Breathing and Exchange of Gases
Welcome to HSLC Guru! In this article, we present comprehensive notes and question answers for Class 11 Biology Chapter 17 — Breathing and Exchange of Gases, prepared strictly according to the ASSEB (Assam State School Education Board) syllabus. This chapter explains how living organisms obtain oxygen for cellular respiration and release carbon dioxide, focusing on the human respiratory system, the mechanism of breathing, exchange and transport of gases, regulation of respiration, and respiratory disorders. Each section is designed in clear English medium language to help students prepare effectively for exams.
Summary
All living cells require continuous supply of oxygen (O₂) to oxidise nutrients and release energy in the form of ATP. The carbon dioxide (CO₂) produced during this process must be removed from the body. The process of exchange of O₂ from atmosphere with CO₂ produced by the cells is called breathing or respiration. Different animals use different respiratory organs depending on their habitat and level of organisation. Lower invertebrates such as sponges, coelenterates and flatworms exchange gases through simple diffusion across their general body surface. Earthworms use their moist cuticle, while aquatic arthropods and most fishes use gills (branchial respiration). Insects have a network of tracheal tubes that directly transport atmospheric air to the tissues. Terrestrial vertebrates including amphibians, reptiles, birds and mammals use well-developed lungs (pulmonary respiration). Frogs respire through both moist skin (cutaneous respiration) and lungs.
The human respiratory system consists of a pair of external nostrils that open into the nasal chamber. Air passes from nasal chamber through pharynx, larynx (sound box), trachea (wind pipe), and primary, secondary and tertiary bronchi. The bronchi divide into smaller bronchioles which terminate in thin-walled, vascularised, balloon-like structures called alveoli. The alveoli and their ducts together form the lungs. The lungs are situated in the thoracic chamber, covered by a double-layered pleura with pleural fluid in between. The trachea is supported by C-shaped cartilaginous rings that prevent its collapse. The branching network of bronchi, bronchioles and alveoli is called the bronchial tree. Respiration involves four steps — breathing (pulmonary ventilation), exchange of gases between alveoli and blood, transport of gases by blood, and exchange of gases between blood and tissues, followed by cellular respiration.
The mechanism of breathing involves two stages — inspiration and expiration. During inspiration, the diaphragm contracts and flattens, while the external intercostal muscles contract, lifting the ribs and sternum upward and outward. This increases the volume of the thoracic cavity and lungs, lowering intra-pulmonary pressure below atmospheric pressure, so air flows into the lungs. During expiration, the diaphragm and external intercostal muscles relax, decreasing the thoracic volume and lungs, increasing intra-pulmonary pressure above atmospheric pressure, forcing air out. A healthy human breathes 12–16 times per minute. Volume of air involved in breathing movements can be measured using a spirometer. Important respiratory volumes and capacities are: Tidal Volume (TV) ≈ 500 mL, Inspiratory Reserve Volume (IRV) ≈ 2500–3000 mL, Expiratory Reserve Volume (ERV) ≈ 1000–1100 mL, Residual Volume (RV) ≈ 1100–1200 mL, Inspiratory Capacity (IC = TV + IRV), Expiratory Capacity (EC = TV + ERV), Functional Residual Capacity (FRC = ERV + RV), Vital Capacity (VC = IRV + TV + ERV) and Total Lung Capacity (TLC = VC + RV ≈ 6000 mL).
The exchange of gases takes place at the alveoli and tissues by simple diffusion based on differences in partial pressures (pO₂ and pCO₂). pO₂ is high (104 mm Hg) in alveoli and low (40 mm Hg) in deoxygenated blood, so O₂ diffuses from alveoli into blood. pCO₂ is high (45 mm Hg) in deoxygenated blood and low (40 mm Hg) in alveoli, so CO₂ diffuses out. The diffusion membrane is very thin (less than 1 mm). Transport of gases — about 97% of O₂ is carried by haemoglobin (RBCs) as oxyhaemoglobin and 3% in dissolved form in plasma. About 70% of CO₂ is transported as bicarbonate (HCO₃⁻), 20–25% as carbamino-haemoglobin and 7% in dissolved form. The oxyhaemoglobin dissociation curve is sigmoid; high pO₂, low pCO₂, low H⁺ and low temperature favour formation of oxyhaemoglobin in alveoli, while opposite conditions in tissues favour dissociation (Bohr effect). Respiration is regulated by the respiratory centre located in the medulla oblongata, with a pneumotaxic centre in the pons and a chemosensitive area sensitive to CO₂ and H⁺. Common respiratory disorders include asthma (inflammation of bronchi), emphysema (damage to alveolar walls), and occupational respiratory disorders like silicosis and asbestosis caused by long-term exposure to industrial dust.
Question and Answers
Very Short Answer Questions (1 Mark)
Q1. Define breathing.
Answer: Breathing is the process of exchange of oxygen from the atmosphere with carbon dioxide produced by the cells of the body.
Q2. What is the respiratory organ in insects?
Answer: A network of tracheal tubes that directly transport atmospheric air to body tissues.
Q3. Name the structural and functional unit of the lung.
Answer: Alveolus is the structural and functional unit of the lung.
Q4. What is tidal volume?
Answer: The volume of air inspired or expired during a normal respiration. It is approximately 500 mL.
Q5. Where is the respiratory centre located in humans?
Answer: The respiratory centre is located in the medulla oblongata of the brain.
Q6. What is the Bohr effect?
Answer: The shift of the oxyhaemoglobin dissociation curve to the right with increase in pCO₂ and H⁺ ions, favouring release of O₂ at tissues, is called the Bohr effect.
Q7. Name the enzyme that catalyses CO₂ + H₂O → H₂CO₃ in RBCs.
Answer: The enzyme is carbonic anhydrase.
Q8. What is vital capacity?
Answer: Vital capacity is the maximum volume of air a person can breathe out after a forced inspiration. VC = IRV + TV + ERV.
Q9. Name two occupational respiratory disorders.
Answer: Silicosis and asbestosis are two occupational respiratory disorders.
Q10. What is the partial pressure of O₂ in alveoli?
Answer: The partial pressure of oxygen (pO₂) in alveoli is about 104 mm Hg.
Short Answer Questions (2–3 Marks)
Q1. List the respiratory organs found in different animals.
Answer: Sponges, coelenterates and flatworms exchange gases through their body surface by diffusion. Earthworms respire through their moist cuticle. Insects use a tracheal system. Aquatic arthropods and fishes use gills. Frogs use moist skin and lungs. Reptiles, birds and mammals use well-developed lungs for pulmonary respiration.
Q2. Trace the path of air from external nostrils to alveoli.
Answer: External nostrils → Nasal chamber → Pharynx → Larynx → Trachea → Primary bronchi → Secondary bronchi → Tertiary bronchi → Bronchioles → Terminal bronchioles → Alveolar ducts → Alveoli.
Q3. Differentiate between inspiration and expiration.
Answer: Inspiration is an active process in which the diaphragm contracts and flattens, the external intercostal muscles contract lifting the ribs and sternum upward, increasing thoracic volume and decreasing intra-pulmonary pressure so that air enters the lungs. Expiration is a passive process where the diaphragm and intercostal muscles relax, decreasing thoracic volume and increasing intra-pulmonary pressure, so air is forced out of the lungs.
Q4. Define IRV, ERV and RV with their values.
Answer: Inspiratory Reserve Volume (IRV) is the additional volume of air inspired by forced inspiration (2500–3000 mL). Expiratory Reserve Volume (ERV) is the additional volume of air expelled by forced expiration (1000–1100 mL). Residual Volume (RV) is the volume of air still remaining in the lungs even after a forced expiration (1100–1200 mL).
Q5. What is functional residual capacity (FRC)?
Answer: Functional Residual Capacity is the volume of air that remains in the lungs after a normal expiration. FRC = ERV + RV ≈ 2100–2300 mL. It prevents the alveoli from collapsing and allows continuous gas exchange between breaths.
Q6. How is CO₂ transported in blood?
Answer: CO₂ is transported in three forms — about 70% as bicarbonate ions (HCO₃⁻) in plasma, formed by the action of carbonic anhydrase in RBCs; about 20–25% as carbamino-haemoglobin bound to the globin part of haemoglobin; and about 7% in dissolved form in plasma.
Long Answer Questions (5–7 Marks)
Q1. Describe the human respiratory system in detail.
Answer: The human respiratory system consists of a pair of external nostrils opening above the upper lip. They lead to the nasal chamber, which is divided into right and left halves by a nasal septum. The nasal chamber opens into the pharynx, a common passage for food and air. From pharynx, air passes through the larynx (sound box) which contains vocal cords. A leaf-like cartilaginous flap called the epiglottis covers the glottis during swallowing to prevent food from entering the larynx. The larynx leads to the trachea, a long tube supported by C-shaped cartilaginous rings to prevent collapse. At the level of the 5th thoracic vertebra, the trachea divides into right and left primary bronchi. Each primary bronchus enters the lung and divides repeatedly into secondary, tertiary bronchi and finally bronchioles. The terminal bronchioles end in thin-walled, balloon-like, highly vascularised alveoli where exchange of gases occurs. The lungs are situated in the thoracic cavity, enclosed by two pleural membranes with pleural fluid between them, which reduces friction during breathing. The thoracic cavity is bound on top by the neck, on the sides by ribs, and below by the dome-shaped diaphragm.
Q2. Explain the mechanism of breathing in humans.
Answer: Breathing involves two stages — inspiration and expiration — both governed by changes in pressure inside the thoracic cavity. During inspiration, the diaphragm contracts and flattens, increasing the vertical volume of the thoracic cavity. The external intercostal muscles contract simultaneously, raising the ribs and sternum, which increases the antero-posterior axis. The overall increase in thoracic volume causes a corresponding increase in pulmonary volume, which reduces intra-pulmonary pressure to below atmospheric pressure. As a result, atmospheric air rushes into the lungs through the respiratory passage. During expiration, the diaphragm relaxes and returns to its dome shape, while the external intercostal muscles relax, lowering the ribs and sternum. This decreases the thoracic volume and pulmonary volume, raising intra-pulmonary pressure slightly above atmospheric pressure, forcing air out of the lungs. A healthy person breathes 12–16 times per minute. Air pressure differences and elastic recoil of the lungs are essential for normal breathing.
Q3. Describe respiratory volumes and capacities with their values.
Answer: Respiratory volumes and capacities can be measured using a spirometer. (i) Tidal Volume (TV) – Volume of air inspired or expired during normal breathing, about 500 mL. (ii) Inspiratory Reserve Volume (IRV) – Additional volume of air inspired by forceful inspiration, 2500–3000 mL. (iii) Expiratory Reserve Volume (ERV) – Additional volume of air expired by forceful expiration, 1000–1100 mL. (iv) Residual Volume (RV) – Volume of air remaining in the lungs after forcible expiration, 1100–1200 mL. Capacities are formed by combining two or more volumes: (a) Inspiratory Capacity (IC) = TV + IRV; (b) Expiratory Capacity (EC) = TV + ERV; (c) Functional Residual Capacity (FRC) = ERV + RV; (d) Vital Capacity (VC) = IRV + TV + ERV (≈ 4500 mL); (e) Total Lung Capacity (TLC) = VC + RV (≈ 6000 mL).
Q4. Explain the transport of oxygen and carbon dioxide in human blood.
Answer: About 97% of oxygen is transported by red blood cells, while about 3% is carried in the dissolved form in plasma. Each haemoglobin molecule binds reversibly with four molecules of O₂ to form oxyhaemoglobin (Hb + 4O₂ ⇌ Hb(O₂)₄). The binding depends mainly on partial pressure of O₂ (pO₂); high pO₂, low pCO₂, low H⁺ concentration and low temperature in alveoli favour formation of oxyhaemoglobin, while opposite conditions in tissues favour dissociation. Carbon dioxide transport takes place in three ways — about 70% as bicarbonate ions (HCO₃⁻) formed by the action of the enzyme carbonic anhydrase: CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻; about 20–25% as carbamino-haemoglobin formed by binding of CO₂ to the globin part of haemoglobin; and 7% as dissolved CO₂ in plasma. At alveoli, where pCO₂ is low, CO₂ is liberated from carbamino-haemoglobin and bicarbonate, ready for diffusion into alveolar air.
Q5. Discuss the regulation of respiration and common respiratory disorders.
Answer: Respiration is regulated by the respiratory rhythm centre located in the medulla oblongata of the brain. A pneumotaxic centre in the pons varolii of the brain can moderate the function of the respiratory rhythm centre. Adjacent to the rhythm centre is the chemosensitive area which is highly sensitive to CO₂ and H⁺ ions. Increased levels of CO₂ and H⁺ activate this area, which signals the rhythm centre to alter the rate of respiration. Receptors associated with the aortic arch and carotid artery also recognise changes in CO₂ and H⁺ levels and send signals to the rhythm centre. The role of O₂ in regulation is insignificant. Common respiratory disorders include — (i) Asthma: difficulty in breathing causing wheezing due to inflammation of bronchi and bronchioles. (ii) Emphysema: a chronic disorder in which alveolar walls are damaged, decreasing the respiratory surface; cigarette smoking is a major cause. (iii) Occupational respiratory disorders: caused by long-term exposure to harmful substances. Industries involving stone-grinding or breaking, asbestos, etc. produce dust which damages the lungs causing inflammation, fibrosis and lung cancer. Examples are silicosis and asbestosis.
Multiple Choice Questions (MCQ)
Q1. The structural and functional unit of lung is —
(a) Bronchus (b) Bronchiole (c) Alveolus (d) Trachea
Answer: (c) Alveolus
Q2. The volume of air inspired or expired during normal breathing is —
(a) Vital Capacity (b) Tidal Volume (c) Residual Volume (d) IRV
Answer: (b) Tidal Volume
Q3. The respiratory rhythm centre is located in the —
(a) Cerebrum (b) Cerebellum (c) Medulla oblongata (d) Hypothalamus
Answer: (c) Medulla oblongata
Q4. Insects respire through —
(a) Skin (b) Gills (c) Tracheal system (d) Lungs
Answer: (c) Tracheal system
Q5. Vital capacity = —
(a) TV + IRV (b) TV + ERV (c) IRV + ERV + RV (d) IRV + TV + ERV
Answer: (d) IRV + TV + ERV
Q6. The percentage of CO₂ transported as bicarbonate is —
(a) 7% (b) 20% (c) 70% (d) 97%
Answer: (c) 70%
Q7. The enzyme that catalyses the formation of bicarbonate from CO₂ is —
(a) Carbonic anhydrase (b) ATP synthase (c) Pepsin (d) Amylase
Answer: (a) Carbonic anhydrase
Q8. Partial pressure of O₂ in alveoli is approximately —
(a) 40 mm Hg (b) 95 mm Hg (c) 104 mm Hg (d) 159 mm Hg
Answer: (c) 104 mm Hg
Q9. The rightward shift of oxyhaemoglobin curve in tissues is called —
(a) Hill effect (b) Bohr effect (c) Pasteur effect (d) Haldane effect
Answer: (b) Bohr effect
Q10. Which disease is caused by stone-grinding industries?
(a) Asthma (b) Silicosis (c) Bronchitis (d) Pneumonia
Answer: (b) Silicosis
Fill in the Blanks
Q1. The trachea is supported by ________ shaped cartilaginous rings.
Answer: C
Q2. Total Lung Capacity = Vital Capacity + ________.
Answer: Residual Volume
Q3. The leaf-like flap covering the glottis during swallowing is called ________.
Answer: Epiglottis
Q4. About ________ % of oxygen is carried by haemoglobin.
Answer: 97
Q5. The pneumotaxic centre is located in the ________.
Answer: Pons varolii
True or False
Q1. Earthworms respire through gills. Answer: False (through moist cuticle).
Q2. Diaphragm contracts and flattens during inspiration. Answer: True.
Q3. Tidal volume of a healthy human is about 500 mL. Answer: True.
Q4. Asthma is caused by exposure to silica dust. Answer: False (silicosis is caused by silica; asthma is due to allergic inflammation).
Q5. The respiratory rhythm centre lies in the medulla oblongata. Answer: True.
Glossary
| Term | Meaning |
|---|---|
| Breathing | The mechanical process of taking in O₂ and giving out CO₂. |
| Alveoli | Tiny air sacs in the lungs where gaseous exchange takes place. |
| Trachea | The wind pipe that carries air from larynx to bronchi. |
| Diaphragm | A dome-shaped muscle below the lungs that aids breathing. |
| Pleura | Double-layered membrane covering the lungs with fluid between layers. |
| Tidal Volume | Volume of air inspired or expired in normal breathing (~500 mL). |
| Vital Capacity | Maximum air that can be expired after forceful inspiration. |
| Residual Volume | Air that remains in lungs even after forced expiration. |
| Oxyhaemoglobin | Compound formed when haemoglobin combines with oxygen. |
| Carbamino-haemoglobin | Compound formed by binding of CO₂ with haemoglobin. |
| Carbonic anhydrase | Enzyme in RBCs that catalyses CO₂ + H₂O ⇌ H₂CO₃. |
| Bohr Effect | Right shift of O₂ dissociation curve due to high CO₂ and H⁺. |
| Partial Pressure | Pressure exerted by an individual gas in a mixture of gases. |
| Asthma | Difficulty in breathing due to inflammation of bronchi. |
| Emphysema | Chronic disorder where alveolar walls are damaged. |
| Silicosis | Lung disease caused by long-term inhalation of silica dust. |
| Asbestosis | Lung disease caused by inhaling asbestos fibres. |
| Medulla Oblongata | Region of brain containing the respiratory rhythm centre. |
| Pneumotaxic Centre | Region in pons that moderates respiratory rhythm. |
| Spirometer | Instrument used for measuring respiratory volumes. |
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