Breathing and Exchange of Gases — NEET Zoology MCQs with answers

Breathing and Exchange of Gases is a Class 11 Zoology chapter in the NEET (UG) syllabus. NEET720 has 1,014 reviewed practice questions on it, each with a quick answer and a step-by-step explanation. The 8 questions below are free and fixed, so you can bookmark this page; the full chapter, plus mistake tracking and spaced revision, is in the app.

Topics covered

Respiratory Organs and Human Respiratory System · Mechanism of Breathing and Lung Volumes · Exchange and Transport of Gases · Regulation of Respiration and Disorders · Diffusion of Gases · Respiratory Volumes and Capacities · Respiratory Organs in Animals · Disorders of Respiratory System · Pulmonary ventilation · Partial pressures · Respiratory tract · Disorders · Applied physiology · Disorders of the Respiratory System · Respiratory Organs Across Animal Groups · Numerical - Respiratory volumes · Numerical - Gas exchange · Numerical - Breathing rate · Respiratory pigments (comparative) · Numerical - Oxygen consumption · Numerical - Alveolar ventilation · Breathing mechanism · Respiratory system anatomy · Respiratory pigments · Human Respiratory System · Respiratory Volumes · Regulation of Breathing · Respiratory Disorders · Respiratory Regulation · Comparative respiration · Respiratory tract structures · Gas exchange · Oxygen dissociation curve · Respiratory Organs · Mechanism of Breathing · Exchange of Gases · Regulation of Respiration · Gaseous exchange · Lung volumes and capacities · Comparative respiratory organs

8 free Breathing and Exchange of Gases practice questions with answers

Choose an answer in your head before opening it. Each explanation says why the correct option is right and, where relevant, why the tempting wrong option is wrong.

1. Question 1 · easy · Exchange and Transport of Gases

   The exchange of O2 and CO2 across the alveolar and tissue surfaces occurs mainly by which process, and what is the principal factor determining its direction?

   • A.Active transport, determined by ATP availability
   • B.Simple diffusion, determined by the pressure (concentration) gradient of the gas
   • C.Facilitated diffusion, determined by carrier protein number
   • D.Osmosis, determined by the water potential gradient
   Show answer and explanation

   Answer: B. Simple diffusion, determined by the pressure (concentration) gradient of the gas

   O2 and CO2 move across respiratory membranes by simple diffusion, driven along their pressure gradient, from a region of higher partial pressure to lower partial pressure.

   NCERT states that exchange of gases occurs by simple diffusion, mainly along a pressure (concentration) gradient, with solubility of gases and the thickness of the membrane as additional determining factors. No ATP, carrier proteins, or osmotic gradients are involved in this exchange; it is purely a passive physical process governed by differences in partial pressure of O2 and CO2 across the membrane.

   Common mistake: Assuming gas exchange needs energy or transport proteins like solute transport does.

   Key point: Gas exchange at alveoli and tissues is simple diffusion driven by the partial pressure gradient, not active transport.

2. Question 2 · medium · Exchange and Transport of Gases

   Besides the pressure gradient of the gas, which two additional factors influence the rate of diffusion of O2 and CO2 across the respiratory membrane, according to NCERT?

   • A.Body temperature and blood pH only
   • B.Heart rate and blood volume
   • C.Solubility of the gases and the thickness of the membrane across which diffusion takes place
   • D.Number of red blood cells and haemoglobin concentration only
   Show answer and explanation

   Answer: C. Solubility of the gases and the thickness of the membrane across which diffusion takes place

   NCERT specifies that solubility of gases in the membrane fluid and the thickness of the diffusion membrane, along with pressure gradient and surface area, determine the rate of gas diffusion.

   The rate of diffusion of respiratory gases depends on: (i) the pressure/concentration gradient of the gas, (ii) the solubility of the gases involved (CO2 is about 20-25 times more soluble than O2, which is why CO2 diffuses faster despite a smaller pressure gradient), (iii) the thickness of the membrane across which diffusion occurs, and (iv) the total surface area available for diffusion. Options A, B and D describe factors relevant to other aspects of respiratory/circulatory physiology (Bohr effect, cardiac output, O2-carrying capacity) but not to the rate of diffusion itself.

   Common mistake: Mixing up factors that shift the oxygen dissociation curve with factors that govern diffusion rate.

   Key point: Diffusion rate depends on pressure gradient, gas solubility, membrane thickness, and surface area — remember all four.

3. Question 3 · medium · Exchange and Transport of Gases

   A student claims: "Since the alveolar surface area in an emphysema patient is drastically reduced, the pressure gradient of O2 between alveolar air and blood must also fall, which is why gas exchange is impaired." What is wrong with this reasoning?

   • A.Nothing is wrong; reduced surface area directly lowers the pressure gradient
   • B.Reduced alveolar surface area impairs gas exchange by lowering the total area available for diffusion, not by altering the partial pressure gradient of the gases, which remains set by atmospheric and blood gas composition
   • C.Emphysema increases the pressure gradient, so gas exchange should actually improve
   • D.Surface area has no effect on diffusion; only the gradient and solubility matter
   Show answer and explanation

   Answer: B. Reduced alveolar surface area impairs gas exchange by lowering the total area available for diffusion, not by altering the partial pressure gradient of the gases, which remains set by atmospheric and blood gas composition

   Surface area and pressure gradient are independent determinants of diffusion rate; losing alveolar surface area (as in emphysema) reduces total gas exchanged without changing the underlying partial pressure gradient.

   Diffusion rate depends on pressure gradient, solubility, membrane thickness, and surface area as separate, independent factors. In emphysema, alveolar walls break down, reducing the total surface area available for gas exchange, but this does not itself change the partial pressures of O2 and CO2 in inspired air or in blood arriving at the alveoli. The impairment arises because less total area is available for diffusion to occur across, not because the gradient driving each gas molecule has changed. Confusing 'less area' with 'less gradient' is a common student error that conflates two distinct variables in the diffusion equation.

   Common mistake: Treating all diffusion-limiting factors as interchangeable rather than as independent variables.

   Key point: Surface area and partial pressure gradient are separate variables — a drop in one does not imply a drop in the other.

4. Question 4 · easy · Exchange and Transport of Gases

   Statement I: The exchange of O2 and CO2 across the respiratory membrane occurs by simple diffusion. Statement II: The direction of diffusion of a gas is always from the region of its lower partial pressure to the region of its higher partial pressure. In light of the above statements, choose the correct answer.

   • A.Both Statement I and Statement II are true
   • B.Statement I is true but Statement II is false
   • C.Statement I is false but Statement II is true
   • D.Both Statement I and Statement II are false
   Show answer and explanation

   Answer: B. Statement I is true but Statement II is false

   Gas exchange does occur by simple diffusion (Statement I true), but gases actually diffuse from higher to lower partial pressure, not lower to higher, making Statement II false.

   Statement I is a correct, direct NCERT fact: O2 and CO2 exchange across the alveolar and tissue membranes by simple diffusion. Statement II inverts the fundamental diffusion rule — diffusion always proceeds down the concentration/pressure gradient, i.e., from a region of HIGHER partial pressure to a region of LOWER partial pressure, never the reverse. This is the basic physics underlying all passive gas exchange in the body.

   Common mistake: Reversing the direction rule for passive diffusion.

   Key point: Gases diffuse from higher partial pressure to lower partial pressure — never the reverse.

5. Question 5 · medium · Exchange and Transport of Gases

   The diffusion membrane across which gas exchange occurs at the alveoli is composed of which layers?

   • A.Squamous epithelium of the alveoli and endothelium of the alveolar capillaries, together forming a very thin barrier
   • B.Ciliated columnar epithelium of the bronchioles and smooth muscle of the capillary wall
   • C.Cuboidal epithelium of the alveoli and basement membrane of the bronchi
   • D.Stratified squamous epithelium of the trachea and connective tissue of the pleura
   Show answer and explanation

   Answer: A. Squamous epithelium of the alveoli and endothelium of the alveolar capillaries, together forming a very thin barrier

   The gas-exchange membrane is a very thin barrier formed by the squamous epithelium lining the alveoli together with the endothelium of the surrounding capillaries.

   NCERT describes the diffusion membrane as extremely thin, made up of three layers in close apposition: the squamous epithelial lining of the alveoli, the endothelium of the alveolar capillaries, and a thin basement membrane between them. This overall thinness minimizes the diffusion distance and maximizes the efficiency of gas exchange. The other options describe unrelated respiratory tract linings (ciliated columnar epithelium of bronchioles, tracheal stratified epithelium) that are not part of the actual alveolar-capillary diffusion barrier.

   Common mistake: Confusing the alveolar gas-exchange membrane with the epithelial lining of conducting airways like bronchioles or trachea.

   Key point: The alveolar-capillary membrane is extremely thin — squamous epithelium plus capillary endothelium — enabling efficient, rapid gas diffusion.

6. Question 6 · medium · Exchange and Transport of Gases

   Why is the extreme thinness of the alveolar-capillary diffusion membrane physiologically important for efficient gas exchange?

   • A.It increases the solubility of O2 and CO2 in blood plasma
   • B.It shortens the diffusion distance, so gases can cross rapidly even though diffusion itself is a relatively slow process over larger distances
   • C.It increases the partial pressure gradient of O2 and CO2 across the membrane
   • D.It allows active transport of gases to occur more efficiently
   Show answer and explanation

   Answer: B. It shortens the diffusion distance, so gases can cross rapidly even though diffusion itself is a relatively slow process over larger distances

   A thinner membrane means a shorter path length for diffusion, so gas molecules cross rapidly despite diffusion being an inherently slow process over long distances.

   According to Fick's principle of diffusion, the rate of diffusion is inversely proportional to the thickness of the membrane being crossed. Because the alveolar-capillary membrane is only a fraction of a micrometre thick, the diffusion distance is minimized, allowing large volumes of O2 and CO2 to be exchanged very rapidly during the brief time blood spends in the pulmonary capillaries. Membrane thickness does not affect gas solubility (an inherent chemical property) or the pressure gradient (set by the difference in partial pressures on either side), and the process remains passive diffusion, not active transport.

   Common mistake: Attributing the efficiency benefit of a thin membrane to solubility or gradient changes rather than to shortened diffusion distance.

   Key point: Diffusion rate is inversely related to membrane thickness — thinness is what makes rapid gas exchange possible.

7. Question 7 · medium · Exchange and Transport of Gases

   What are the approximate partial pressures of O2 in alveolar air and in deoxygenated blood arriving at the alveoli, and in which direction does O2 diffuse as a result?

   • A.pO2 in alveolar air ≈ 104 mmHg, pO2 in deoxygenated blood ≈ 40 mmHg; O2 diffuses from alveolar air into the blood
   • B.pO2 in alveolar air ≈ 40 mmHg, pO2 in deoxygenated blood ≈ 104 mmHg; O2 diffuses from blood into the alveolar air
   • C.pO2 in alveolar air ≈ 45 mmHg, pO2 in deoxygenated blood ≈ 104 mmHg; O2 diffuses from blood into alveolar air
   • D.pO2 in alveolar air ≈ 104 mmHg, pO2 in deoxygenated blood ≈ 45 mmHg; O2 diffuses from blood into alveolar air
   Show answer and explanation

   Answer: A. pO2 in alveolar air ≈ 104 mmHg, pO2 in deoxygenated blood ≈ 40 mmHg; O2 diffuses from alveolar air into the blood

   Alveolar air has a higher pO2 (~104 mmHg) than deoxygenated blood arriving at the lungs (~40 mmHg), so O2 diffuses from the alveoli into the blood.

   At the alveoli, the partial pressure of O2 in alveolar air is approximately 104 mmHg, while the pO2 in deoxygenated blood arriving via the pulmonary artery is approximately 40 mmHg. Because gases diffuse from a region of higher partial pressure to lower partial pressure, O2 diffuses from the alveolar air into the pulmonary capillary blood, oxygenating it. This is the fundamental alveolar exchange step that loads oxygen onto haemoglobin before the blood returns to the heart and is pumped to the tissues.

   Common mistake: Swapping alveolar and blood pO2 values or reversing the diffusion direction.

   Key point: At the alveoli, pO2(alveolar air, ~104) > pO2(blood, ~40), so O2 diffuses INTO the blood.

8. Question 8 · medium · Exchange and Transport of Gases

   What are the approximate partial pressures of CO2 in deoxygenated blood arriving at the alveoli and in alveolar air, and in which direction does CO2 diffuse as a result?

   • A.pCO2 in blood ≈ 45 mmHg, pCO2 in alveolar air ≈ 104 mmHg; CO2 diffuses from alveolar air into blood
   • B.pCO2 in blood ≈ 40 mmHg, pCO2 in alveolar air ≈ 45 mmHg; CO2 diffuses from alveolar air into blood
   • C.pCO2 in blood ≈ 104 mmHg, pCO2 in alveolar air ≈ 40 mmHg; CO2 diffuses from blood into alveolar air
   • D.pCO2 in blood ≈ 45 mmHg, pCO2 in alveolar air ≈ 40 mmHg; CO2 diffuses from blood into alveolar air
   Show answer and explanation

   Answer: D. pCO2 in blood ≈ 45 mmHg, pCO2 in alveolar air ≈ 40 mmHg; CO2 diffuses from blood into alveolar air

   Deoxygenated blood has a slightly higher pCO2 (~45 mmHg) than alveolar air (~40 mmHg), so CO2 diffuses from the blood into the alveoli to be exhaled.

   Deoxygenated blood arriving at the alveoli carries CO2 at a partial pressure of approximately 45 mmHg, picked up from tissue metabolism, whereas alveolar air has a pCO2 of approximately 40 mmHg. Since gases diffuse from higher to lower partial pressure, CO2 diffuses out of the blood into the alveolar air, from where it is expelled during expiration. This complements the O2 exchange step: at the alveoli, O2 moves air→blood while CO2 moves blood→air, simultaneously oxygenating the blood and removing metabolic CO2.

   Common mistake: Reversing the blood and alveolar pCO2 values, which flips the direction of CO2 movement.

   Key point: At the alveoli, pCO2(blood, ~45) > pCO2(alveolar air, ~40), so CO2 diffuses OUT of the blood into the alveoli.

Questions about Breathing and Exchange of Gases for NEET

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