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This page provides complete NCERT Solutions for CBSE Class 11 Biology Chapter 17 — Breathing and Exchange of Gases. All intext questions (organised by page number) and chapter-end exercise questions are answered in detail. Use the section headings to jump directly to the question you need help with, or read through the full page for a comprehensive revision of the chapter.
Key Concepts (Quick Recap)
- Respiratory Pathway: Nostrils → Nasal cavity → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli
- Inspiration: Diaphragm contracts (flattens), external intercostal muscles contract, thoracic volume increases, intrapulmonary pressure decreases, air flows in
- Expiration: Diaphragm relaxes (domes), external intercostal muscles relax, thoracic volume decreases, intrapulmonary pressure increases, air flows out
- Gas Exchange: Occurs at alveoli by diffusion — O₂ moves from alveolar air (higher pO₂) to blood (lower pO₂), CO₂ moves from blood (higher pCO₂) to alveolar air (lower pCO₂)
- O₂ Transport: 98.5% as oxyhaemoglobin (HbO₂), 1.5% dissolved in plasma
- CO₂ Transport: 7% dissolved in plasma, 23% as carbaminohaemoglobin, 70% as bicarbonate (HCO₃⁻)
- Bohr Effect: Increased CO₂ / decreased pH reduces Hb affinity for O₂, promoting O₂ release in tissues
- Lung Volumes: TV = 500 mL, IRV = 3000 mL, ERV = 1100 mL, RV = 1200 mL; Vital Capacity = TV + IRV + ERV ≈ 4600 mL
- Regulation: Neural (respiratory rhythm centre in medulla, pneumotaxic centre in pons) and chemical (chemosensitive areas in medulla respond to CO₂, H⁺)
- Disorders: Asthma (bronchial inflammation and spasm), Emphysema (alveolar wall damage), Occupational respiratory diseases (silicosis, asbestosis, black lung disease)
Intext Questions
Page 277
In insects, respiration occurs through a network of tubes called tracheae. These tubes deliver oxygen directly to the tissues. Air enters through openings called spiracles on the body surface and travels through the tracheal tubes. Tracheoles, the finest branches of tracheae, are in direct contact with the body cells, enabling efficient gas exchange without relying on a circulatory system for oxygen transport.
The human respiratory system is a complete pathway from the nostrils to the lungs. Air enters through the nostrils, passes through the nasal cavity (where it is filtered, warmed, and humidified), then through the pharynx, larynx (voice box), trachea (windpipe), bronchi, bronchioles, and finally reaches the alveoli in the lungs where gas exchange occurs.
Page 283
During inspiration, the diaphragm contracts and moves downward (flattens), which increases the volume of the thoracic cavity. The external intercostal muscles also contract, raising the ribs and sternum. This increase in thoracic volume decreases the intrapulmonary pressure below atmospheric pressure, causing air to rush into the lungs.
During normal expiration (a passive process), the diaphragm relaxes and returns to its dome shape, reducing the thoracic volume. The external intercostal muscles also relax, lowering the ribs. This decreases thoracic volume and increases intrapulmonary pressure above atmospheric pressure, forcing air out of the lungs.
Page 288
Gas exchange in the alveoli occurs by simple diffusion. Oxygen moves from the alveolar air (where pO₂ is approximately 104 mm Hg) into the pulmonary blood (where pO₂ is approximately 40 mm Hg) down its concentration gradient. Carbon dioxide moves from the pulmonary blood (where pCO₂ is approximately 45 mm Hg) into the alveolar air (where pCO₂ is approximately 40 mm Hg), also down its concentration gradient.
The oxygen-haemoglobin dissociation curve shifts to the right when pCO₂ increases, pH decreases (more acidic), or temperature increases. This rightward shift indicates reduced affinity of haemoglobin for oxygen, which promotes oxygen unloading in tissues with high metabolic activity (where CO₂ is high, pH is low, and temperature is elevated). This is known as the Bohr effect.
Chapter-End Exercise
VC = TV + IRV + ERV = 500 + 3000 + 1100 = 4600 mL (approximately)
Significance: 1. It indicates the maximum amount of air available for gas exchange. 2. It is a measure of lung function and respiratory health. 3. It is used in diagnosing respiratory disorders — reduced vital capacity indicates restrictive lung diseases like fibrosis. 4. Athletes generally have higher vital capacity due to better lung development. 5. It decreases with age and in conditions like emphysema and asthma.
FRC = ERV + RV = 1100 + 1200 = 2300 mL (approximately)
This volume of air remains in the lungs after a normal expiration. It is important because: 1. It prevents the lungs from collapsing completely. 2. It ensures continuous gas exchange even between breaths. 3. It dilutes the incoming air, preventing sudden changes in alveolar gas composition.
Note: The Residual Volume (RV ≈ 1200 mL) always remains in the lungs and can never be expelled voluntarily.
1. Large surface area: The alveoli provide an enormous surface area (about 70–100 m²) for gas exchange due to their numerous, thin-walled, balloon-like structure.
2. Thin barrier: The alveolar wall is extremely thin (about 0.5 micrometres), consisting of only squamous epithelial cells and basement membrane, which minimises the diffusion distance.
3. Rich blood supply: Each alveolus is surrounded by a dense network of pulmonary capillaries, ensuring close proximity between air and blood.
4. Favourable concentration gradient: The difference in partial pressures of O₂ and CO₂ between alveolar air and pulmonary blood creates a continuous concentration gradient for diffusion.
5. Moist surface: The inner lining of alveoli is moist, which is essential for gases to dissolve and diffuse.
The other parts of the respiratory system (trachea, bronchi, bronchioles) are conducting zones — they are thick-walled and not designed for gas exchange. They serve only to transport air to and from the alveoli.
1. As oxyhaemoglobin (98.5%): The major portion of oxygen binds reversibly with haemoglobin (Hb) in the red blood cells to form oxyhaemoglobin (HbO₂). Each haemoglobin molecule can bind up to 4 oxygen molecules. The binding is reversible and depends on partial pressure of oxygen (pO₂). In the lungs, where pO₂ is high, oxygen binds to Hb. In the tissues, where pO₂ is low, oxygen dissociates from Hb and is released to the cells.
Hb + O₂ ⇌ HbO₂
2. As dissolved gas (1.5%): A small fraction of oxygen is transported as dissolved gas in the blood plasma. This is the only form in which oxygen can diffuse from capillaries into tissue cells.
The oxygen-haemoglobin dissociation curve is sigmoid (S-shaped), which reflects the cooperative binding of oxygen to haemoglobin. The curve shifts right (decreased affinity) when pCO₂ increases, pH decreases, or temperature increases (Bohr effect).
1. As bicarbonate (HCO₃⁻) — approximately 70%: CO₂ enters RBCs and reacts with water in the presence of the enzyme carbonic anhydrase to form carbonic acid (H₂CO₃), which dissociates into H⁺ and HCO₃⁻. The HCO₃⁻ ions diffuse out of RBCs into the plasma and are transported to the lungs. In the lungs, the reverse reaction occurs, releasing CO₂ into the alveolar air.
CO₂ + H₂O →(Carbonic anhydrase)→ H₂CO₃ → H⁺ + HCO₃⁻
2. As carbaminohaemoglobin — approximately 23%: CO₂ binds directly to the amino group of haemoglobin, forming carbaminohaemoglobin (HbCO₂). This binding is reversible. In the lungs, where pCO₂ is low, CO₂ dissociates from haemoglobin and is released.
Hb-NH₂ + CO₂ → Hb-NHCOOH (carbaminohaemoglobin)
3. As dissolved CO₂ — approximately 7%: A small fraction of CO₂ is transported as dissolved gas in the blood plasma.
Inspiration (active process): 1. The inspiratory centre in the medulla oblongata sends signals to the diaphragm and external intercostal muscles. 2. The diaphragm contracts and flattens (moves downward). 3. The external intercostal muscles contract, raising the ribs upward and outward. 4. This increases the volume of the thoracic cavity in both the dorso-ventral and lateral dimensions. 5. The increased thoracic volume causes the lungs to expand (due to adhesion between the pleural membranes). 6. As the lungs expand, the intrapulmonary pressure decreases below atmospheric pressure. 7. Air rushes into the lungs down the pressure gradient.
Expiration (normally passive): 1. The inspiratory centre is inhibited, and the diaphragm and external intercostal muscles relax. 2. The diaphragm returns to its dome shape (moves upward). 3. The ribs move downward and inward. 4. This decreases the thoracic volume. 5. The decreased thoracic volume compresses the lungs. 6. The intrapulmonary pressure increases above atmospheric pressure. 7. Air is forced out of the lungs.
During forced expiration (e.g., during exercise), the internal intercostal muscles and abdominal muscles also contract to further reduce thoracic volume.
Significance: 1. Prevents lung collapse: RV prevents the alveoli from collapsing completely, keeping them partially inflated. This maintains the structural integrity of the lungs. 2. Continuous gas exchange: The remaining air ensures that gas exchange continues even between breaths, maintaining a steady supply of oxygen to the blood. 3. Dilution of fresh air: RV dilutes the incoming air, preventing sudden large changes in alveolar gas composition that could be harmful. 4. Reserve for emergencies: The residual air can be partially utilised during conditions of increased oxygen demand.
1. In tissues (high pCO₂): The tissues produce CO₂ as a by-product of metabolism. The increased pCO₂ in tissues decreases the affinity of haemoglobin for oxygen, causing more oxygen to be released from oxyhaemoglobin. This is beneficial because metabolically active tissues need more oxygen. The oxygen-haemoglobin dissociation curve shifts to the right.
2. In lungs (low pCO₂): In the alveoli, where pCO₂ is lower, the affinity of haemoglobin for oxygen increases, facilitating oxygen binding. The curve shifts to the left.
3. Effect on dissociation curve: An increase in pCO₂, decrease in pH, or increase in temperature shifts the oxygen-haemoglobin dissociation curve to the right (Bohr effect), promoting oxygen unloading. A decrease in pCO₂ shifts the curve to the left, promoting oxygen loading.
This mechanism ensures that oxygen is efficiently loaded in the lungs (where pCO₂ is low) and efficiently unloaded in the tissues (where pCO₂ is high).
1. Decreased atmospheric pressure: At high altitudes, the atmospheric pressure is lower, resulting in lower pO₂ in the air. This means less oxygen is available per breath.
2. Hypoxia: The reduced pO₂ leads to decreased oxygen saturation of haemoglobin, causing hypoxia (oxygen deficiency in tissues).
3. Increased breathing rate: The chemosensitive areas in the medulla detect the increased CO₂ and H⁺ concentration (and decreased pO₂), and increase the breathing rate to compensate for the oxygen deficit.
4. Increased heart rate: The heart pumps faster to deliver more blood to tissues in an attempt to meet their oxygen demands.
5. Increased RBC production: Over time, the kidneys release erythropoietin, which stimulates the bone marrow to produce more RBCs. This increases the oxygen-carrying capacity of the blood (acclimatisation).
6. Increased 2,3-BPG: The concentration of 2,3-bisphosphoglycerate in RBCs increases, which decreases haemoglobin's affinity for oxygen, promoting oxygen release to tissues.
7. Mountain sickness: Initially, a person may experience headache, dizziness, nausea, and shortness of breath (mountain sickness) due to the reduced oxygen availability.
1. Increased CO₂ production: During exercise, muscles metabolise more glucose and produce more CO₂, which dissolves in blood to form carbonic acid, lowering blood pH.
2. Increased O₂ demand: Working muscles require more oxygen for aerobic respiration to produce more ATP.
3. Chemosensory stimulation: The increased pCO₂ and decreased pH are detected by chemosensitive areas in the medulla oblongata and by aortic and carotid bodies. These send signals to the respiratory centre to increase both the rate and depth of breathing.
4. Neural stimulation: Proprioceptors in muscles and joints send signals to the respiratory centre about increased muscular activity.
5. Adrenaline release: During exercise, adrenaline is released, which directly stimulates the respiratory centre.
The combined effect is an increase in both the breathing rate (from about 12–16 breaths/min to 30–40 breaths/min during intense exercise) and the tidal volume, ensuring adequate oxygen supply and CO₂ removal.
Internal Links
- [Chapter 17 Guide — Breathing and Exchange of Gases](/school/cbse/class/11/biology/cbse-breathing-and-exchange-of-gases)
- [All Class 11 Biology Solutions](/school/cbse/class/11/biology/solutions)
- [Class 11 Biology Subject Guide](/school/cbse/class/11/biology)