Breathing Gas Transport

8 MCQs9-step worked example
Source: NCERT Human PhysiologyPYQ coverage: NEET 2021, 2022, 2023, 2024Official key: NTA-verifiedLast updated: 25 Sep 2026

Breathing Gas Transport, explained for NEET

The high-frequency trap in gas transport questions is confusing the direction of the oxyhaemoglobin dissociation curve shift with its physiological consequence. A right shift does not mean "more oxygen in blood" — it means haemoglobin releases oxygen more readily to tissues.

Oxygen transport. About 97% of O₂ is carried as oxyhaemoglobin (HbO₂); ~3% dissolves in plasma. Each haemoglobin molecule binds up to 4 O₂ molecules. Binding is cooperative — the sigmoid shape of the dissociation curve reflects this. At alveolar pO₂ (~104 mmHg), saturation is ~98%. At tissue pO₂ (~40 mmHg), saturation drops to ~75%, releasing O₂ for cellular respiration (NCERT Class 11 Biology Chapter 14, page 189).

The Bohr effect. In active tissues: ↑pCO₂, ↑H⁺ (lower pH), ↑temperature, ↑2,3-BPG → curve shifts right → haemoglobin's O₂ affinity decreases → more O₂ unloaded. In the lungs: low CO₂, higher pH, lower temperature → curve shifts left → Hb binds O₂ tightly.

CO₂ transport. Three modes: (1) ~70% as bicarbonate ions (HCO₃⁻) formed via carbonic anhydrase in RBCs (CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻); (2) ~23% as carbaminohaemoglobin (CO₂ bound to Hb's amino groups); (3) ~7% dissolved in plasma.

Chloride shift. As HCO₃⁻ exits RBCs into plasma, Cl⁻ enters to maintain electrical neutrality — this is the chloride shift (Hamburger phenomenon).

Regulation and disorders. The respiratory rhythm centre in the medulla sets the basic breathing pattern; a pneumotaxic centre in the pons can moderate it by reducing inspiration duration. A chemosensitive area next to the rhythm centre is highly sensitive to CO₂ and H⁺ — a rise in either activates it, signalling the rhythm centre to adjust breathing; aortic-arch and carotid receptors send similar signals. Oxygen's role here is minor (NCERT Class 11 Biology, Chapter 14, page 190). Among disorders: asthma is difficulty in breathing with wheezing from bronchial/bronchiolar inflammation; emphysema damages alveolar walls, shrinking the respiratory surface, often from smoking (NCERT Class 11 Biology, Chapter 14, page 190). Occupational disorders arise in dust-heavy industries (grinding, stone-breaking), where prolonged exposure causes fibrosis and lung damage — protective masks are advised (NCERT Class 11 Biology, Chapter 14, page 191).

Watch-out: NEET distractors frequently swap the percentages (placing bicarbonate at 23% and carbamino at 70%) or state that CO₂ binds to the iron of haemoglobin (it binds to amino groups, not the haem iron).

Can you answer these Breathing Gas Transport MCQs?

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

What percentage of oxygen transported in blood is carried as oxyhaemoglobin?

Show answer and why every option is right or wrong

Answer: A. Approximately 97% of O₂ is transported bound to haemoglobin as oxyhaemoglobin; only ~3% is dissolved in plasma (NCERT Class 11 Biology Chapter 14, page 189).

Why B is wrong: B — 70% refers to the proportion of CO₂ transported as bicarbonate, not O₂ transport.

Why C is wrong: C — 3% is the dissolved fraction in plasma, not the haemoglobin-bound fraction.

Why D is wrong: D — 23% refers to CO₂ transported as carbaminohaemoglobin, not O₂ transport.

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MCQ 2Easy RecallPractice

The majority of carbon dioxide in blood is transported as:

Show answer and why every option is right or wrong

Answer: D. About 70% of CO₂ is converted to bicarbonate ions in RBCs by carbonic anhydrase and transported in plasma (NCERT Class 11 Biology Chapter 14, page 190).

Why A is wrong: A — Only ~7% of CO₂ is transported dissolved in plasma.

Why B is wrong: B — Carbaminohaemoglobin accounts for ~23%, not the majority.

Why C is wrong: C — CO₂ binds to amino groups of haemoglobin, not the haem iron. This is a common distractor exploiting term confusion.

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MCQ 3Easy RecallPractice

The chloride shift (Hamburger phenomenon) involves movement of which ion into the RBC?

Show answer and why every option is right or wrong

Answer: D. As bicarbonate leaves the RBC, chloride ions (Cl⁻) move in to maintain electrical neutrality — this exchange is the chloride shift (NCERT Class 11 Biology, Chapter 14).

Why A is wrong: A — HCO₃⁻ moves OUT of the RBC into plasma; the shift replaces it with Cl⁻ moving inward.

Why B is wrong: B — Na⁺ is not the ion involved in the chloride shift; it plays roles in nerve impulse and kidney function.

Why C is wrong: C — K⁺ is the major intracellular cation but is not exchanged in the chloride shift mechanism.

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MCQ 4Direct ApplicationPractice

During vigorous exercise, which of the following changes in active muscle tissue causes a RIGHT shift of the oxyhaemoglobin dissociation curve?

Show answer and why every option is right or wrong

Answer: C. Active tissues produce more CO₂ (raising pCO₂), generate H⁺ (lowering pH), and increase temperature — all factors shift the curve rightward, promoting O₂ release to tissues. This is the Bohr effect (NCERT Class 11 Biology Chapter 14, page 189).

Why A is wrong: A — Decreased pCO₂ and increased pH describe lung conditions and would cause a LEFT shift, not right (trap: confusing tissue vs. lung environment).

Why B is wrong: B — Decreased temperature and decreased 2,3-BPG both cause a LEFT shift (increased Hb affinity for O₂), the opposite of what exercise produces.

Why D is wrong: D — Increased pH and decreased pCO₂ are lung-like conditions causing a LEFT shift — this distractor reverses the Bohr effect direction.

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MCQ 5Direct ApplicationPractice

In the lungs, the oxyhaemoglobin dissociation curve shifts LEFT compared to tissues. Which factor in the alveolar environment is responsible?

Show answer and why every option is right or wrong

Answer: B. The lungs have low pCO₂ (CO₂ is being expired) and consequently higher pH. Both factors shift the curve left, increasing Hb's affinity for O₂ so it loads efficiently (NCERT Class 11 Biology, Chapter 14).

Why A is wrong: A — High pCO₂ is a tissue condition that shifts the curve RIGHT, not left (trap: reversing site-specific conditions).

Why C is wrong: C — Low pH means acidic/tissue conditions → right shift. Lungs have relatively HIGHER pH.

Why D is wrong: D — 2,3-BPG is elevated in conditions of chronic hypoxia and shifts curve RIGHT. It is not characteristically high in normal alveolar capillaries.

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MCQ 6Direct ApplicationPractice

CO₂ binds to haemoglobin to form carbaminohaemoglobin. To which part of the haemoglobin molecule does CO₂ attach?

Show answer and why every option is right or wrong

Answer: B. CO₂ binds to the free amino groups (–NH₂) of the globin polypeptide chains, forming carbamino compounds. It does NOT bind to the haem iron — that site is for O₂ (NCERT Class 11 Biology, Chapter 14).

Why A is wrong: A — The haem iron binds O₂ (and CO in poisoning), not CO₂. This is a high-frequency distractor exploiting the O₂/CO₂ binding-site confusion.

Why C is wrong: C — The porphyrin ring is the structural scaffold holding Fe²⁺; it is not the CO₂ binding site.

Why D is wrong: D — The distal histidine stabilizes bound O₂ through hydrogen bonding; CO₂ attaches to terminal amino groups on globin chains, not histidine.

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MCQ 7Concept TrapPractice

A patient with chronic obstructive pulmonary disease (COPD) shows chronically elevated blood CO₂. Regarding the oxyhaemoglobin dissociation curve, which statement is correct?

Show answer and why every option is right or wrong

Answer: A. Chronically elevated CO₂ → increased H⁺ → right shift of the dissociation curve → reduced Hb-O₂ affinity → more O₂ released to tissues. This is an application of the Bohr effect to a clinical scenario (NCERT Class 11 Biology, Chapter 14).

Why B is wrong: B — Left shift means INCREASED affinity (holds O₂ tighter). High CO₂ does the opposite — it causes a right shift (trap: reversing shift direction and consequence).

Why C is wrong: C — CO₂ significantly affects Hb-O₂ binding via the Bohr effect; it does not only influence dissolved O₂.

Why D is wrong: D — CO₂ does NOT bind to haem iron (that's O₂/CO). Also the direction is wrong — high CO₂ causes right shift, not left.

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MCQ 8CalculationPractice

In an exercising individual, pO₂ in active muscles drops to 20 mmHg. At this pO₂, haemoglobin saturation is approximately 25%. If each 100 mL of fully oxygenated blood carries 20 mL O₂ (bound to Hb), approximately how much O₂ is released per 100 mL blood passing through these muscles?

Show answer and why every option is right or wrong

Answer: C. Arriving blood: ~98% saturated ≈ 20 mL O₂ per 100 mL. Leaving muscle at 25% saturation: 0.25 × 20 = 5 mL O₂ still bound. O₂ released = 20 − 5 = 15 mL per 100 mL blood. This requires reading the dissociation curve AND calculating the difference (NCERT Class 11 Biology, Chapter 14).

Why A is wrong: A — 5 mL is the O₂ still REMAINING bound to Hb at 25% saturation, not the amount released (trap: confusing remaining with released).

Why B is wrong: B — 10 mL would correspond to 50% O₂ release. At 25% saturation, 75% of bound O₂ has been released (0.75 × 20 = 15 mL).

Why D is wrong: D — 20 mL would mean 100% O₂ release (0% saturation). At 25% saturation, Hb still retains some O₂.

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How do you solve a Breathing Gas Transport question? A worked example

Pattern: Dissociation curve interpretation + quantitative O₂ release (anchored to NEET pattern: unit bundle)

  1. 1

    Given

    • Blood entering tissue capillaries: Hb saturation = 97%, carrying 19.4 mL O₂ per 100 mL blood (bound to Hb)• Blood leaving resting tissue: Hb saturation = 75%• Blood leaving exercising muscle: Hb saturation = 30%• O₂ carrying capacity at 100% saturation = 20 mL O₂ per 100 mL blood

  2. 2

    Required

    Calculate (a) O₂ delivered to resting tissue per 100 mL blood, and (b) O₂ delivered to exercising muscle per 100 mL blood.

  3. 3

    Concept

    The oxyhaemoglobin dissociation curve relates pO₂ to Hb saturation. O₂ released = O₂ arriving − O₂ remaining. The Bohr effect shifts the curve right in active tissue, dropping saturation further at the same venous pO₂.

  4. 4

    Formula

    O₂ released = (saturation_arterial − saturation_venous) × O₂ capacity at 100%

  5. 5

    Substitution

    (a) Resting: (0.97 − 0.75) × 20 mL = 0.22 × 20
    (b) Exercise: (0.97 − 0.30) × 20 mL = 0.67 × 20

  6. 6

    Calculation

    (a) 0.22 × 20 = 4.4 mL O₂ per 100 mL blood (resting)
    (b) 0.67 × 20 = 13.4 mL O₂ per 100 mL blood (exercising)

  7. 7

    Final answer

    • Resting tissue receives ~4.4 mL O₂ per 100 mL blood• Exercising muscle receives ~13.4 mL O₂ per 100 mL blood — approximately 3× more, due to the rightward Bohr shift
    Note: The percentage values (97%, 75%, 30%) are given-data treated as exact for this calculation; they do not limit significant figures.

  8. 8

    Common trap

    Confusing the SHIFT DIRECTION with the CONSEQUENCE. A right shift does NOT mean "more O₂ in blood." It means lower affinity → O₂ unloads more readily → lower venous saturation → more O₂ delivered to tissue. The curve shifting right REDUCES Hb saturation at any given pO₂.

  9. 9

    Similar NEET-style question

    "At a tissue pO₂ of 40 mmHg, Hb saturation is 75%. If the patient has a fever (raising tissue temperature), would Hb saturation at the same pO₂ be higher or lower than 75%? Explain using the Bohr effect."

    Answer: Lower. Fever → ↑temperature → right shift → lower affinity → saturation drops below 75% at 40 mmHg → more O₂ released.

    ---

What to remember before solving Breathing Gas Transport questions

Key Fact

Gas transport

O2 transport: about 97% of O2 is carried by haemoglobin in RBCs (each haemoglobin molecule carries a maximum of four O2 molecules); the remaining 3% is dissolved in plasma. Binding depends mainly on pO2; pCO2, H+ concentration and temperature also interfere with it. The O2 dissociation curve (percentage saturation of haemoglobin against pO2) is sigmoid. In the alveoli, high pO2, low pCO2, lesser H+ concentration and lower temperature favour the formation of oxyhaemoglobin; in the tissues, low pO2, high pCO2, high H+ concentration and higher temperature favour its dissociation. CO2 transport: nearly 20-25% by RBCs (carbamino-haemoglobin), 70% as bicarbonate, about 7% dissolved in plasma.

-- NCERT Class 11 Biology, Chapter 14, p. 189

Where do students lose marks on Breathing Gas Transport?

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

Oxyhaemoglobin curve shift direction

Category: Graph Interpretation

Right shift (lower affinity, more O2 release) caused by ↑CO2, ↑H+, ↑temperature, ↑2,3-BPG (Bohr effect). Left = opposite.

When it triggers

Question on tissue-vs-lung haemoglobin behaviour, exercise effect.

How to avoid

In tissues (high CO2/low pH/high T) → right shift → release O2. In lungs → left → bind O2.

More in Human Physiology: 32 exam traps and mistakes · 2 formulas · 1 question pattern from its other lessons.

Breathing Gas Transport questions from past NEET papers

5 questions from NEET 2021, 2022, 2023, 2024. Answers verified against NTA official keys.

NEET 2023

Which one of the following statements is NOT correct?

1Algal blooms caused by excess of organic matter in water improve water quality and promote fisheries
2Water hyacinth grows abundantly in eutrophic water bodies and leads to an imbalance in the ecosystem dynamics of the water body
3The amount of some toxic substances of industrial waste water increases in the organisms at successive trophic levels
4The micro-organisms involved in biodegradation of organic matter in a sewage polluted water body consume a lot of oxygen causing the death of aquatic organisms
NTA Answer: Option 1(final)

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All 71 past-paper questions from Human Physiology →

Sources

NCERT refs: Class 11 Biology Chapter 14, p.189 | Class 11 Biology Chapter 14, p.190 | Class 11 Biology Chapter 14, p.191

Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.

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