Breathing and Exchange of Gases — Practice Questions with Answers
30 free MCQs on Breathing and Exchange of Gases with worked answers and explanations. The mechanics of breathing, lung volumes, gas transport in blood, the Bohr effect, and respiratory regulation.
Below are 30 practice questions on Breathing and Exchange of Gases, sorted Easy → Hard. Tap “Show answer & explanation” under any question to check yourself. Want the full theory first? Read the Breathing and Exchange of Gases notes.
The S-shaped (sigmoid) curve reflects cooperative binding between Hb's four O₂ sites; the Bohr effect shifts the whole curve to the right under conditions found in actively respiring tissue (more CO₂, more acid, more heat), making haemoglobin release oxygen more readily exactly where it's needed.
Easy — 10 questions
Q1.
Gas exchange in the lungs occurs at the:
A Trachea
B Bronchi
C Alveoli
D Diaphragm
Show answer & explanation
Answer: C. Alveoli
Why: Alveoli are tiny air sacs in the lungs where O2 and CO2 are exchanged between air and blood (capillaries).
Q2.
Breathing in (inhalation) is caused by the diaphragm:
A Relaxing and moving up
B Contracting and moving down
C Staying still
D Moving sideways
Show answer & explanation
Answer: B. Contracting and moving down
Why: During inhalation, the diaphragm contracts and flattens (moves down), increasing chest volume, decreasing pressure, and drawing air in.
Q3.
The respiratory pigment found in human red blood cells is:
A Chlorophyll
B Hemocyanin
C Myoglobin
D Hemoglobin
Show answer & explanation
Answer: D. Hemoglobin
Why: Hemoglobin, present in RBCs, binds and transports most of the O2 in blood. Each molecule can carry up to four O2 molecules.
Q4.
The pleural membrane that directly covers the lung surface is the:
A Pericardium
B Parietal pleura
C Visceral pleura
D Peritoneum
Show answer & explanation
Answer: C. Visceral pleura
Why: The visceral pleura lines the lung surface itself, while the parietal pleura lines the inner wall of the thoracic cavity.
Q5.
The conducting part of the human respiratory system includes the:
A Diaphragm, ribs and intercostal muscles under typical physiological conditions
B Alveoli, alveolar ducts and alveolar sacs in most observed cases
C Pulmonary artery, vein and capillary bed according to standard texts
D Nasal passage, pharynx, larynx, trachea and bronchi
Show answer & explanation
Answer: D. Nasal passage, pharynx, larynx, trachea and bronchi
Why: The conducting part carries air from outside to the alveoli, including nostrils, nasal passage, pharynx, larynx, trachea, bronchi and bronchioles.
Q6.
Most of the carbon dioxide transported in blood is carried as:
A Bicarbonate ions
B Carbonic acid crystals
C Dissolved gas in plasma
D Carbaminohemoglobin
Show answer & explanation
Answer: A. Bicarbonate ions
Why: About 70% of CO2 is transported as bicarbonate ions (HCO3-) formed in RBCs with the help of carbonic anhydrase; a smaller fraction binds hemoglobin as carbaminohemoglobin.
Q7.
The volume of air that remains in the lungs even after a forceful exhalation is called:
A Inspiratory reserve volume
B Tidal volume
C Residual volume
D Vital capacity
Show answer & explanation
Answer: C. Residual volume
Why: Residual volume is the air that always remains in the lungs after maximum exhalation, preventing the lungs from collapsing.
Q8.
Exhalation (expiration) occurs mainly because the volume of the thoracic cavity:
A Decreases as the diaphragm and intercostal muscles relax
B Increases as the diaphragm contracts downward
C Increases due to outward movement of the rib cage
D Stays roughly constant while internal pressure rises
Show answer & explanation
Answer: A. Decreases as the diaphragm and intercostal muscles relax
Why: During expiration, the diaphragm and external intercostal muscles relax, reducing thoracic volume and increasing pulmonary pressure, pushing air out.
Q9.
The exchange of O2 and CO2 between alveoli and blood occurs mainly by:
A Active transport across alveolar walls
B Facilitated transport using carrier proteins
C Bulk flow driven by blood pressure
D Simple diffusion along a pressure gradient
Show answer & explanation
Answer: D. Simple diffusion along a pressure gradient
Why: Gases move across the alveolar-capillary membrane by simple diffusion, driven by differences in their partial pressures on either side.
Q10.
The rhythm of breathing in humans is primarily controlled by a centre located in the:
A Medulla oblongata, in the brainstem
B Cerebrum, in the frontal region
C Cerebellum, behind the brainstem
D Spinal cord, near the cervical region
Show answer & explanation
Answer: A. Medulla oblongata, in the brainstem
Why: The respiratory rhythm centre in the medulla oblongata generates the basic rhythm of breathing; a pneumotaxic centre in the pons fine-tunes it.
Medium — 10 questions
Q11.
During the Bohr effect, increased CO2 in blood:
A Raises hemoglobin's affinity for O2 by stabilizing the relaxed conformation
B Decreases hemoglobin affinity for O2 (more O2 released to active tissues)
C Leaves the oxygen-hemoglobin binding curve completely unshifted
D Denatures hemoglobin's quaternary structure within red blood cells
Show answer & explanation
Answer: B. Decreases hemoglobin affinity for O2 (more O2 released to active tissues)
Why: Bohr effect: increased CO2 (and H+) decreases Hb affinity for O2, promoting O2 release to metabolically active tissues.
Q12.
Tidal volume is:
A The maximum air volume the lungs can hold after forced inhalation
B Volume of air inhaled/exhaled in a normal breath (~500 mL)
C The air remaining in lungs after the most forceful possible exhalation
D The maximum air that can be exhaled after the deepest possible inhalation
Show answer & explanation
Answer: B. Volume of air inhaled/exhaled in a normal breath (~500 mL)
Why: Tidal volume: volume of air moved in or out per normal breath, about 500 mL at rest.
Q13.
Surfactant in alveoli prevents:
A Gas exchange across the alveolar membrane surface
B Alveoli collapse by reducing surface tension
C Infection from inhaled bacteria and airborne pathogens
D Blood clotting occurring within the lung capillary network
Show answer & explanation
Answer: B. Alveoli collapse by reducing surface tension
Why: Pulmonary surfactant (from type II pneumocytes) reduces surface tension inside alveoli, preventing collapse during exhalation.
Q14.
Carbon dioxide is transported in blood mainly as:
A Dissolved CO2 in plasma, accounting for the majority of transported gas
B Carbaminohemoglobin formed by binding to globin chains exclusively
C Bicarbonate ions (HCO3-) in plasma (~70%)
D Carbonic acid accumulating freely in plasma without further dissociation
Show answer & explanation
Answer: C. Bicarbonate ions (HCO3-) in plasma (~70%)
Why: About 70% of CO2 is transported as bicarbonate (HCO3-) in plasma. About 20% bound to Hb (as carbaminoHb). 10% dissolved.
Q15.
Residual volume refers to the volume of air that:
A Can be forcibly exhaled after a normal tidal expiration
B Is inhaled or exhaled during normal quiet breathing
C Remains in the lungs even after the most forceful expiration
D Can be additionally inhaled after a normal tidal inspiration
Show answer & explanation
Answer: C. Remains in the lungs even after the most forceful expiration
Why: Residual volume is the air that remains in the lungs after the most forceful possible exhalation, preventing the alveoli from collapsing completely.
Q16.
Most of the oxygen transported in the blood is carried:
A Bound to plasma albumin proteins according to standard texts
B Dissolved directly in blood plasma in most observed cases
C As bicarbonate ions in plasma under typical physiological conditions
D Bound to haemoglobin within red blood cells
Show answer & explanation
Answer: D. Bound to haemoglobin within red blood cells
Why: About 97% of oxygen in blood is transported bound to haemoglobin as oxyhaemoglobin within red blood cells; only a small fraction dissolves directly in plasma.
Q17.
The partial pressure of oxygen is higher in the alveoli than in the pulmonary capillary blood arriving at the lungs. This pressure difference causes oxygen to:
A Diffuse from the blood into the alveoli
B Remain in the alveoli without net movement
C Diffuse from the alveoli into the blood
D Bind irreversibly to carbon dioxide in the alveolar air
Show answer & explanation
Answer: C. Diffuse from the alveoli into the blood
Why: Because alveolar oxygen partial pressure is higher than in deoxygenated capillary blood, oxygen diffuses down its concentration gradient from the alveoli into the blood.
Q18.
Contraction of the diaphragm during inspiration causes it to:
A Pull the lungs downward without changing thoracic volume
B Dome upward, decreasing the volume of the thoracic cavity
C Remain stationary while the rib cage does all the work
D Flatten, increasing the volume of the thoracic cavity
Show answer & explanation
Answer: D. Flatten, increasing the volume of the thoracic cavity
Why: During inspiration, the diaphragm contracts and flattens, increasing the volume of the thoracic cavity and lowering intrapulmonary pressure, drawing air into the lungs.
Q19.
Hyperventilation can lead to a drop in blood CO2 levels, which in turn causes:
A A rise in blood pH, since less carbonic acid is formed
B Increased binding of CO2 to haemoglobin as carbaminohaemoglobin
C No change in pH, since CO2 does not affect blood acidity
D A fall in blood pH, due to excess bicarbonate accumulation
Show answer & explanation
Answer: A. A rise in blood pH, since less carbonic acid is formed
Why: Excessive removal of CO2 during hyperventilation reduces carbonic acid formation in blood, raising blood pH (respiratory alkalosis).
Q20.
Which respiratory centre, located in the medulla, primarily sets the basic rhythm of breathing?
A Medullary rhythm/respiratory centre
B Carotid body chemoreceptors
C Aortic body chemoreceptors
D Pneumotaxic centre in the pons
Show answer & explanation
Answer: A. Medullary rhythm/respiratory centre
Why: The medullary respiratory rhythm centre generates the basic rhythm of breathing, while the pneumotaxic centre in the pons can moderate the duration and depth of breaths.
Hard — 10 questions
Q21.
Oxyhemoglobin dissociation curve shifts right (Bohr effect) due to:
A Increased blood pH combined with decreased CO2 stabilizing the relaxed high-affinity state overall in most cases under typical conditions
B Decreased pH, increased CO2, increased temperature, increased 2,3-BPG (decreased Hb-O2 affinity, more O2 released to tissues)
C A fall in core body temperature that slows the hemoglobin conformational transition process according to standard textbooks
D Replacement of adult HbA by fetal HbF circulating within the maternal bloodstream during pregnancy in general practice as frequently described
Show answer & explanation
Answer: B. Decreased pH, increased CO2, increased temperature, increased 2,3-BPG (decreased Hb-O2 affinity, more O2 released to tissues)
Why: Right shift (decreased O2 affinity): low pH, high CO2, high temperature, high 2,3-BPG. All occur in metabolically active tissues, ensuring O2 release where needed most.
Q22.
Fetal hemoglobin (HbF) has higher O2 affinity than adult HbA because:
A It carries two additional heme groups per tetramer for extra oxygen binding capacity in most textbook accounts
B Its gamma chains have lower affinity for 2,3-BPG than beta chains of HbA, maintaining higher O2 saturation
C Its heme prosthetic group uses a different iron oxidation state compared to HbA during normal conditions
D It dissolves more readily in fetal plasma due to its smaller overall molecular size as generally observed
Show answer & explanation
Answer: B. Its gamma chains have lower affinity for 2,3-BPG than beta chains of HbA, maintaining higher O2 saturation
Why: HbF (alpha2gamma2) binds 2,3-BPG less strongly than HbA (alpha2beta2). Since 2,3-BPG reduces O2 affinity, HbF has higher O2 affinity, facilitating O2 transfer from maternal to fetal blood.
Q23.
Respiratory distress syndrome in premature infants is due to:
A Excess surfactant production flooding the alveolar lining with lipid film material in the majority of cases studied
B Insufficient surfactant production (type II alveolar cells immature), causing alveolar collapse
C Premature over-development of alveolar septa restricting the gas exchange surface area as widely reported
D Pulmonary edema resulting from cardiac overload unrelated to surfactant levels present in standard practice
Show answer & explanation
Answer: B. Insufficient surfactant production (type II alveolar cells immature), causing alveolar collapse
Why: Neonatal RDS (hyaline membrane disease): premature type II alveolar cells do not produce adequate surfactant. Surface tension causes alveoli to collapse on expiration. Treated with exogenous surfactant.
Q24.
Carotid body peripheral chemoreceptors primarily respond to:
A Fluctuations in arterial blood pressure sensed via stretch receptors
B Low arterial PO2 (hypoxia), high PCO2, and low pH
C Core body temperature shifts relayed from the hypothalamus
D Circulating blood glucose concentration via glucose-sensing neurons
Show answer & explanation
Answer: B. Low arterial PO2 (hypoxia), high PCO2, and low pH
Why: Carotid body (and aortic bodies): peripheral chemoreceptors sensitive to low PO2 (<60 mmHg), high PCO2, and low pH. Signal brain stem respiratory centers to increase ventilation.
Q25.
At high altitude, the body's compensatory increase in red blood cell production is triggered mainly by:
A A fall in carbon dioxide levels within the blood
B Hypoxia, which raises erythropoietin secretion from the kidney
C Increased atmospheric pressure acting directly on bone marrow
D Faster hemoglobin breakdown occurring in the spleen
Show answer & explanation
Answer: B. Hypoxia, which raises erythropoietin secretion from the kidney
Why: Low oxygen levels (hypoxia) at high altitude stimulate the kidneys to secrete more erythropoietin, which increases red blood cell production to improve oxygen-carrying capacity.
Q26.
During strenuous exercise, the rightward shift of the oxygen-hemoglobin dissociation curve aids tissues mainly because it:
A Raises hemoglobin's affinity for carbon monoxide instead
B Lets hemoglobin bind oxygen more tightly in muscle capillaries
C Lets hemoglobin release more oxygen at a given partial pressure
D Stops further oxygen unloading at the tissue level
Show answer & explanation
Answer: C. Lets hemoglobin release more oxygen at a given partial pressure
Why: A rightward-shifted curve (favoured by raised CO2, H+, and temperature during exercise) reflects lower hemoglobin-O2 affinity, so more oxygen is unloaded to meet the higher demand of active tissues.
Q27.
Surfactant secreted by alveolar Type II cells is essential because it:
A Lowers surface tension in alveoli, preventing their collapse
B Raises surface tension in alveoli to keep them rigid under typical physiological conditions
C Pumps oxygen directly into alveolar capillaries according to standard texts
D Converts CO2 into bicarbonate inside the alveoli in general clinical practice
Show answer & explanation
Answer: A. Lowers surface tension in alveoli, preventing their collapse
Why: Pulmonary surfactant, a phospholipid film, lowers surface tension at the air-water interface in alveoli, preventing their collapse, especially during exhalation when alveolar volume decreases.
Q28.
A decrease in blood pH (acidosis) stimulates an increase in the rate and depth of breathing mainly through:
A Inhibition of the pneumotaxic centre located in the pons
B Reduced oxygen-carrying capacity of plasma proteins
C Chemoreceptors signalling the medullary centre to raise ventilation
D Direct contraction of the diaphragm with little nervous input
Show answer & explanation
Answer: C. Chemoreceptors signalling the medullary centre to raise ventilation
Why: Central and peripheral chemoreceptors detect a fall in blood pH (rise in H+/CO2) and signal the medullary respiratory centre, which increases the rate and depth of breathing to expel more CO2.
Q29.
In a person with emphysema, gas exchange efficiency declines mainly because:
A The diaphragm loses most of its ability to contract in most reference accounts
B Damaged alveolar walls merge, cutting the surface area for diffusion
C The trachea becomes blocked by excess mucus build-up as frequently documented
D Hemoglobin gradually loses its ability to bind oxygen under normal conditions
Show answer & explanation
Answer: B. Damaged alveolar walls merge, cutting the surface area for diffusion
Why: Emphysema involves breakdown of alveolar walls, which reduces the surface area available for gas exchange and impairs efficient diffusion of O2 and CO2.
Q30.
Why does carbon monoxide (CO) poisoning severely impair oxygen transport even at low CO concentrations?
A CO reacts with plasma proteins to form toxic precipitates in typical laboratory settings
B CO destroys red blood cells within minutes of exposure as generally observed
C CO drives an excessive rise in red blood cell production under usual circumstances
D CO binds hemoglobin far more strongly than oxygen, blocking O2 sites
Show answer & explanation
Answer: D. CO binds hemoglobin far more strongly than oxygen, blocking O2 sites
Why: Carbon monoxide binds hemoglobin with about 200 times greater affinity than oxygen, forming stable carboxyhemoglobin and effectively blocking the sites needed for oxygen transport.