Free Human Respiratory System MCQs with Answers

11. The passage common to both food and air is the

• A. trachea
• B. larynx
• C. pharynx
• D. bronchus

Explanation: The pharynx receives air from the nose and food from the mouth and directs them onwards, which is why swallowing must temporarily close the airway. The trachea and bronchi carry air only.

12. The trachea divides at its lower end into

• A. alveoli
• B. alveolar ducts
• C. pleural sacs
• D. two bronchi

Explanation: Each primary bronchus enters a lung and branches into ever finer bronchioles ending in alveoli. The division mirrors the need to serve both lungs from a single windpipe.

13. The double membrane covering each lung is the

• A. pleura
• B. pericardium
• C. peritoneum
• D. meninges

Explanation: The pleura secretes fluid that lets the lungs slide smoothly against the chest wall during breathing. The pericardium wraps the heart and the peritoneum the gut, so each serous membrane is named for its organ.

14. The normal breathing rate of a resting adult is about

• A. 6 per minute
• B. 16 per minute
• C. 40 per minute
• D. 75 per minute

Explanation: Quiet breathing runs at roughly 12 to 20 cycles per minute, around 16. The figure 75 per minute belongs to the heart rate, and examiners often offer it as the trap.

15. During a forced inspiration, in addition to the diaphragm, which muscles contract to raise the ribs?

• A. the abdominal muscles only
• B. the internal intercostal muscles only
• C. the external intercostal muscles
• D. the smooth muscles of the alveoli

Explanation: The external intercostals lift the rib cage up and out, enlarging the thorax with the diaphragm's downward pull. The internal intercostals and abdominal muscles act in forced expiration instead.

16. The terminal bronchioles end in bunches of thin walled sacs called

• A. glomeruli
• B. villi
• C. cisternae
• D. alveoli

Explanation: Alveoli hang in grape like clusters at the end of the airways, each wrapped in capillaries. Glomeruli filter blood in the kidney and villi absorb in the intestine, so similar sounding structures serve different systems.

17. The air we breathe in is warmed, moistened and filtered of dust mainly in the

• A. nasal cavity
• B. trachea only
• C. bronchi
• D. alveoli

Explanation: Hairs and mucus trap particles while blood vessels warm the air and secretions humidify it before it reaches delicate lung tissue. Mouth breathing bypasses this conditioning, which is why it dries the throat.

18. An asthma attack, in which breathing becomes difficult with wheezing, is caused by

• A. fluid filling the alveoli
• B. constriction of the bronchi and bronchioles with excess mucus
• C. destruction of the cartilage rings of the trachea
• D. paralysis of the diaphragm

Explanation: The airway smooth muscle contracts and the lining swells with mucus, narrowing the tubes so air whistles through. Inhalers relax the muscle, confirming that constriction, not destruction, is the problem.

19. Oxygen diffuses from the alveolar air into the blood of the capillaries because

• A. the blood actively pumps it in
• B. the alveolar walls are impermeable to nitrogen
• C. the partial pressure of oxygen is higher in the alveoli than in the blood
• D. plasma proteins pull it across

Explanation: Gases always diffuse down their partial pressure gradients, and incoming alveolar air carries more oxygen than the arriving deoxygenated blood. At the tissues the gradient reverses, so oxygen leaves the blood there.

20. Most of the oxygen carried in the blood is transported

• A. dissolved in the plasma
• B. as bicarbonate ions
• C. as carboxyhaemoglobin
• D. bound to haemoglobin inside red blood cells

Explanation: About 97 per cent of oxygen rides on haemoglobin as oxyhaemoglobin, while only a small fraction dissolves in plasma. Bicarbonate carries carbon dioxide, not oxygen, and carboxyhaemoglobin is the poisonous CO complex.