🫁 Breathing & Exchange of Gases – MCQs

Part 1 — Q1–Q25

Q1. The primary muscle of normal (quiet) inspiration is:

a) External intercostals
b) Diaphragm
c) Internal intercostals
d) Abdominal muscles

Answer: b) Diaphragm
Explanations:

a) External intercostals assist inspiration by elevating ribs, but are secondary.
b) Diaphragm — correct; contraction increases thoracic volume and causes most of quiet inspiration.
c) Internal intercostals are primarily active in forced expiration.
d) Abdominal muscles assist forced expiration, not quiet inspiration.

Q2. Surfactant reduces surface tension in alveoli mainly by:

a) Increasing alveolar fluid
b) Breaking hydrogen bonds between water molecules
c) Increasing alveolar pressure
d) Stimulating type II pneumocyte proliferation

Answer: b) Breaking hydrogen bonds between water molecules
Explanations:

a) Increasing fluid would increase, not reduce, surface tension.
b) Surfactant (phospholipids + proteins) inserts at air–liquid interface, disrupting water hydrogen bonding → reduces surface tension; correct.
c) It lowers surface tension and helps prevent collapse, not increase alveolar pressure.
d) Type II cells produce surfactant; surfactant does not primarily stimulate their proliferation.

Q3. The functional residual capacity (FRC) equals:

a) Tidal volume + Inspiratory reserve volume
b) Expiratory reserve volume + Residual volume
c) Inspiratory reserve volume + Residual volume
d) Tidal volume + Expiratory reserve volume

Answer: b) Expiratory reserve volume + Residual volume
Explanations:

a) That sum equals inspiratory capacity, not FRC.
b) FRC = ERV + RV — correct; volume remaining in lungs after quiet expiration.
c) IRV + RV is not standard lung capacity.
d) TV + ERV = expiratory capacity? (actually it’s not standard); incorrect.

Q4. The major form in which CO₂ is transported in blood is:

a) Dissolved CO₂ in plasma
b) Bound to hemoglobin as carbaminohemoglobin
c) As bicarbonate ions (HCO₃⁻) in plasma
d) Combined with plasma proteins

Answer: c) As bicarbonate ions (HCO₃⁻) in plasma
Explanations:

a) Dissolved CO₂ accounts for ~5–10%, not the major form.
b) Carbamino-Hb carries some (~20–30%) but less than bicarbonate.
c) CO₂ is hydrated (via carbonic anhydrase) to H₂CO₃ then HCO₃⁻ — major (≈60–70%) form; correct.
d) Only small amounts bind nonspecifically to plasma proteins.

Q5. The Bohr effect describes:

a) The effect of CO₂ on hemoglobin oxygen affinity (right shift with ↑CO₂/↓pH)
b) The effect of O₂ on hemoglobin CO₂ affinity
c) Increased hemoglobin affinity for O₂ at higher temperatures
d) CO poisoning reducing O₂ carrying capacity

Answer: a) The effect of CO₂ on hemoglobin oxygen affinity (right shift with ↑CO₂/↓pH)
Explanations:

a) Bohr effect: ↑CO₂/↓pH reduces Hb–O₂ affinity → O₂ offloading in tissues — correct.
b) That’s Haldane effect (effect of O₂ on CO₂ binding), not Bohr.
c) Higher temperature decreases O₂ affinity (right shift), but that’s not the Bohr effect definition.
d) CO poisoning is unrelated to the Bohr effect.

Q6. Haldane effect refers to:

a) Effect of CO₂ on hemoglobin oxygen affinity
b) Effect of O₂ binding on CO₂ uptake/release by hemoglobin
c) Surfactant function at alveoli
d) Neural control of breathing

Answer: b) Effect of O₂ binding on CO₂ uptake/release by hemoglobin
Explanations:

a) That’s Bohr effect.
b) Haldane effect: oxygenation of Hb reduces its ability to carry CO₂ and H⁺, promoting CO₂ release in lungs — correct.
c) Unrelated.
d) Unrelated.

Q7. Which receptor primarily senses arterial PO₂ and stimulates ventilation when PO₂ falls?

a) Central chemoreceptors in medulla
b) Peripheral chemoreceptors in carotid and aortic bodies
c) Baroreceptors in carotid sinus
d) Pulmonary stretch receptors

Answer: b) Peripheral chemoreceptors in carotid and aortic bodies
Explanations:

a) Central chemoreceptors mainly respond to ↑PCO₂ / ↓pH in CSF, not direct O₂ sensing.
b) Peripheral chemoreceptors (carotid & aortic bodies) respond to ↓PaO₂ (especially <60 mmHg) — correct.
c) Baroreceptors sense BP changes.
d) Pulmonary stretch receptors respond to lung inflation.

Q8. Which would increase alveolar ventilation (VA) without changing metabolic CO₂ production?

a) Increase in dead space only
b) Increase in tidal volume with same respiratory rate
c) Increase in respiratory rate but decrease in tidal volume so minute ventilation unchanged
d) Increase in tidal volume while reducing anatomic dead space proportionally

Answer: d) Increase in tidal volume while reducing anatomic dead space proportionally
Explanations:

a) Increasing dead space reduces effective alveolar ventilation.
b) Increasing TV increases VA (because VA = (TV − VD) × f) — could be correct but statement b lacks dead space consideration; d is clearer.
c) If minute ventilation unchanged but TV decreased and rate increased, alveolar ventilation may fall due to proportionally larger dead space fraction.
d) Increasing TV while effectively lowering proportion of dead space increases (TV − VD) per breath → VA increases; correct.

Q9. Pulmonary surfactant is produced by:

a) Type I pneumocytes
b) Type II pneumocytes
c) Alveolar macrophages
d) Clara cells

Answer: b) Type II pneumocytes
Explanations:

a) Type I cells are thin for gas exchange; do not secrete surfactant.
b) Type II pneumocytes synthesize & secrete surfactant (phospholipids, proteins) — correct.
c) Macrophages clear debris, not surfactant production.
d) Clara (club) cells in bronchioles secrete protective proteins but not main surfactant.

Q10. A rightward shift of the oxygen–hemoglobin dissociation curve means:

a) Increased hemoglobin affinity for O₂
b) Decreased hemoglobin affinity for O₂ (easier O₂ release)
c) No change in O₂ delivery to tissues
d) Hemoglobin cannot bind O₂ at all

Answer: b) Decreased hemoglobin affinity for O₂ (easier O₂ release)
Explanations:

a) Left shift = increased affinity; so (a) is wrong.
b) Right shift (caused by ↑CO₂, ↑H⁺, ↑temp, ↑2,3-BPG) reduces affinity → facilitates O₂ unloading; correct.
c) It changes O₂ unloading; so not correct.
d) Curve shift doesn’t abolish binding.

Q11. Venous blood returning to the lungs has:

a) High PO₂ and low PCO₂
b) Low PO₂ and high PCO₂
c) Same PO₂ and PCO₂ as arterial blood
d) Extremely low pH (<6.8) normally

Answer: b) Low PO₂ and high PCO₂
Explanations:

a) Opposite of venous composition.
b) Venous blood carries O₂ to tissues and returns CO₂-rich, so low PO₂, high PCO₂; correct.
c) Arterial and venous differ.
d) Normal venous pH is ~7.35–7.4; pH <6.8 is pathological.

Q12. Which law describes the amount of gas dissolved in a liquid is proportional to its partial pressure?

a) Boyle’s law
b) Charles’s law
c) Henry’s law
d) Dalton’s law

Answer: c) Henry’s law
Explanations:

a) Boyle: pressure × volume constant (at constant T).
b) Charles: volume ∝ temperature (at constant P).
c) Henry’s law — correct; solubility ∝ partial pressure of gas.
d) Dalton: total pressure = sum of partial pressures.

Q13. The driving force for gas diffusion across the alveolar membrane is:

a) Concentration gradient of gases in air only
b) Difference in partial pressures of gas between alveolar air and capillary blood
c) Blood flow only
d) Thickness of alveolar cells only

Answer: b) Difference in partial pressures of gas between alveolar air and capillary blood
Explanations:

a) Concentration in air alone isn’t the physiological driver.
b) Fick’s law: diffusion rate ∝ area × (ΔP) / thickness — ΔP = partial pressure difference; correct.
c) Perfusion affects overall exchange but not the diffusion driving force.
d) Thickness influences rate but is not the driving force.

Q14. Physiologic dead space includes:

a) Anatomic dead space only (airways)
b) Alveoli that are ventilated but not perfused + anatomic dead space
c) Only alveoli perfectly matched for V/Q
d) Only alveoli perfectly perfused and unventilated

Answer: b) Alveoli that are ventilated but not perfused + anatomic dead space
Explanations:

a) Physiologic dead space = anatomic dead space + alveolar dead space.
b) Correct; alveoli with no perfusion (e.g., pulmonary embolism) contribute to dead space.
c/d) Incorrect descriptions.

Q15. Central chemoreceptors respond primarily to changes in:

a) Arterial PO₂
b) CSF pH (reflecting arterial PCO₂)
c) Arterial glucose
d) Systemic blood pressure

Answer: b) CSF pH (reflecting arterial PCO₂)
Explanations:

a) Peripheral chemoreceptors sense PaO₂.
b) Central chemoreceptors (medulla) detect H⁺ in CSF produced from CO₂ diffusion → respond mainly to arterial PCO₂ indirectly; correct.
c/d) Not primary stimuli for central chemoreceptors.

Q16. At high altitude, which change helps maintain arterial oxygen content?

a) Decreased 2,3-BPG in RBCs
b) Increased ventilation (hyperventilation) lowering PaCO₂
c) Decreased hemoglobin concentration
d) Lower pulmonary vascular resistance

Answer: b) Increased ventilation (hyperventilation) lowering PaCO₂
Explanations:

a) 2,3-BPG increases at altitude, facilitating O₂ unloading; decreased 2,3-BPG would be maladaptive.
b) Hyperventilation increases alveolar PO₂ → helps maintain PaO₂; correct.
c) Hemoglobin often increases (polycythemia) chronically; decreased Hb would worsen oxygenation.
d) Pulmonary resistance typically increases (hypoxic vasoconstriction), not lower.

Q17. The FEV₁/FVC ratio is reduced most characteristically in:

a) Restrictive lung disease
b) Obstructive lung disease (e.g., asthma, COPD)
c) Normal healthy lungs
d) High altitude acclimatization

Answer: b) Obstructive lung disease (e.g., asthma, COPD)
Explanations:

a) Restrictive disease: both FEV₁ and FVC reduced proportionally → ratio normal or increased.
b) Obstructive disease: FEV₁ reduced more than FVC → FEV₁/FVC decreased; correct.
c/d) Not associated.

Q18. Pulmonary surfactant deficiency in premature infants leads to:

a) Pulmonary edema only
b) Infant respiratory distress syndrome due to alveolar collapse (atelectasis)
c) Excessive lung compliance making breathing effortless
d) Increased surfactant production by type I pneumocytes

Answer: b) Infant respiratory distress syndrome due to alveolar collapse (atelectasis)
Explanations:

a) Not the primary effect.
b) Without surfactant, high surface tension causes alveoli to collapse → poor gas exchange & hypoxemia — correct.
c) Lack of surfactant decreases compliance (stiffer lungs), not increase.
d) Type II, not type I, produce surfactant; premature type II are immature.

Q19. Oxygen content of arterial blood depends on:

a) Only PaO₂ (dissolved O₂)
b) Hemoglobin concentration and % saturation plus dissolved O₂ (PaO₂)
c) Only hemoglobin concentration
d) Only cardiac output

Answer: b) Hemoglobin concentration and % saturation plus dissolved O₂ (PaO₂)
Explanations:

a) Dissolved O₂ is a small fraction; major content bound to Hb.
b) Total O₂ content = (Hb × 1.34 × SaO₂) + (0.003 × PaO₂) — correct.
c/d) Cardiac output affects delivery, not content per unit blood.

Q20. In pulmonary circulation, compared to systemic circulation, pulmonary vascular resistance is:

a) Much higher
b) Much lower
c) About equal
d) Zero

Answer: b) Much lower
Explanations:

Pulmonary circulation is low-pressure, low-resistance to accommodate entire cardiac output with lower work; resistance is much lower than systemic. (a/c/d incorrect)

Q21. The primary control center for voluntary alteration of breathing (e.g., speaking, breath-holding) is located in:

a) Medulla oblongata only
b) Cerebral cortex
c) Carotid bodies
d) Pons only

Answer: b) Cerebral cortex
Explanations:

a/d) Medulla and pons provide automatic rhythmic control, but voluntary control originates in cerebral cortex which can override medullary centers temporarily.
c) Carotid bodies are chemoreceptors, not voluntary control centers.

Q22. During exercise, what happens to alveolar ventilation and arterial PCO₂?

a) Alveolar ventilation decreases; PaCO₂ rises markedly
b) Alveolar ventilation increases; PaCO₂ remains near normal or decreases slightly
c) Alveolar ventilation unchanged; PaCO₂ rises
d) Alveolar ventilation increases; PaCO₂ increases dramatically

Answer: b) Alveolar ventilation increases; PaCO₂ remains near normal or decreases slightly
Explanations:

b) Exercise increases metabolic CO₂ production; ventilation increases proportionally (or more) to maintain PaCO₂ near normal or slightly decreased — correct.
Other options inconsistent with normal physiology.

Q23. A ventilation–perfusion (V/Q) ratio of zero (V = 0) in a lung region represents:

a) Dead space
b) Shunt (perfusion without ventilation)
c) Perfect match
d) Normal lung function

Answer: b) Shunt (perfusion without ventilation)
Explanations:

a) Dead space = ventilation without perfusion → V/Q = ∞.
b) V/Q = 0 = shunt (e.g., airway obstruction), blood passes unoxygenated — correct.
c/d) Not correct.

Q24. Which statement about the oxygen–hemoglobin dissociation curve is true?

a) It is linear across physiological PO₂ range
b) It is sigmoid due to cooperative binding of O₂ to Hb
c) It shifts only with pH changes, not temperature or 2,3-BPG
d) Left shifts facilitate O₂ release to tissues

Answer: b) It is sigmoid due to cooperative binding of O₂ to Hb
Explanations:

a) Curve is sigmoid, not linear.
b) Cooperative binding (each O₂ binding increases affinity for next) → sigmoid curve; correct.
c) Curve shifts with pH, temperature, CO₂, and 2,3-BPG.
d) Left shift increases affinity and hinders O₂ release; opposite of stated.

Q25. Which of the following increases the affinity of hemoglobin for oxygen (left shift)?

a) Increased H⁺ (lower pH)
b) Increased temperature
c) Increased 2,3-BPG
d) Decreased CO₂ (lower PCO₂)

Answer: d) Decreased CO₂ (lower PCO₂)
Explanations:

a) ↑H⁺ (acidosis) → right shift (↓affinity).
b) ↑temperature → right shift (↓affinity).
c) ↑2,3-BPG → right shift (↓affinity).
d) ↓PCO₂ and ↑pH → left shift (↑Hb–O₂ affinity) — correct.

Human Physiology – Breathing and Exchange of Gases Part 2: MCQs (Q26–Q50)

Q26. Which of the following muscles is primarily responsible for normal inspiration?
a) Intercostal muscles
b) Diaphragm
c) Abdominal muscles
d) Sternocleidomastoid

Answer: b) Diaphragm
Explanation: The diaphragm contracts and flattens during inspiration, increasing thoracic volume and drawing air in.

Q27. Which structure prevents the collapse of the trachea?
a) Elastic fibers
b) Smooth muscle
c) Cartilaginous rings
d) Pleural fluid

Answer: c) Cartilaginous rings
Explanation: C-shaped cartilaginous rings in the trachea maintain airway patency and prevent collapse.

Q28. During expiration, alveolar pressure is:
a) Lower than atmospheric pressure
b) Equal to atmospheric pressure
c) Higher than atmospheric pressure
d) Zero

Answer: c) Higher than atmospheric pressure
Explanation: Expiration occurs when alveolar pressure becomes greater than atmospheric pressure, pushing air out.

Q29. Oxygen is transported in blood mainly as:
a) Dissolved in plasma
b) Bound to hemoglobin
c) As oxyhemocyanin
d) Bound to platelets

Answer: b) Bound to hemoglobin
Explanation: About 97% of oxygen is carried as oxyhemoglobin in RBCs; only ~3% is dissolved in plasma.

Q30. The chloride shift occurs in:
a) Lungs only
b) Tissues only
c) Both lungs and tissues
d) Heart

Answer: b) Tissues only
Explanation: In tissues, chloride ions enter RBCs in exchange for bicarbonate ions to maintain ionic balance.

Q31. Which enzyme catalyzes the conversion of CO₂ and water to carbonic acid?
a) Carbonic anhydrase
b) Catalase
c) Dehydrogenase
d) Peptidase

Answer: a) Carbonic anhydrase
Explanation: Carbonic anhydrase, present in RBCs, rapidly converts CO₂ and H₂O into H₂CO₃.

Q32. Which lung volume cannot be measured by spirometry?
a) Tidal volume
b) Inspiratory reserve volume
c) Residual volume
d) Expiratory reserve volume

Answer: c) Residual volume
Explanation: Residual volume (air left after maximum expiration) cannot be measured by spirometry.

Q33. Vital capacity is equal to:
a) TV + IRV + ERV
b) TV + IRV + RV
c) ERV + RV + TV
d) IRV + ERV + RV

Answer: a) TV + IRV + ERV
Explanation: Vital capacity = tidal volume + inspiratory reserve volume + expiratory reserve volume.

Q34. Total lung capacity in a healthy adult is approximately:
a) 3000 mL
b) 4000 mL
c) 5000 mL
d) 6000 mL

Answer: d) 6000 mL
Explanation: TLC = VC + RV; average is about 6000 mL in healthy adults.

Q35. Which of the following is an obstructive lung disease?
a) Pneumonia
b) Asthma
c) Tuberculosis
d) Pulmonary fibrosis

Answer: b) Asthma
Explanation: Asthma causes obstruction of airflow due to bronchial constriction, making it an obstructive lung disease.

Q36. The primary respiratory control center is located in the:
a) Cerebellum
b) Medulla oblongata
c) Pons
d) Hypothalamus

Answer: b) Medulla oblongata
Explanation: The medulla oblongata regulates the basic rhythm of respiration.

Q37. Which form of hemoglobin has the highest affinity for oxygen?
a) Deoxyhemoglobin
b) Carbaminohemoglobin
c) Fetal hemoglobin (HbF)
d) Myoglobin

Answer: c) Fetal hemoglobin (HbF)
Explanation: HbF has a higher oxygen affinity than adult hemoglobin, aiding oxygen transfer across the placenta.

Q38. Which factor shifts the oxygen dissociation curve to the right?
a) Increased pH
b) Decreased temperature
c) Decreased CO₂
d) Increased 2,3-BPG

Answer: d) Increased 2,3-BPG
Explanation: High 2,3-BPG reduces hemoglobin affinity for oxygen, shifting the curve to the right.

Q39. In the lungs, exchange of gases occurs mainly by:
a) Diffusion
b) Active transport
c) Osmosis
d) Filtration

Answer: a) Diffusion
Explanation: Gas exchange occurs by passive diffusion due to partial pressure gradients.

Q40. The Bohr effect refers to:
a) Increased CO₂ decreases O₂ binding to Hb
b) Increased CO₂ increases O₂ binding to Hb
c) Decreased CO₂ increases O₂ release from Hb
d) Decreased CO₂ decreases O₂ binding to Hb

Answer: a) Increased CO₂ decreases O₂ binding to Hb
Explanation: In tissues, high CO₂ lowers pH, promoting oxygen release from hemoglobin (Bohr effect).

Q41. The Haldane effect is:
a) O₂ presence enhances CO₂ binding to Hb
b) O₂ presence reduces CO₂ binding to Hb
c) CO₂ enhances O₂ binding to Hb
d) CO₂ reduces O₂ binding to Hb

Answer: b) O₂ presence reduces CO₂ binding to Hb
Explanation: In lungs, oxygenation of blood reduces hemoglobin’s affinity for CO₂ (Haldane effect).

Q42. Partial pressure of O₂ in alveoli is about:
a) 159 mmHg
b) 104 mmHg
c) 95 mmHg
d) 40 mmHg

Answer: b) 104 mmHg
Explanation: Alveolar pO₂ is ~104 mmHg, which drives diffusion into blood (venous pO₂ ~40 mmHg).

Q43. During hypoventilation, what happens to blood CO₂ levels?
a) Decrease
b) Increase
c) Remain constant
d) Become zero

Answer: b) Increase
Explanation: Hypoventilation leads to accumulation of CO₂ (hypercapnia) and respiratory acidosis.

Q44. Which of the following is a restrictive lung disease?
a) Asthma
b) Emphysema
c) Pulmonary fibrosis
d) Chronic bronchitis

Answer: c) Pulmonary fibrosis
Explanation: Restrictive diseases reduce lung expansion and compliance, as seen in fibrosis.

Q45. Which type of respiration occurs without oxygen?
a) Aerobic respiration
b) Anaerobic respiration
c) Pulmonary respiration
d) Cellular respiration

Answer: b) Anaerobic respiration
Explanation: Anaerobic respiration occurs in absence of oxygen, producing lactic acid in muscles.

Q46. Maximum amount of CO₂ is transported as:
a) Dissolved in plasma
b) Carbaminohemoglobin
c) Bicarbonate ions
d) Free gas in blood

Answer: c) Bicarbonate ions
Explanation: About 70% of CO₂ is transported as bicarbonate ions in plasma.

Q47. Which part of the respiratory system is lined with ciliated epithelium?
a) Alveoli
b) Bronchi
c) Pleura
d) Diaphragm

Answer: b) Bronchi
Explanation: Bronchi are lined with ciliated columnar epithelium that helps trap and remove dust/mucus.

Q48. Hypercapnia refers to:
a) Low O₂ in blood
b) High O₂ in blood
c) Low CO₂ in blood
d) High CO₂ in blood

Answer: d) High CO₂ in blood
Explanation: Hypercapnia is an abnormally elevated level of CO₂ in the blood.

Q49. The normal respiratory rate in a healthy adult is:
a) 6–8 breaths/min
b) 10–12 breaths/min
c) 12–16 breaths/min
d) 20–25 breaths/min

Answer: c) 12–16 breaths/min
Explanation: Average adult resting respiratory rate is 12–16 breaths per minute.

Q50. Surfactant is secreted by:
a) Type I alveolar cells
b) Type II alveolar cells
c) Ciliated epithelial cells
d) Goblet cells

Answer: b) Type II alveolar cells
Explanation: Type II pneumocytes secrete surfactant, which reduces surface tension and prevents alveolar collapse.

Human Physiology – Breathing and Exchange of Gases Part 3: MCQs (Q51–Q75)

Q51. Which respiratory volume represents the maximum amount of air that can be inhaled after a normal inspiration?
a) Tidal volume
b) Inspiratory reserve volume
c) Expiratory reserve volume
d) Residual volume

Answer: b) Inspiratory reserve volume

Explanation: IRV is the extra volume of air that can be forcibly inhaled beyond normal inspiration.

Q52. Vital capacity can be represented as:
a) TV + ERV + IRV
b) TV + ERV + RV
c) IRV + ERV + RV
d) IRV + TV + RV

Answer: a) TV + ERV + IRV

Explanation: Vital capacity is the maximum volume of air exhaled after maximum inhalation, excluding residual volume.

Q53. The average respiratory rate in a healthy adult human is:
a) 12–16 breaths/min
b) 6–8 breaths/min
c) 20–24 breaths/min
d) 25–30 breaths/min

Answer: a) 12–16 breaths/min

Explanation: A healthy adult at rest breathes about 12–16 times per minute.

Q54. The primary gas that stimulates breathing is:
a) Oxygen
b) Carbon dioxide
c) Nitrogen
d) Carbon monoxide

Answer: b) Carbon dioxide

Explanation: Elevated CO₂ increases H⁺ ions, stimulating the respiratory center in the medulla.

Q55. Hemoglobin has the highest affinity for:
a) Oxygen
b) Carbon dioxide
c) Carbon monoxide
d) Nitrogen

Answer: c) Carbon monoxide

Explanation: CO binds to hemoglobin with ~200 times greater affinity than O₂, forming carboxyhemoglobin.

Q56. The chloride shift is associated with:
a) Entry of chloride into RBCs
b) Exit of chloride from RBCs
c) Exchange of chloride for oxygen
d) Exchange of chloride for nitrogen

Answer: a) Entry of chloride into RBCs

Explanation: In tissues, HCO₃⁻ diffuses out of RBCs, replaced by Cl⁻ ions to maintain ionic balance (Hamburger’s phenomenon).

Q57. The oxygen dissociation curve shifts to the right when:
a) Temperature decreases
b) pH increases
c) CO₂ increases
d) Oxygen pressure increases

Answer: c) CO₂ increases

Explanation: High CO₂ and low pH reduce hemoglobin affinity for O₂, favoring oxygen release (Bohr effect).

Q58. Which organ regulates respiratory rhythm?
a) Cerebellum
b) Medulla oblongata
c) Hypothalamus
d) Pons

Answer: b) Medulla oblongata

Explanation: The medulla contains the respiratory centers that regulate the rate and depth of breathing.

Q59. In alveoli, the partial pressure of oxygen (pO₂) is approximately:
a) 159 mmHg
b) 104 mmHg
c) 95 mmHg
d) 40 mmHg

Answer: b) 104 mmHg

Explanation: Alveolar pO₂ is about 104 mmHg due to mixing of atmospheric air with residual air.

Q60. During inspiration, the diaphragm:
a) Contracts and moves downward
b) Relaxes and moves upward
c) Contracts and moves upward
d) Relaxes and moves downward

Answer: a) Contracts and moves downward

Explanation: Contraction increases thoracic cavity volume, reducing pressure to draw air in.

Q61. The Haldane effect refers to:
a) Increased O₂ affinity in lungs
b) Increased CO₂ unloading in lungs due to oxygenation
c) Increased CO₂ affinity in tissues
d) Decreased O₂ affinity in tissues

Answer: b) Increased CO₂ unloading in lungs due to oxygenation

Explanation: Oxygenation of blood in lungs reduces hemoglobin’s affinity for CO₂, aiding CO₂ release.

Q62. The respiratory pigment in humans is:
a) Hemocyanin
b) Hemerythrin
c) Hemoglobin
d) Myoglobin

Answer: c) Hemoglobin

Explanation: Hemoglobin, an iron-containing protein in RBCs, transports oxygen and carbon dioxide.

Q63. The majority of oxygen is transported in the blood as:
a) Dissolved in plasma
b) Oxyhemoglobin
c) Bicarbonate
d) Carboxyhemoglobin

Answer: b) Oxyhemoglobin

Explanation: About 97% of O₂ is carried bound to hemoglobin.

Q64. Respiratory quotient (RQ) for carbohydrates is:
a) 1.0
b) 0.7
c) 0.8
d) 0.9

Answer: a) 1.0

Explanation: In carbohydrate metabolism, CO₂ released equals O₂ consumed (RQ = 1).

Q65. The condition of difficult or labored breathing is called:
a) Dyspnea
b) Apnea
c) Hyperpnea
d) Tachypnea

Answer: a) Dyspnea

Explanation: Dyspnea is difficulty in breathing, often seen in asthma and other respiratory disorders.

Q66. The thin membrane covering the lungs is called:
a) Pericardium
b) Pleura
c) Peritoneum
d) Endocardium

Answer: b) Pleura

Explanation: Pleura is a double-layered membrane that surrounds the lungs and reduces friction during breathing.

Q67. The exchange of gases between alveoli and blood is known as:
a) Internal respiration
b) External respiration
c) Cellular respiration
d) Ventilation

Answer: b) External respiration

Explanation: It is the exchange of O₂ and CO₂ across alveolar and capillary membranes.

Q68. Which one of the following structures prevents food from entering the trachea?
a) Glottis
b) Epiglottis
c) Pharynx
d) Larynx

Answer: b) Epiglottis

Explanation: Epiglottis acts as a flap, covering the glottis during swallowing to prevent entry of food.

Q69. The functional unit of lungs for gas exchange is:
a) Bronchi
b) Alveoli
c) Bronchioles
d) Trachea

Answer: b) Alveoli

Explanation: Alveoli provide a large surface area and thin walls for efficient gas exchange.

Q70. The binding of O₂ with hemoglobin is:
a) Irreversible
b) Reversible
c) Covalent
d) Permanent

Answer: b) Reversible

Explanation: Oxyhemoglobin formation is reversible, allowing O₂ release in tissues.

Q71. Which law explains the diffusion of gases across the alveolar membrane?
a) Boyle’s law
b) Dalton’s law
c) Henry’s law
d) Fick’s law

Answer: d) Fick’s law

Explanation: Fick’s law states that the rate of diffusion is proportional to the surface area and partial pressure gradient.

Q72. In humans, the lungs are located in the:
a) Abdominal cavity
b) Thoracic cavity
c) Cranial cavity
d) Pelvic cavity

Answer: b) Thoracic cavity

Explanation: Lungs are enclosed in the thoracic cavity, protected by the rib cage.

Q73. Which respiratory disorder is characterized by inflammation of the bronchi?
a) Asthma
b) Bronchitis
c) Emphysema
d) Pneumonia

Answer: b) Bronchitis

Explanation: Bronchitis is inflammation of the bronchi, causing cough and difficulty in breathing.

Q74. Pneumonia primarily affects:
a) Alveoli
b) Bronchi
c) Trachea
d) Larynx

Answer: a) Alveoli

Explanation: Pneumonia is infection of alveoli, leading to fluid accumulation and reduced gas exchange.

Q75. Which condition is caused by destruction of alveolar walls, reducing respiratory surface area?
a) Asthma
b) Tuberculosis
c) Emphysema
d) Pneumonia

Answer: c) Emphysema

Explanation: Emphysema destroys alveoli, reducing surface area and causing breathlessness.

Part 4: Human Physiology – Breathing and Exchange of Gases (Q76–Q100)

Q76. Which of the following has the highest affinity for hemoglobin?

a) Oxygen
b) Carbon dioxide
c) Carbon monoxide
d) Nitrogen

Answer: c) Carbon monoxide
👉 Carbon monoxide has about 200–250 times higher affinity for hemoglobin than oxygen, forming carboxyhemoglobin, which reduces oxygen transport.

Q77. Which pigment is responsible for oxygen transport in humans?

a) Hemocyanin
b) Hemoglobin
c) Myoglobin
d) Hemerythrin

Answer: b) Hemoglobin
👉 Hemoglobin is the red pigment present in RBCs, responsible for oxygen transport in humans.

Q78. Myoglobin is found in:

a) Liver cells
b) Muscle cells
c) RBCs
d) Nerve cells

Answer: b) Muscle cells
👉 Myoglobin stores oxygen in muscle tissues for immediate energy needs during muscle contraction.

Q79. Chloride shift occurs due to:

a) Entry of HCO₃⁻ into RBC
b) Entry of Cl⁻ into RBC
c) Exit of Cl⁻ from RBC
d) Exit of HCO₃⁻ from plasma

Answer: b) Entry of Cl⁻ into RBC
👉 To maintain ionic balance, when HCO₃⁻ leaves RBC into plasma, Cl⁻ enters — called the chloride shift.

Q80. Bohr’s effect refers to:

a) High CO₂ lowers O₂ affinity
b) Low CO₂ increases O₂ affinity
c) CO binds to Hb
d) Haldane effect

Answer: a) High CO₂ lowers O₂ affinity
👉 Bohr effect states that increased CO₂ and H⁺ concentration reduces hemoglobin’s affinity for oxygen, aiding O₂ release to tissues.

Q81. Haldane effect is:

a) Deoxygenated blood carries more CO₂
b) Oxygenated blood carries more CO₂
c) CO poisoning of Hb
d) Effect of low pressure

Answer: a) Deoxygenated blood carries more CO₂
👉 The Haldane effect states that deoxygenated blood has a higher capacity to bind CO₂, promoting CO₂ transport.

Q82. Oxygen dissociation curve shifts to the right when:

a) CO₂ decreases
b) pH increases
c) Temperature decreases
d) CO₂ increases

Answer: d) CO₂ increases
👉 High CO₂, low pH, and high temperature shift the curve right, decreasing Hb affinity for O₂.

Q83. How much CO₂ is carried in dissolved form in plasma?

a) 7%
b) 23%
c) 70%
d) 50%

Answer: a) 7%
👉 Only about 7% CO₂ is transported dissolved in plasma.

Q84. The majority of CO₂ is transported as:

a) Carbaminohemoglobin
b) Carbonic acid
c) Bicarbonate ions
d) Dissolved CO₂

Answer: c) Bicarbonate ions
👉 About 70% CO₂ is carried as bicarbonate ions in plasma.

Q85. Carbaminohemoglobin is formed by the binding of:

a) O₂ with Hb
b) CO₂ with Hb
c) CO with Hb
d) HCO₃⁻ with Hb

Answer: b) CO₂ with Hb
👉 When CO₂ binds with hemoglobin, it forms carbaminohemoglobin.

Q86. At high altitudes, oxygen availability decreases because:

a) Hemoglobin concentration decreases
b) Atmospheric pressure decreases
c) CO₂ concentration increases
d) RBC count decreases

Answer: b) Atmospheric pressure decreases
👉 Lower atmospheric pressure reduces partial pressure of oxygen, leading to hypoxia at high altitudes.

Q87. Which organ detects changes in blood O₂ concentration?

a) Kidney
b) Carotid body and aortic body
c) Brain cortex
d) Pancreas

Answer: b) Carotid body and aortic body
👉 Peripheral chemoreceptors in carotid and aortic bodies sense low O₂ levels.

Q88. Respiratory quotient (RQ) for carbohydrate is:

a) 0.7
b) 0.8
c) 1.0
d) 1.2

Answer: c) 1.0
👉 Carbohydrate metabolism produces equal amounts of CO₂ and O₂ consumed, hence RQ = 1.

Q89. Respiratory quotient (RQ) for fats is:

a) 0.7
b) 0.8
c) 1.0
d) 1.2

Answer: a) 0.7
👉 Fat metabolism consumes more O₂ compared to CO₂ produced, giving an RQ of 0.7.

Q90. Which enzyme catalyzes the conversion of CO₂ to HCO₃⁻?

a) Pepsin
b) Carbonic anhydrase
c) Amylase
d) Trypsin

Answer: b) Carbonic anhydrase
👉 Carbonic anhydrase inside RBCs rapidly converts CO₂ and H₂O into H₂CO₃ (carbonic acid), which dissociates into HCO₃⁻.

Q91. Which of the following increases breathing rate?

a) Low CO₂
b) High CO₂
c) Low H⁺
d) High pH

Answer: b) High CO₂
👉 Increased CO₂ stimulates medullary respiratory centers, increasing breathing rate.

Q92. Which part of the brain controls voluntary breathing?

a) Cerebrum
b) Medulla oblongata
c) Hypothalamus
d) Pons

Answer: a) Cerebrum
👉 Voluntary control of breathing (holding breath, deep inhalation) is regulated by the cerebrum.

Q93. Pneumotaxic center is located in:

a) Medulla
b) Pons
c) Cerebellum
d) Midbrain

Answer: b) Pons
👉 The pneumotaxic center in the pons regulates the rate and pattern of breathing.

Q94. Apnea is defined as:

a) Rapid breathing
b) Temporary cessation of breathing
c) Labored breathing
d) Deep breathing

Answer: b) Temporary cessation of breathing
👉 Apnea is a condition where breathing stops temporarily.

Q95. Which respiratory volume remains after maximal expiration?

a) Tidal volume
b) Residual volume
c) Expiratory reserve volume
d) Inspiratory reserve volume

Answer: b) Residual volume
👉 Residual volume is the air that remains in the lungs after maximum exhalation, preventing lung collapse.

Q96. Which factor directly affects the diffusion of gases across alveolar membranes?

a) Blood pressure
b) Partial pressure gradient
c) Heart rate
d) Hemoglobin concentration

Answer: b) Partial pressure gradient
👉 The exchange of gases in alveoli depends on the difference in partial pressures of gases.

Q97. At rest, oxygen utilization from blood is about:

a) 25%
b) 50%
c) 75%
d) 90%

Answer: a) 25%
👉 At rest, tissues utilize only about 25% of oxygen carried by blood.

Q98. Which respiratory disorder is characterized by hypersensitivity and constriction of bronchioles?

a) Asthma
b) Emphysema
c) Pneumonia
d) Tuberculosis

Answer: a) Asthma
👉 In asthma, bronchioles constrict due to allergic reactions, leading to difficulty in breathing.

Q99. Which respiratory disorder is caused by damage to alveolar walls?

a) Tuberculosis
b) Asthma
c) Emphysema
d) Pneumonia

Answer: c) Emphysema
👉 Emphysema is caused by the destruction of alveolar walls, reducing respiratory surface area.

Q100. Which gas is most important in regulating breathing rate?

a) Oxygen
b) Carbon dioxide
c) Nitrogen
d) Carbon monoxide

Answer: b) Carbon dioxide
👉 Breathing rate is mainly regulated by CO₂ concentration in blood, detected by chemoreceptors.

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CBSE Class 11 Biology MCQs