Class 11 Biology Chapter 14 Breathing and Exchange of Gases – Extra Questions with Answers

These questions cover the mechanics of breathing under Boyle’s law, how most CO2 travels as bicarbonate in the blood, and the medulla’s role in regulating respiratory rhythm.

Last Updated: September 23, 2026

Very Short Answer Questions (1 mark)

Q1. What is the main muscle involved in breathing?
Ans: Diaphragm.

Q2. What percentage of CO₂ is transported as bicarbonate ions?
Ans: About 70%.

Q3. What law governs the pressure-volume relationship during breathing?
Ans: Boyle’s Law.

Q4. Where is the respiratory rhythm centre located?
Ans: Medulla oblongata.

Q5. What is tidal volume?
Ans: The volume of air inspired or expired during a normal (quiet) breath, approximately 500 mL.

Short Answer Questions (2–3 marks)

Q6. Describe the process of inspiration.
Ans: The diaphragm contracts and flattens, external intercostal muscles contract, increasing thoracic cavity volume, which decreases pulmonary pressure below atmospheric pressure, causing air to flow into the lungs.

Q7. Explain how oxygen and carbon dioxide are exchanged at the alveoli.
Ans: Gases diffuse across the thin alveolar membrane along their partial pressure gradients: O₂ moves from the alveoli (higher pO₂) into the blood (lower pO₂), and CO₂ moves from the blood (higher pCO₂) into the alveoli (lower pCO₂).

Q8. What is the main factor that stimulates the respiratory rhythm centre to increase breathing rate?
Ans: An increase in blood CO₂ concentration (and associated decrease in pH), rather than a decrease in O₂ levels.

Higher-Order Thinking / Application Questions

Q9. Explain why the respiratory centre in the medulla oblongata is primarily sensitive to blood CO₂ levels rather than O₂ levels, discussing the physiological advantage of this arrangement.
Ans: Under normal physiological conditions, the human body maintains a relatively large reserve of oxygen bound to haemoglobin (oxyhaemoglobin saturation typically remains high, around 95-98%, even during moderate exertion), meaning blood O₂ levels tend to change relatively little and relatively slowly under most everyday circumstances. In contrast, CO₂ is produced continuously as a metabolic byproduct of cellular respiration, and even small changes in metabolic rate (such as during exercise) cause proportionally much larger and more immediate changes in blood CO₂ concentration, which also directly and rapidly affects blood pH (since CO₂ combines with water to form carbonic acid, which dissociates into H⁺ and bicarbonate ions). Because CO₂/pH levels change more sensitively, rapidly, and proportionally with the body’s actual metabolic and respiratory needs than O₂ levels do, using CO₂ (via pH-sensitive chemoreceptors) as the primary trigger for adjusting breathing rate and depth provides a much more responsive, fine-tuned, and physiologically relevant regulatory mechanism for matching ventilation to the body’s real-time metabolic demand, compared to relying on the comparatively sluggish and less sensitive changes in O₂ levels.

Q10. Explain why haemoglobin’s oxygen-binding affinity changes with pH and CO₂ concentration (the Bohr effect), and describe why this is physiologically beneficial for oxygen delivery to actively respiring tissues.
Ans: The Bohr effect describes how haemoglobin’s affinity for oxygen decreases as CO₂ concentration increases and/or pH decreases (becomes more acidic), which occurs because increased CO₂ and H⁺ ions bind to haemoglobin at sites other than the oxygen-binding site, inducing a conformational change in the haemoglobin molecule that reduces its oxygen-binding affinity, promoting oxygen release. This is physiologically advantageous because actively respiring tissues (such as contracting muscles during exercise) have a high metabolic rate, producing large amounts of CO₂ as a byproduct of cellular respiration, which locally lowers the pH (more acidic) and raises CO₂ concentration in the surrounding blood capillaries. As blood passes through these metabolically active tissues, this local increase in CO₂/decrease in pH triggers the Bohr effect, reducing haemoglobin’s oxygen affinity precisely at the tissues that need oxygen the most, causing haemoglobin to release more of its bound oxygen exactly where and when it is most urgently required. This elegant physiological feedback mechanism ensures that oxygen delivery is automatically enhanced in proportion to a tissue’s actual metabolic activity and oxygen demand, without requiring any conscious or separate neural regulation for this fine-tuned, localized oxygen delivery adjustment.

Frequently Asked Questions

How would you explain why oxygen diffuses from alveoli into blood capillaries?
Oxygen moves from a region of higher partial pressure in the alveoli to a region of lower partial pressure in the blood, following simple diffusion along the pressure gradient.

If a person moves to a high altitude, how does the body typically adjust to lower oxygen availability?
The body increases red blood cell production over time to carry more oxygen and compensate for the lower partial pressure of oxygen at altitude.

Written by Satish

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