MDCAT Biology chapter 13: Biology · Respiration MCQs with answers
By Team Megalecture · Updated 4 Nov 2026
26 exam-style questions on Biology · Respiration, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.
Try each one, then open the answer. These ten stay the same; the test draws new ones every time.
1. A swimmer holds her breath under water. After some time she is forced to come up and breathe, mainly because
A
lactic acid builds up in her lungs and paralyses her diaphragm
B
rising CO2 in her blood stimulates the breathing centre
C
rising oxygen in her lungs stimulates the breathing centre
D
water vapour fills her alveoli and blocks the exchange of gases
Show answer
Answer: B. As CO2 builds up, the blood becomes more acidic; chemoreceptors detect this and the breathing centre in the medulla oblongata drives breathing more strongly. A rise in carbon dioxide, not oxygen, is the main stimulus.
2. A long-term smoker becomes breathless even when walking slowly. Tests show that many alveolar walls have broken down, leaving fewer, larger air spaces with a smaller total surface area. The disorder is
A
asthma
B
emphysema
C
tuberculosis
D
pneumonia
Show answer
Answer: B. In emphysema the alveolar walls break down and the lungs lose their elasticity, so the surface for gas exchange is reduced. Asthma narrows the bronchioles but does not destroy the alveoli.
3. Which component of tobacco smoke contains most of the cancer-causing chemicals (carcinogens)?
A
carbon monoxide
B
carbon dioxide
C
tar
D
nicotine
Show answer
Answer: C. Tar condenses in the airways and contains the main carcinogens linked to lung cancer. Nicotine is the addictive drug, and carbon monoxide reduces oxygen transport.
4. Inside red blood cells, carbon dioxide combines rapidly with water to form carbonic acid. The enzyme that speeds up this reaction is
A
succinate dehydrogenase
B
carbonic anhydrase
C
catalase
D
ribulose carboxylase
Show answer
Answer: B. Carbonic anhydrase in red cells makes the reaction very fast; carbonic acid then dissociates into H+ and HCO3−, and most CO2 travels as bicarbonate in the plasma.
5. In the capillaries of respiring tissues, oxyhaemoglobin releases its oxygen mainly because
A
the oxygen concentration in the tissues is low
B
the red cells burst there and release their oxygen
C
the tissue cells pump oxygen out of the red cells
D
haemoglobin is broken down in the tissue capillaries
Show answer
Answer: A. Respiring cells use up oxygen, so its concentration (partial pressure) in the tissues is low; there oxyhaemoglobin dissociates and the oxygen diffuses into the cells. The red cells stay intact and return to the lungs.
6. As bicarbonate ions diffuse out of red blood cells into the plasma, the electrical balance of the cells is kept by
A
sodium ions moving out of the red cells
B
hydrogen ions moving into the plasma
C
chloride ions moving into the red cells
D
potassium ions moving into the plasma
Show answer
Answer: C. This exchange of HCO3− for Cl− is the chloride shift. The H+ ions formed stay inside the red cell, where haemoglobin buffers them.
7. Which sequence shows the path of inhaled air correctly?
Answer: D. Air passes from the nasal cavity into the pharynx, through the larynx into the trachea, which divides into two bronchi; these branch into bronchioles that end in alveoli.
8. The vaccine used to protect against tuberculosis is
A
MMR
B
BCG
C
OPV
D
DPT
Show answer
Answer: B. BCG (bacillus Calmette-Guérin) is a weakened strain of Mycobacterium bovis, a close relative of the tuberculosis bacterium. DPT protects against diphtheria, pertussis and tetanus, MMR against measles, mumps and rubella, and OPV against polio.
9. Blood leaves the lungs with its haemoglobin 95% saturated with oxygen. In resting muscle the saturation falls to 75%; in exercising muscle, where CO2 is higher, it falls to 15%. How many times more oxygen is unloaded in the exercising muscle than in the resting muscle?
A
3 times
B
4 times
C
5 times
D
6 times
Show answer
Answer: B. Unloaded at rest = 95 − 75 = 20%; in exercise = 95 − 15 = 80%; 80 ÷ 20 = 4. Dividing the saturations (75 ÷ 15 = 5) compares what is left on the haemoglobin, not what is released.
10. A man breathes 12 times a minute and takes in 6 dm3 of air each minute. His tidal volume is
A
72 cm3
B
200 cm3
C
500 cm3
D
2000 cm3
Show answer
Answer: C. Tidal volume = volume per minute ÷ breaths per minute = 6000 cm3 ÷ 12 = 500 cm3. Dividing the wrong way round (12 ÷ 6 = 2 dm3) gives 2000 cm3, and multiplying 6 by 12 gives 72.
26, each with the answer and a short explanation, across 2 topics: Respiratory system and breathing, Gas transport and disorders. Every chapter test draws ten of them, so you can take it many times.
Is the MDCAT Biology chapter 13 test free?
Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.
How is the chapter test marked?
The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.
