Respiratory System – NDA Biology PYQs

Question [NDA 2006-I]
During the anaerobic respiration that takes place in our muscles while doing rigorous exercise, what is the end product formed?
(a) Acetic acid
(b) Carbon dioxide
(c) Ethanol
(d) Lactic acid

Answer: (d)
In human muscles, anaerobic respiration produces lactic acid. Ethanol is produced during fermentation in yeast — not in human muscle cells. Carbon dioxide is the product of aerobic respiration. Acetic acid is not a product of muscle metabolism. Lactic acid is the correct and only answer for human muscle anaerobic respiration.

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Question [NDA 2008-I]
In which one of the following animals is skin a respiratory organ?
(a) Cockroach
(b) Shark
(c) Whale
(d) Frog

Answer: (d)
The frog breathes through both its lungs and its moist skin. Skin respiration is a supplement to lung breathing in frogs. Cockroaches breathe through spiracles. Sharks and fish breathe through gills. Whales breathe through lungs like mammals. The frog is the standard answer to this question.

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Question [NDA 2018-I]
Which one of the following parts of the body does NOT take part in the process of breathing?
(a) Bronchi
(b) Diaphragm
(c) Trachea
(d) Bowman’s capsule

Answer: (d)
Bowman’s capsule is a structure in the kidney’s nephron. It filters blood to form urine. It has no connection to the respiratory system. Bronchi, trachea, and diaphragm are all directly involved in breathing. Bowman’s capsule is placed here as a deliberate trap — testing whether students know their organ systems.

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Question [NDA 2024-II]
In which part or organ of the human body is cartilage NOT found?
(a) Nose
(b) Trachea
(c) Knee joint
(d) Bronchiole

Answer: (d)
Bronchioles have no cartilage — only smooth muscle walls. The nose, trachea, and bronchi all have cartilage to keep them rigid and open. The knee joint has cartilage as a cushion between the bones. Bronchioles are the first structure in the airway to lack cartilage.

Question [NDA 2025-I]
The lactic acid that gets accumulated in the muscle cells during rigorous exercise causing muscle cramps is produced from —
(a) ATP
(b) Ethanol
(c) Glucose
(d) Pyruvate

Answer: (d)
Lactic acid is produced from pyruvate. During intense exercise without sufficient oxygen, glucose is first broken down to pyruvate, then pyruvate is converted to lactic acid. ATP is not the source — it is the energy product. Ethanol is the anaerobic product in yeast, not in humans. Glucose is converted to pyruvate first, not directly to lactic acid.

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Question [NDA 2025-I]
The amount of air breathed in and out during a normal respiration by human lungs is referred to as —
(a) Vital capacity
(b) Inspiratory volume
(c) Total lung capacity
(d) Tidal volume

Answer: (d)
Tidal volume is the amount of air breathed in and out during normal, resting breathing (approximately 500 mL). Vital capacity is the maximum air that can be expelled after a maximum inhalation — much larger. Inspiratory volume is the extra air that can be inhaled beyond tidal volume. Total lung capacity is the absolute maximum the lungs can hold.

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