Science — CBSE Class 10 board question

(i) Rings of cartilage in the throat ensure that the air-passage (trachea) does not collapse, keeping it open for continuous airflow to the lungs.

(ii) When air is taken in (inhalation), the ribs move upward and outward, and the diaphragm moves downward and becomes flattened. These movements increase the volume of the chest cavity. As a result, air pressure inside decreases and air is sucked into the lungs, filling the expanded alveoli.

(iii) During heavy exercise, the muscle cells may not receive sufficient oxygen. In the absence of adequate oxygen, pyruvate (formed from glucose) is converted into lactic acid instead of being broken down aerobically. This lactic acid builds up in the muscle cells, causing muscle cramps. The cramps disappear once the oxygen supply is restored and lactic acid is broken down.

Source: Chapter 5, Section 5.3 — Respiration

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• (i) is a direct one-liner fact from the text — just state the function clearly.
• (ii) Name both structures (ribs AND diaphragm), state the direction of their movement, link it to increased chest volume → decreased pressure → air sucked in. Two marks = two clear steps.
• (iii) The key chain is: heavy exercise → lack of O₂ → anaerobic pathway in muscles → pyruvate → lactic acid → cramps. The word lactic acid is essential for full marks. Examiners also expect you to mention that it is a 3-carbon molecule (optional but good) and that it is the build-up that causes cramps.