Free Respiration MCQs with Answers

31. 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.

32. 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.

33. 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.

34. 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.

35. 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.

36. 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.

37. 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.

38. 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.

39. 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.

40. 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.