All Free Biology MCQs with Answers

761. 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.

762. 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.

763. 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.

764. 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.

765. 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.

766. 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.

767. 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.

768. 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.

769. 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.

770. 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.