Breathing and Exchange of Gases

What to remember

• Inspiration is an active process: the diaphragm and the external intercostal muscles contract, the thoracic volume increases, the pulmonary pressure falls below atmospheric pressure, and air moves in. Expiration is usually passive.
• Gases move by diffusion along partial pressure gradients: alveolar pO₂ is about 104 mm Hg and pCO₂ about 40 mm Hg; deoxygenated blood has pO₂ about 40 mm Hg and pCO₂ about 45 mm Hg. CO₂ is about 20–25 times more soluble than O₂, so a small gradient is enough for CO₂.
• Oxygen travels mainly as oxyhaemoglobin (about 97%); carbon dioxide travels mainly as bicarbonate (about 70%), with about 20–25% as carbamino-haemoglobin and about 7% dissolved in plasma. Carbonic anhydrase in red blood cells speeds up the CO₂ and water reaction.
• Breathing rhythm is set by the respiratory rhythm centre in the medulla, moderated by the pneumotaxic centre in the pons, and adjusted by the chemosensitive area (sensitive to CO₂ and H⁺) and by receptors in the aortic arch and carotid artery.

Respiratory organs across animal groups

Animals exchange O₂ and CO₂ with the environment. The organ used depends on the habitat and the body plan.

Human respiratory system

The system has a conducting part and an exchange part.

Path of air: external nostrils → nasal chamber → nasopharynx → larynx → trachea → right and left primary bronchi → secondary and tertiary bronchi → bronchioles → terminal bronchioles → respiratory bronchioles → alveolar ducts → alveoli.

• The nasal chamber has a mucus-lined passage. Hairs and mucus trap dust, and the air is moistened and brought to body temperature.
• The pharynx is a common passage for food and air.
• The larynx is a cartilaginous box that helps in sound production. It is called the sound box. The glottis is the opening of the larynx and is covered by a thin elastic cartilaginous flap called the epiglottis, which prevents food from entering the larynx during swallowing.
• The trachea is a straight tube extending up to the mid-thoracic cavity. It divides at the level of the 5th thoracic vertebra into right and left primary bronchi. Tracheal rings are incomplete (C-shaped) cartilage rings, which keep the tube open.
• Each bronchus divides repeatedly. The last bronchioles end in very thin, irregular-walled, vascularised bag-like structures called alveoli.

Lungs and pleura. The two lungs are situated in the thoracic chamber. The right lung has three lobes and the left lung has two lobes. Each lung is covered by a double-layered pleura with pleural fluid between the layers. The fluid reduces friction on the lung surface during movement.

Thoracic chamber. It is an anatomically air-tight chamber. Its boundaries are the vertebral column dorsally, the sternum ventrally, the ribs laterally, and the dome-shaped diaphragm on the lower side. This arrangement means that any change in the volume of the thoracic cavity is reflected in the lung (pulmonary) cavity. The lungs cannot be directly emptied or filled, so the volume of the thoracic cavity must be changed.

Alveolar wall. The diffusion membrane (respiratory membrane) has three layers: the thin squamous epithelium of the alveoli, the endothelium of the alveolar capillaries, and the basement substance between them. Its total thickness is less than a millimetre.

Mechanism of breathing

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

• Quiet expiration is mainly passive. Additional muscles in the abdomen can help to increase strength of inspiration and expiration.
• An average healthy human breathes 12–16 times per minute.
• The volume of air breathed in and out is estimated by a spirometer, which helps in clinical assessment of lung function.

Respiratory volumes and capacities

Volumes are single amounts. Capacities are sums of two or more volumes.

• Vital capacity is the maximum volume of air a person can breathe out after a forced inspiration.
• A healthy human can inspire or expire approximately 6000 to 8000 mL of air per minute (TV × breathing rate; 500 mL × 12–16 gives this figure).
• Residual volume cannot be expired, so TLC is always larger than VC.

Exchange of gases

Alveoli are the primary sites of gas exchange. Exchange also occurs between blood and tissues. O₂ and CO₂ are exchanged by simple diffusion, driven by a pressure or concentration gradient. Factors that affect diffusion are the pressure gradient, the solubility of the gases, and the thickness of the membranes involved.

• In the alveoli, O₂ diffuses from alveolar air (104) to deoxygenated blood (40). CO₂ diffuses from blood (45) to alveolar air (40).
• In the tissues, O₂ diffuses from blood to tissues (pO₂ 40 in tissue, lower than in oxygenated blood), and CO₂ diffuses from tissues (45) to blood (40).
• The solubility of CO₂ is 20–25 times higher than that of O₂. So the amount of CO₂ that diffuses through the membrane per unit difference in partial pressure is much higher than that of O₂.
• The diffusion membrane is very thin, and the total surface of alveoli is large, which favours diffusion.

Transport of gases

Transport of oxygen. About 97% of O₂ is transported by red blood cells as oxyhaemoglobin. The remaining 3% is carried in a dissolved state through the plasma. Haemoglobin is a red-coloured, iron-containing pigment of the RBCs. Each haemoglobin molecule can carry a maximum of four molecules of O₂. Binding of O₂ with haemoglobin is mainly related to the partial pressure of O₂. Other factors are pCO₂, H⁺ concentration and temperature.

• A graph of percentage saturation of haemoglobin with O₂ against pO₂ is the oxygen dissociation curve. It is sigmoid (S-shaped).
• In the alveoli, there is high pO₂, low pCO₂, lesser H⁺ concentration and lower temperature. These favour the formation of oxyhaemoglobin.
• In the tissues, there is low pO₂, high pCO₂, high H⁺ concentration and higher temperature. These favour the dissociation of oxygen from oxyhaemoglobin.
• So the curve shifts to the right (more release of O₂ to tissues) when pCO₂ rises, H⁺ rises (pH falls) or temperature rises. The curve shifts to the left under the opposite conditions.
• Every 100 mL of oxygenated blood can deliver around 5 mL of O₂ to the tissues under normal physiological conditions.

Transport of carbon dioxide.

• Binding of CO₂ with haemoglobin depends on pCO₂ and the amount of oxygenated haemoglobin. In the tissues, pCO₂ is high and pO₂ is low, so more CO₂ binds. In the alveoli, pCO₂ is low and pO₂ is high, so CO₂ dissociates from carbamino-haemoglobin.
• RBCs contain a very high concentration of the enzyme carbonic anhydrase. A minute quantity is present in plasma too. The reaction is: CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺.
• At the tissue site, where pCO₂ is high, CO₂ combines with water to form carbonic acid, which dissociates into bicarbonate and H⁺. At the alveolar site, where pCO₂ is low, the reaction proceeds in the opposite direction and CO₂ is released.
• Every 100 mL of deoxygenated blood delivers approximately 4 mL of CO₂ to the alveoli.

Regulation of respiration

Humans can maintain the respiratory rhythm and adjust it to meet the needs of the body. This is done by the neural system.

• The role of oxygen in the regulation of respiratory rhythm is quite insignificant.

Disorders of the respiratory system

• Asthma: a difficulty in breathing causing wheezing due to inflammation of bronchi and bronchioles.
• Emphysema: a chronic disorder in which the alveolar walls are damaged, due to which respiratory surface is decreased. One of the major causes is cigarette smoking.
• Occupational respiratory disorders: in certain industries, such as grinding or stone-breaking, so much dust is produced that the defence mechanism of the body cannot fully cope with the situation. Long exposure can cause inflammation leading to fibrosis (proliferation of fibrous connective tissue) and serious lung damage. Workers in such industries should wear protective masks.

NEET traps

• Residual volume is not part of vital capacity. VC = ERV + TV + IRV; TLC = VC + RV.
• Functional residual capacity is ERV + RV, not TV + RV. Expiratory capacity is TV + ERV.
• Inspiratory capacity is TV + IRV; do not add ERV to it.
• The pneumotaxic centre is in the pons, but the rhythm centre is in the medulla. The pneumotaxic centre moderates; it does not generate the rhythm.
• The chemosensitive area responds to CO₂ and H⁺, not mainly to O₂. The role of O₂ in regulation is insignificant.
• Most O₂ travels as oxyhaemoglobin (about 97%), but most CO₂ travels as bicarbonate (about 70%), not as carbamino-haemoglobin (about 20–25%).
• A rise in temperature, pCO₂ or H⁺ shifts the dissociation curve to the right (more O₂ release), not to the left.
• Carbonic anhydrase is abundant in RBCs; the CO₂ reaction forward direction occurs at tissues and reverse direction at alveoli.
• Partial pressure of O₂ in alveoli (104 mm Hg) differs from that in the atmosphere (159 mm Hg); pO₂ of deoxygenated blood and tissue are the same (40 mm Hg).
• Emphysema damages alveolar walls; asthma is inflammation of bronchi and bronchioles. Do not interchange them.

One-liners

• Sponges, coelenterates and flatworms exchange gases over the body surface.
• Insects use tracheal tubes; fishes use gills; earthworms use the moist cuticle.
• The glottis is protected by the epiglottis during swallowing.
• The trachea divides into two primary bronchi at about the level of the 5th thoracic vertebra.
• The pleura is double-layered with pleural fluid between its layers.
• The right lung has three lobes and the left lung has two lobes.
• The diffusion membrane has three layers and is thinner than a millimetre.
• Tidal volume is about 500 mL.
• Vital capacity is about 3500–4000 mL and total lung capacity about 5000–6000 mL.
• Alveolar pO₂ is about 104 mm Hg and pCO₂ about 40 mm Hg.
• Each haemoglobin molecule can bind a maximum of four O₂ molecules.
• 100 mL of oxygenated blood delivers about 5 mL of O₂ to tissues.
• 100 mL of deoxygenated blood delivers about 4 mL of CO₂ to alveoli.
• The oxygen dissociation curve is sigmoid.
• Smoking is a major cause of emphysema.

Practice questions

1. Which structure is the respiratory organ of insects?

   1. Gills
   2. Tracheal tubes
   3. Moist cuticle
   4. Body surface only
   Answer

   B. Tracheal tubes

   Insects exchange gases through a network of tracheal tubes.

2. Which structure covers the glottis and prevents food from entering the larynx during swallowing?

   1. Thyroid cartilage
   2. Pharynx
   3. Tracheal ring
   4. Epiglottis
   Answer

   D. Epiglottis

   The glottis is covered by a thin elastic cartilaginous flap called the epiglottis.

3. Vital capacity is equal to

   1. ERV + RV
   2. TV + ERV
   3. ERV + TV + IRV
   4. TV + IRV
   Answer

   C. ERV + TV + IRV

   Vital capacity is the maximum air expired after a forced inspiration: ERV + TV + IRV.

4. The approximate value of functional residual capacity in a healthy adult is

   1. 2100–2300 mL
   2. 3000–3500 mL
   3. 5000–6000 mL
   4. 1500–1600 mL
   Answer

   A. 2100–2300 mL

   FRC = ERV + RV, which is about 2100–2300 mL.

5. The partial pressure of oxygen in the alveoli is about

   1. 40 mm Hg
   2. 104 mm Hg
   3. 159 mm Hg
   4. 0.3 mm Hg
   Answer

   B. 104 mm Hg

   Alveolar pO₂ is about 104 mm Hg; atmospheric pO₂ is 159 mm Hg.

6. The largest share of carbon dioxide in blood is transported as

   1. Bicarbonate
   2. Carbonic anhydrase
   3. Dissolved CO₂ in plasma
   4. Carbamino-haemoglobin
   Answer

   A. Bicarbonate

   About 70% of CO₂ is carried as bicarbonate.

7. The pneumotaxic centre is located in the

   1. Hypothalamus
   2. Medulla
   3. Cerebellum
   4. Pons
   Answer

   D. Pons

   The pneumotaxic centre lies in the pons region of the brain.

8. How many lobes does the human left lung have?

   1. Four
   2. Three
   3. Two
   4. One
   Answer

   C. Two

   The left lung has two lobes; the right lung has three.

9. Under normal conditions, 100 mL of oxygenated blood delivers about how much O₂ to the tissues?

   1. 10 mL
   2. 4 mL
   3. 5 mL
   4. 20 mL
   Answer

   C. 5 mL

   About 5 mL of O₂ is delivered per 100 mL of oxygenated blood.

10. The volume of air that remains in the lungs after a forceful expiration is about

    1. 1100–1200 mL
    2. 500 mL
    3. 3500–4000 mL
    4. 2500–3000 mL
    Answer

    A. 1100–1200 mL

    This is the residual volume, about 1100–1200 mL.

11. Which receptors send signals to the rhythm centre about changes in CO₂ and H⁺ concentration?

    1. Receptors in the alveoli
    2. Receptors in the diaphragm
    3. Receptors in the pleura
    4. Receptors in the aortic arch and carotid artery
    Answer

    D. Receptors in the aortic arch and carotid artery

    Receptors in the aortic arch and carotid artery recognise changes in CO₂ and H⁺.

12. The maximum number of O₂ molecules that one haemoglobin molecule can carry is

    1. Two
    2. Four
    3. Six
    4. Eight
    Answer

    B. Four

    Each haemoglobin molecule can carry a maximum of four O₂ molecules.

13. Actual gas exchange between air and blood in the lungs takes place in the

    1. Alveoli
    2. Trachea
    3. Primary bronchi
    4. Terminal bronchioles
    Answer

    A. Alveoli

    Alveoli (with their ducts) form the exchange part; the rest is conducting.

14. Total lung capacity is equal to

    1. ERV + RV
    2. TV + ERV
    3. TV + IRV
    4. VC + RV
    Answer

    D. VC + RV

    TLC = VC + RV, i.e., the sum of all four volumes.

15. The enzyme carbonic anhydrase occurs in very high concentration in

    1. Plasma only
    2. Platelets
    3. White blood cells
    4. Red blood cells
    Answer

    D. Red blood cells

    RBCs contain a very high concentration of carbonic anhydrase; plasma has only a minute quantity.

16. About what percentage of CO₂ is carried as carbamino-haemoglobin?

    1. 70%
    2. 7%
    3. 20–25%
    4. 97%
    Answer

    C. 20–25%

    About 20–25% of CO₂ is bound to haemoglobin as carbamino-haemoglobin.

17. Given below are two statements: Statement I: Inspiration occurs when the diaphragm and the external intercostal muscles contract. Statement II: During inspiration the intra-pulmonary pressure rises above the atmospheric pressure. Choose the correct option.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    C. Statement I is correct but Statement II is incorrect

    Statement I is correct. In inspiration the pulmonary volume rises and the pressure falls below atmospheric, so Statement II is incorrect.

18. Given below are two statements: Statement I: Residual volume can be expelled by forceful expiration. Statement II: Total lung capacity is the sum of vital capacity and residual volume. Choose the correct option.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    D. Statement I is incorrect but Statement II is correct

    Residual volume remains after forceful expiration, so Statement I is incorrect. TLC = VC + RV is correct.

19. Given below are two statements: Statement I: CO₂ is about 20–25 times more soluble than O₂. Statement II: Because of this, more CO₂ than O₂ diffuses through the membrane per unit difference in partial pressure. Choose the correct option.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    A. Both Statement I and Statement II are correct

    Both are correct; higher solubility gives faster CO₂ diffusion per unit pressure difference.

20. Which of the following statements are correct? 1. The left lung has three lobes. 2. The pleura is double-layered with pleural fluid between the layers. 3. Tracheal rings are incomplete cartilage rings.

    1. 1 and 2 only
    2. 1 and 3 only
    3. 2 and 3 only
    4. 1, 2 and 3
    Answer

    C. 2 and 3 only

    The left lung has two lobes, so 1 is wrong. 2 and 3 are correct.

21. Which of the following statements about the oxygen dissociation curve are correct? 1. It is sigmoid in shape. 2. A rise in temperature shifts it to the left. 3. Low pCO₂ favours binding of O₂ to haemoglobin.

    1. 1 and 2 only
    2. 1 and 3 only
    3. 2 and 3 only
    4. 1, 2 and 3
    Answer

    B. 1 and 3 only

    A rise in temperature shifts the curve to the right, so 2 is wrong. 1 and 3 are correct.

22. Given below are two statements: Statement I: Asthma is caused by damage to the alveolar walls. Statement II: The chemosensitive area is highly sensitive to O₂ rather than to CO₂. Choose the correct option.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    B. Both Statement I and Statement II are incorrect

    Asthma is inflammation of bronchi and bronchioles; the chemosensitive area is sensitive to CO₂ and H⁺. Both statements are incorrect.

23. Given below are two statements: Statement I: The respiratory rhythm centre is present in the medulla. Statement II: A signal from the pneumotaxic centre in the pons can reduce the duration of inspiration. Choose the correct option.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    A. Both Statement I and Statement II are correct

    Both statements are correct.

24. Which of the following statements are correct? 1. About 3% of O₂ is carried dissolved in plasma. 2. About 7% of CO₂ is carried dissolved in plasma. 3. About 70% of CO₂ is carried as carbamino-haemoglobin.

    1. 1 and 2 only
    2. 2 and 3 only
    3. 1 and 3 only
    4. 1, 2 and 3
    Answer

    A. 1 and 2 only

    About 70% of CO₂ is carried as bicarbonate, so 3 is wrong.

25. Given below are two statements: Statement I: Alveolar pCO₂ is about 45 mm Hg. Statement II: Deoxygenated blood and tissues both have a pO₂ of about 40 mm Hg. Choose the correct option.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    D. Statement I is incorrect but Statement II is correct

    Alveolar pCO₂ is about 40 mm Hg, so Statement I is incorrect. Statement II is correct.

26. Which of the following statements are correct? 1. The diaphragm contracts during inspiration. 2. Pleural fluid reduces friction on the lung surface. 3. The diffusion membrane has four layers.

    1. 1 and 3 only
    2. 1 and 2 only
    3. 2 and 3 only
    4. 1, 2 and 3
    Answer

    B. 1 and 2 only

    The diffusion membrane has three layers, so 3 is wrong.

27. Assertion (A): The pulmonary pressure falls below atmospheric pressure during inspiration. Reason (R): The volume of the thoracic cavity increases during inspiration. Choose the correct option.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    A. Both A and R are true and R is the correct explanation of A

    Larger volume lowers the pressure, so air flows in.

28. Assertion (A): Oxyhaemoglobin releases more O₂ in the tissues. Reason (R): Tissues have low pCO₂ and low H⁺ concentration. Choose the correct option.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    C. A is true but R is false

    A is true, but tissues have high pCO₂ and high H⁺, so R is false.

29. Assertion (A): Alveoli are part of the conducting region of the respiratory system. Reason (R): Alveoli are the sites of gas exchange. Choose the correct option.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    D. A is false but R is true

    Alveoli are part of the exchange region, so A is false. R is true.

30. Assertion (A): CO₂ moves from the tissues into the blood. Reason (R): The pCO₂ in the tissues (about 45 mm Hg) is higher than in the blood reaching them (about 40 mm Hg). Choose the correct option.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    A. Both A and R are true and R is the correct explanation of A

    Diffusion follows the partial pressure gradient.

31. Assertion (A): The receptors in the aortic arch and carotid artery are mainly sensitive to O₂. Reason (R): The role of O₂ in the regulation of respiratory rhythm is quite insignificant. Choose the correct option.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    D. A is false but R is true

    These receptors recognise changes in CO₂ and H⁺, so A is false. R is true.

32. Match List I with List II. List I: a. Tidal volume b. Inspiratory reserve volume c. Expiratory reserve volume d. Residual volume. List II: i. About 500 mL ii. About 2500–3000 mL iii. About 1000–1100 mL iv. About 1100–1200 mL.

    1. a-iii, b-ii, c-i, d-iv
    2. a-ii, b-i, c-iii, d-iv
    3. a-i, b-ii, c-iii, d-iv
    4. a-i, b-ii, c-iv, d-iii
    Answer

    C. a-i, b-ii, c-iii, d-iv

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

33. Match List I with List II. List I: a. Inspiratory capacity b. Expiratory capacity c. Functional residual capacity d. Vital capacity. List II: i. TV + ERV ii. ERV + RV iii. TV + IRV iv. ERV + TV + IRV.

    1. a-iii, b-i, c-iv, d-ii
    2. a-iii, b-i, c-ii, d-iv
    3. a-i, b-iii, c-ii, d-iv
    4. a-ii, b-iv, c-iii, d-i
    Answer

    B. a-iii, b-i, c-ii, d-iv

    IC = TV + IRV, EC = TV + ERV, FRC = ERV + RV, VC = ERV + TV + IRV.

34. Match List I with List II. List I: a. Earthworm b. Insects c. Fishes d. Amphibians. List II: i. Tracheal tubes ii. Gills iii. Moist cuticle iv. Lungs and moist skin.

    1. a-iii, b-ii, c-i, d-iv
    2. a-iii, b-i, c-ii, d-iv
    3. a-iv, b-i, c-ii, d-iii
    4. a-i, b-iii, c-ii, d-iv
    Answer

    B. a-iii, b-i, c-ii, d-iv

    Earthworm: moist cuticle; insects: tracheal tubes; fishes: gills; amphibians: lungs and skin.

35. Match List I with List II. List I: a. Atmospheric pO₂ b. Alveolar pCO₂ c. Deoxygenated blood pCO₂ d. Alveolar pO₂. List II: i. 40 mm Hg ii. 45 mm Hg iii. 104 mm Hg iv. 159 mm Hg.

    1. a-iv, b-i, c-iii, d-ii
    2. a-iii, b-i, c-ii, d-iv
    3. a-iv, b-i, c-ii, d-iii
    4. a-iv, b-ii, c-i, d-iii
    Answer

    C. a-iv, b-i, c-ii, d-iii

    159, 40, 45 and 104 mm Hg respectively.

36. Match List I with List II. List I: a. Rhythm centre b. Pneumotaxic centre c. Chemosensitive area d. Receptors for CO₂ and H⁺. List II: i. Pons ii. Medulla iii. Adjacent to the rhythm centre iv. Aortic arch and carotid artery.

    1. a-i, b-ii, c-iii, d-iv
    2. a-ii, b-i, c-iv, d-iii
    3. a-iii, b-i, c-ii, d-iv
    4. a-ii, b-i, c-iii, d-iv
    Answer

    D. a-ii, b-i, c-iii, d-iv

    Rhythm centre in medulla; pneumotaxic centre in pons; chemosensitive area next to rhythm centre; receptors in aortic arch and carotid artery.

37. Match List I with List II. List I: a. Asthma b. Emphysema c. Occupational lung disorder d. Epiglottis. List II: i. Alveolar walls damaged ii. Inflammation of bronchi and bronchioles iii. Fibrosis after long dust exposure iv. Covers the glottis.

    1. a-ii, b-i, c-iv, d-iii
    2. a-i, b-ii, c-iii, d-iv
    3. a-ii, b-i, c-iii, d-iv
    4. a-iii, b-i, c-ii, d-iv
    Answer

    C. a-ii, b-i, c-iii, d-iv

    Asthma: bronchial inflammation; emphysema: alveolar damage; occupational disorder: fibrosis; epiglottis covers the glottis.

38. A person has a vital capacity of 4000 mL, a tidal volume of 500 mL and an expiratory reserve volume of 1000 mL. What is the inspiratory reserve volume?

    1. 3500 mL
    2. 2500 mL
    3. 3000 mL
    4. 2000 mL
    Answer

    B. 2500 mL

    IRV = VC − TV − ERV = 4000 − 500 − 1000 = 2500 mL.

39. A person's total lung capacity is 5800 mL and vital capacity is 4600 mL. What is the residual volume?

    1. 1200 mL
    2. 1000 mL
    3. 1100 mL
    4. 1300 mL
    Answer

    A. 1200 mL

    RV = TLC − VC = 5800 − 4600 = 1200 mL.

40. A long-term smoker develops breathlessness because the walls of the alveoli are damaged and the respiratory surface is reduced. Which disorder is this?

    1. Fibrosis
    2. Asthma
    3. Emphysema
    4. Occupational lung disorder
    Answer

    C. Emphysema

    Damage to alveolar walls with reduced respiratory surface due to smoking is emphysema.

41. During heavy exercise, the working muscles show a rise in temperature and pCO₂. What is the effect on the oxygen dissociation curve and O₂ release?

    1. Curve shifts to the left; more O₂ is released
    2. Curve shifts to the right; more O₂ is released
    3. Curve does not shift; O₂ release is unchanged
    4. Curve shifts to the right; less O₂ is released
    Answer

    B. Curve shifts to the right; more O₂ is released

    Higher temperature, pCO₂ and H⁺ shift the curve right and favour dissociation of O₂.

42. A man breathes 14 times per minute with a tidal volume of 500 mL. How much air does he inspire per minute?

    1. 7000 mL
    2. 6500 mL
    3. 8500 mL
    4. 5000 mL
    Answer

    A. 7000 mL

    Minute volume = 500 mL × 14 = 7000 mL.

43. A worker in a stone-breaking industry develops fibrosis of the lungs after many years. What is the most likely reason?

    1. Damage to the alveolar walls caused by cigarette smoke
    2. Inflammation of the bronchi caused by an allergy
    3. Failure of the rhythm centre in the medulla
    4. Dust in such quantity that the body's defence mechanism cannot cope, leading to inflammation
    Answer

    D. Dust in such quantity that the body's defence mechanism cannot cope, leading to inflammation

    Heavy dust exposure overwhelms the defence mechanism and causes inflammation and fibrosis.

44. A person's blood shows a rise in CO₂ and H⁺ concentration. Which response is expected?

    1. The pneumotaxic centre stops the rhythm centre completely
    2. Breathing is slowed because O₂ is high
    3. The aortic arch receptors stop sending signals
    4. The chemosensitive area and arterial receptors stimulate the rhythm centre, increasing breathing
    Answer

    D. The chemosensitive area and arterial receptors stimulate the rhythm centre, increasing breathing

    The chemosensitive area and aortic and carotid receptors detect CO₂ and H⁺ and signal the rhythm centre to increase breathing for their removal.