₂ from alveoli to tissues and CO₂ from tissues to alveoli.

Worked example

The pCO₂ difference between deoxygenated blood and alveoli is only 5 mm Hg, while the pO₂ difference is 64 mm Hg. How can CO₂ still be removed efficiently? (2 marks)

Try it first, then show the solutionSolution

1. pO₂: 104 − 40 = 64 mm Hg; pCO₂: 45 − 40 = 5 mm Hg. (1)
2. CO₂ is 20-25 times more soluble than O₂, so even a small partial pressure gradient moves a large amount across the very thin (<1 mm) membrane. (1)

Answer: High solubility of CO₂ compensates for its small gradient.

Transport of oxygen and carbon dioxide

NCERT section 14.4 Transport of Gases (14.4.1 Transport of Oxygen; 14.4.2 Transport of Carbon dioxide)

Blood carries both gases.

• O₂: about 97% by RBCs (as oxyhaemoglobin); 3% dissolved in plasma.
• CO₂: about 20-25% by RBCs (as carbamino-haemoglobin); about 70% as bicarbonate; about 7% dissolved in plasma.

Oxygen transport:

• Haemoglobin is a red, iron-containing pigment in RBCs. O₂ binds reversibly to form oxyhaemoglobin. Each haemoglobin molecule can carry up to four O₂ molecules.
• Binding depends mainly on pO₂; it is also affected by pCO₂, H⁺ concentration and temperature.
• Oxygen dissociation curve: % saturation of haemoglobin plotted against pO₂ gives a sigmoid curve. It helps explain how pCO₂, H⁺ etc. affect binding.
• In the alveoli: high pO₂, low pCO₂, lower H⁺ and lower temperature favour formation of oxyhaemoglobin.
• In the tissues: low pO₂, high pCO₂, high H⁺ and higher temperature favour dissociation, releasing O₂.
• Every 100 mL of oxygenated blood delivers about 5 mL of O₂ to the tissues under normal physiological conditions.

Carbon dioxide transport:

• Carbamino-haemoglobin: about 20-25% of CO₂ binds to haemoglobin. This depends on pCO₂; pO₂ also affects it. High pCO₂ and low pO₂ (tissues) favour binding; low pCO₂ and high pO₂ (alveoli) favour release of CO₂.
• Bicarbonate: RBCs (and to a small extent plasma) contain carbonic anhydrase, which speeds:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺

• At tissues (high pCO₂), CO₂ forms HCO₃⁻ and H⁺. At alveoli (low pCO₂), the reaction runs backwards, releasing CO₂ and water.
• Every 100 mL of deoxygenated blood delivers about 4 mL of CO₂ to the alveoli.

Worked example

Why does oxyhaemoglobin release O₂ more readily in an exercising muscle than at rest? (3 marks)

Try it first, then show the solutionSolution

1. Exercising muscle uses more O₂ and makes more CO₂, so pO₂ falls and pCO₂ rises. (1)
2. More CO₂ forms more H⁺ (via carbonic anhydrase), and muscle temperature rises. (1)
3. Low pO₂, high pCO₂, high H⁺ and higher temperature all favour dissociation of oxyhaemoglobin, so more O₂ is released. (1)

Answer: Low pO₂, high pCO₂, high H⁺ and warmth all push O₂ off haemoglobin.

Regulation of respiration

NCERT section 14.5 Regulation of Respiration

Humans can adjust their breathing rhythm to the demands of the body tissues, through the neural system.

• Respiratory rhythm centre: a specialised centre in the medulla region of the brain; primarily responsible for this regulation.
• Pneumotaxic centre: in the pons; it can moderate the functions of the rhythm centre. A neural signal from it can reduce the duration of inspiration and thereby alter the respiratory rate.
• Chemosensitive area: next to the rhythm centre; highly sensitive to CO₂ and hydrogen ions. A rise in these substances activates this area, which signals the rhythm centre to make adjustments so that these substances are eliminated.
• Receptors associated with the aortic arch and carotid artery also recognise changes in CO₂ and H⁺ and send signals to the rhythm centre for remedial action.
• The role of oxygen in regulating the respiratory rhythm is quite insignificant.

Flow chart: exercise → CO₂ and H⁺ rise → chemosensitive area and aortic/carotid receptors → respiratory rhythm centre (medulla) → faster, deeper breathing → CO₂ removed → levels return to normal.

Worked example

Predict the change in breathing in each case and name the structure responsible: (a) signals from the pneumotaxic centre are blocked; (b) a person sits in a crowded, closed room where CO₂ rises but O₂ stays almost normal; (c) a person breathes air that is low in O₂ but normal in CO₂ for a short time. (3 marks)

Try it first, then show the solutionSolution

1. (a) The pneumotaxic centre in the pons normally shortens inspiration; without its signal each inspiration lasts longer and the breathing rate changes (slows). (1)
2. (b) Rising CO₂ and H⁺ stimulate the chemosensitive area beside the medullary rhythm centre and the receptors of the aortic arch and carotid artery, so the rhythm centre speeds up breathing. (1)
3. (c) Little change at first: the role of O₂ in regulating the respiratory rhythm is quite insignificant compared with CO₂ and H⁺. (1)

Answer: (a) Longer inspiration, altered (slower) rate; (b) faster breathing via the chemosensitive area and aortic/carotid receptors; (c) little immediate change, since O₂ plays a minor role.

Disorders of the respiratory system

NCERT section 14.6 Disorders of Respiratory System

Asthma:

• Difficulty in breathing causing wheezing, due to inflammation of bronchi and bronchioles.
• Air passages narrow, so less air reaches the alveoli. Triggers often include dust, pollen and smoke.

Emphysema:

• A chronic disorder in which alveolar walls are damaged, so the respiratory surface decreases.
• A major cause is cigarette smoking.
• With fewer, larger air sacs, less O₂ diffuses into blood and the person tires easily.

Occupational respiratory disorders (listed in the 2026-27 syllabus):

• In some industries, especially those involving grinding or stone-breaking (stone quarries, mines, cement, glass and asbestos factories), so much dust is produced that the body's defences cannot cope.
• Long exposure causes inflammation leading to fibrosis (proliferation of fibrous tissue) and serious lung damage. Examples: silicosis (silica dust) and asbestosis (asbestos fibres).
• Workers in such industries should wear protective masks.

Comparison: asthma affects the air passages (reversible narrowing); emphysema and fibrosis damage the alveolar exchange surface (long-term).

Worked example

Distinguish between asthma and emphysema in terms of the part affected and the cause. (2 marks)

Try it first, then show the solutionSolution

1. Asthma: inflammation of bronchi and bronchioles causing wheezing and difficult breathing. (1)
2. Emphysema: chronic damage to alveolar walls reducing the respiratory surface; mainly caused by cigarette smoking. (1)

Answer: Asthma: inflamed airways; emphysema: damaged alveoli (smoking).

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 and Transport of Oxygen and CO2
• Mechanism of Breathing and Lung Volumes

Breathing and Exchange of Gases: key facts

Vital capacity

$\text{VC}=\text{IRV}+\text{TV}+\text{ERV}$

Maximum air breathed out after a forced inspiration.

Total lung capacity

$\text{TLC}=\text{VC}+\text{RV}=\text{RV}+\text{ERV}+\text{TV}+\text{IRV}$

Air in the lungs after a forced inspiration.

Inspiratory, expiratory and functional residual capacity

$\text{IC}=\text{TV}+\text{IRV};\text{EC}=\text{TV}+\text{ERV};\text{FRC}=\text{ERV}+\text{RV}$

Capacities are sums of volumes.

Minute (pulmonary) ventilation

$\text{air per minute}=\text{TV}\times \text{breaths per minute}$

500 mL × 12-16 ≈ 6000-8000 mL/min.

Bicarbonate formation

${\text{CO}}_2+{\text{H}}_2\text{O}\rightleftharpoons {\text{H}}_2{\text{CO}}_3\rightleftharpoons {\text{HCO}}_3^{-}+{\text{H}}^{+}$

Catalysed by carbonic anhydrase; forward at tissues, backward at alveoli.

Oxyhaemoglobin

$\text{Hb}+4{\text{O}}_2\rightleftharpoons {\text{Hb(O}}_2{\text{)}}_4$

Formed at high pO₂ (alveoli); dissociates at low pO₂ (tissues).

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.

• Air path: No Pain, Lots To Breathe, Better Alveoli

  Nostrils, Pharynx, Larynx, Trachea, Bronchi, Bronchioles, Alveoli.

  Use it when: Path of air.

• 5-25-11-12

  TV 500, IRV 2500-3000, ERV 1000-1100, RV 1100-1200 (all mL).

  Use it when: Lung volume values.

• Capacity = sum; VC is 'Very Complete' minus RV

  VC = IRV + TV + ERV; add RV to get TLC.

  Use it when: Capacity formulas.

• 104-40, 95-40, 40-45

  Alveoli pO₂ 104/pCO₂ 40; oxygenated blood 95/40; deoxygenated blood and tissues 40/45.

  Use it when: Partial pressure table.

• 97-3 and 70-23-7

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

  Use it when: Transport percentages.

• Tissues are Hot, Sour and Stuffy

  Higher temperature, higher H⁺, higher pCO₂ and lower pO₂ at tissues make haemoglobin unload O₂.

  Use it when: Dissociation factors.

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 E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 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 the diaphragm relaxes during inspiration

  The upward dome is thought to be the active position.

  ✓Do this: Diaphragm contracts (flattens) in inspiration and relaxes in expiration.

  Usually costs: 1 mark.

• ✗Mistake: Including residual volume in vital capacity

  All volumes are added.

  ✓Do this: VC = IRV + TV + ERV; residual volume can never be breathed out.

  Usually costs: 1 mark.

• ✗Mistake: Writing that most CO₂ is carried by haemoglobin

  Haemoglobin is linked with gas transport in general.

  ✓Do this: About 70% travels as bicarbonate; only 20-25% as carbamino-haemoglobin.

  Usually costs: 1 mark.

• ✗Mistake: Stating that low O₂ is the main stimulus for breathing

  Common-sense assumption.

  ✓Do this: CO₂ and H⁺ are the main stimuli; O₂'s role is insignificant.

  Usually costs: 1 mark.

• ✗Mistake: Mixing the epiglottis and the glottis

  Similar names.

  ✓Do this: Glottis is the opening of the larynx; the epiglottis is the flap that covers it during swallowing.

  Usually costs: ½ mark.

NCERT exercise guide: Breathing and Exchange of Gases

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

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 volume of air remaining in the lungs after a forcible expiration is:
   (a) tidal volume (b) residual volume (c) vital capacity (d) expiratory reserve volume

   Hint

   Cannot be breathed out.

   Answer

   (b) residual volume

2. Q2 · 1 mark · MCQExam level

   About what percentage of CO₂ is carried as bicarbonate?
   (a) 7% (b) 25% (c) 70% (d) 97%

   Hint

   The largest share.

   Answer

   (c) 70%

3. Q3 · 2 marks · Very short answerExam level

   List the five steps involved in respiration. (2 marks)

   Hint

   Ventilation to cellular respiration.

   Answer

   Breathing (ventilation); diffusion of gases across alveolar membrane; transport by blood; diffusion between blood and tissues; utilisation of O₂ by cells (cellular respiration) (2).

4. Q4 · 2 marks · Very short answerHarder exam level

   Why are the trachea and bronchi supported by cartilaginous rings, and why are the rings incomplete? (2 marks)

   Hint

   Keeping airways open.

   Answer

   Rings keep the airways open so they do not collapse when pressure changes during breathing (1). They are incomplete (open at the back), allowing some flexibility of the tube, for example so the oesophagus behind the trachea can expand during swallowing (1).

5. Q5 · 2 marks · Very short answerHarder exam level

   State two functions of the conducting part of the respiratory system other than carrying air. (2 marks)

   Hint

   Clean, wet, warm.

   Answer

   It removes foreign particles from the air (1) and humidifies the air and brings it to body temperature (1).

6. Q6 · 2 marks · Very short answerHarder exam level

   What conditions in the alveoli favour the formation of oxyhaemoglobin? (2 marks)

   Hint

   Four factors.

   Answer

   High pO₂ and low pCO₂ (1); lower H⁺ concentration and lower temperature (1).

7. Q7 · 3 marks · Short answerHarder exam level

   Explain the roles of the pneumotaxic centre and the chemosensitive area in regulating breathing. (3 marks)

   Hint

   Pons; CO₂ and H⁺.

   Answer

   Pneumotaxic centre in the pons moderates the rhythm centre; its signal shortens inspiration and changes the breathing rate (1½). Chemosensitive area next to the rhythm centre senses rising CO₂ and H⁺ and signals the rhythm centre to increase ventilation and remove them; receptors in the aortic arch and carotid artery do the same (1½).

8. Q8 · 3 marks · Short answerHarder exam level

   Using the partial pressure values, explain the exchange of gases at the alveoli. (3 marks)

   Hint

   104/40 vs 40/45.

   Answer

   Alveolar air: pO₂ 104, pCO₂ 40; deoxygenated blood: pO₂ 40, pCO₂ 45 (1). O₂ diffuses from alveoli to blood (104 → 40); CO₂ from blood to alveoli (45 → 40) (1). Diffusion is aided by CO₂'s high solubility and the very thin three-layered membrane; blood leaving has pO₂ 95, pCO₂ 40 (1).

9. Q9 · 5 marks · Long answerHarder exam level

   (a) Explain how O₂ is transported in blood and released at tissues. (b) Use the shape of the oxygen dissociation curve to explain why a small fall in alveolar pO₂ hardly changes how much O₂ blood picks up, while a small fall in tissue pO₂ releases a lot of O₂. (5 marks)

   Hint

   97% Hb; flat top, steep middle.

   Answer

   (a) 97% of O₂ binds haemoglobin (iron-containing, up to four O₂ each) as oxyhaemoglobin; 3% dissolves in plasma (1). In alveoli, high pO₂, low pCO₂, low H⁺ and lower temperature favour loading; in tissues, low pO₂, high pCO₂, high H⁺ and higher temperature favour unloading; 100 mL of blood delivers about 5 mL O₂ (2). (b) The curve of % saturation against pO₂ is sigmoid: it is nearly flat at high pO₂, so a slight drop in alveolar pO₂ (104 mm Hg) still leaves haemoglobin almost saturated (1). Around tissue values (pO₂ about 40 mm Hg) the curve is steep, so a small further fall in pO₂ in active tissue unloads a large amount of O₂ (1).

10. Q10 · 3 marks · Short answerHOTS / competency

    Suppose a person breathes rapidly and deeply for a minute (hyperventilation) and then finds that the urge to breathe disappears for a while. Explain using the regulation of respiration. (3 marks)

    Hint

    What leaves the blood?

    Answer

    Hyperventilation removes extra CO₂ from the blood, lowering pCO₂ and H⁺ (1). The chemosensitive area and aortic/carotid receptors therefore receive little stimulus (1). The rhythm centre slows breathing until CO₂ builds up again; O₂ levels play little role, so the urge to breathe is delayed (1).

11. Q11 · 3 marks · Short answerHOTS / competency

    In emphysema, many small alveoli merge into fewer large sacs. Using the factors affecting diffusion, explain why a patient becomes breathless even at rest. (3 marks)

    Hint

    Surface area.

    Answer

    Diffusion depends on surface area, membrane thickness and partial pressure gradient (1). Merging alveoli greatly reduce the total respiratory surface, so less O₂ can cross into blood per breath (1). To get enough O₂ the patient must breathe faster and harder, feeling breathless even at rest; smoking is a major cause (1).

12. Q12 · 1 mark · Assertion-reasonHOTS / competency

    Assertion (A): The role of oxygen in regulating the respiratory rhythm is quite insignificant.
    Reason (R): The chemosensitive area of the medulla is highly sensitive to CO₂ and H⁺.
    (a) Both A and R are true and R is the correct explanation of A.
    (b) Both A and R are true but R is not the correct explanation of A.
    (c) A is true but R is false.
    (d) A is false but R is true.

    Hint

    Who drives the rhythm?

    Answer

    (a)

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.At which vertebra does the trachea divide?Show answer

   The 5th thoracic vertebra.

2. 2.What reduces friction on the lung surface?Show answer

   Pleural fluid between the two pleural membranes.

3. 3.How many times does a healthy human breathe per minute?Show answer

   12-16 times.

4. 4.Which instrument measures respiratory volumes?Show answer

   Spirometer.

5. 5.What is the approximate residual volume?Show answer

   1100-1200 mL.

6. 6.What is the pO₂ of alveolar air?Show answer

   104 mm Hg.

7. 7.How many times more soluble is CO₂ than O₂?Show answer

   20-25 times.

8. 8.What percentage of O₂ is carried dissolved in plasma?Show answer

   About 3%.

9. 9.How much O₂ does 100 mL of oxygenated blood deliver to tissues?Show answer

   About 5 mL.

10. 10.Which enzyme speeds bicarbonate formation in RBCs?Show answer

    Carbonic anhydrase.

11. 11.Where is the pneumotaxic centre?Show answer

    In the pons.

12. 12.Name the shape of the oxygen dissociation curve.Show answer

    Sigmoid (S-shaped).

Quick revision checklist

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

• ☐Can you name the respiratory organs of an insect, a fish and an earthworm?
• ☐Can you trace air from the nostrils to the alveoli?
• ☐Can you explain inspiration and expiration in four steps each?
• ☐Can you calculate VC and TLC from given volumes?
• ☐Can you write the partial pressure table from memory?
• ☐Can you give the percentages for O₂ and CO₂ transport?
• ☐Can you draw the oxygen dissociation curve and list the factors that shift unloading?
• ☐Can you distinguish asthma, emphysema and occupational lung disease?

Breathing and Exchange of Gases: frequently asked questions