Free Human Respiratory System MCQs with Answers

21. The enzyme that speeds the combination of carbon dioxide and water inside red blood cells is

• A. carbonic anhydrase
• B. catalase
• C. amylase
• D. decarboxylase

Explanation: Carbonic anhydrase makes carbonic acid form rapidly, which then dissociates to bicarbonate and H+, the main vehicle for CO2 transport. Without the enzyme the reaction would be too slow for the transit time through a capillary.

22. The exchange of gases between the blood and the body tissues is called

• A. external respiration
• B. internal respiration
• C. breathing
• D. ventilation

Explanation: Internal, or tissue, respiration is gas exchange at the cells, while external respiration happens at the lungs. Breathing is only the mechanical movement of air in and out.

23. The partial pressure of carbon dioxide is highest in the

• A. alveolar air
• B. arterial blood
• C. respiring tissues
• D. pulmonary vein

Explanation: Cells produce CO2 continuously, so its pressure peaks in the tissues and falls stepwise into venous blood, alveoli and finally the expired air. The gradient is what drives CO2 out of the body.

24. When blood reaches the respiring tissues, haemoglobin

• A. releases carbon dioxide and binds more oxygen
• B. is destroyed by the cells
• C. holds its oxygen more tightly
• D. releases oxygen and picks up carbon dioxide

Explanation: Low oxygen pressure and high CO2 in the tissues make haemoglobin surrender oxygen and take on CO2 and H+, reversing what happened in the lungs. One molecule thus shuttles both gases on the same round trip.

25. Which of the following is NOT a characteristic of an efficient respiratory surface?

• A. It is dry and thick
• B. It is thin
• C. It is kept moist
• D. It has a large surface area

Explanation: Gases must dissolve before diffusing, and distance must be tiny, so efficiency demands a moist, thin, extensive surface. A dry thick barrier would stop diffusion, which is exactly why land animals keep their lungs internal.

26. Between the air in an alveolus and the blood, oxygen must cross

• A. one cell layer
• B. two cell layers
• C. four cell layers
• D. eight cell layers

Explanation: Only the single alveolar epithelium and the single capillary endothelium stand in the way, a combined thickness of about one micrometre. This minimal barrier is what makes rapid diffusion possible.

27. During gaseous exchange in the lungs, carbon dioxide moves

• A. from the alveolar air into the blood
• B. from red blood cells into haemoglobin
• C. from the blood into the alveolar air
• D. from the plasma into the lymph

Explanation: Venous blood arrives with a higher CO2 pressure than alveolar air, so the gas diffuses out to be exhaled. The opposite direction would defeat the entire purpose of ventilation.

28. The sticky brown substance in tobacco smoke that carries most of the carcinogens is

• A. nicotine
• B. carbon monoxide
• C. ash
• D. tar

Explanation: Tar condenses in the airways and contains the chemicals that mutate DNA and cause cancer, while nicotine addicts and carbon monoxide starves tissues of oxygen. Each component has its own signature harm.

29. Which of the following is NOT caused by smoking?

• A. Malaria
• B. Lung cancer
• C. Emphysema
• D. Chronic bronchitis

Explanation: Malaria is transmitted by the Anopheles mosquito and has nothing to do with tobacco. Cancer, emphysema and chronic bronchitis form the classic trio of smoking diseases.

30. Babies born to mothers who smoke during pregnancy often have

• A. a higher than average birth weight
• B. a low birth weight
• C. extra ribs
• D. immunity against respiratory infection

Explanation: Carbon monoxide and nicotine reduce the oxygen and blood supply to the fetus, restricting growth and raising the risk of premature birth. This is one of the strongest arguments against smoking in pregnancy.