Class 11 Biology Notes Chapter 17 (Breathing and exchange of gases) – Biology Book

Detailed Notes with MCQs of Chapter 17: Breathing and Exchange of Gases. This is a crucial chapter, not just for understanding our own bodies but also because questions frequently appear from this section in various government exams. Pay close attention to the definitions, mechanisms, and values mentioned.

Chapter 17: Breathing and Exchange of Gases - Detailed Notes

1. Introduction

• Breathing (Ventilation): The mechanical process of moving air into and out of the lungs. It involves inspiration (inhalation) and expiration (exhalation).
• Respiration: A broader term encompassing:
  • Breathing (External Respiration stage 1)
  • Exchange of gases (O2 and CO2) across the alveolar membrane (External Respiration stage 2).
  • Transport of gases by the blood.
  • Exchange of gases between blood and tissues (Internal Respiration).
  • Utilization of O2 by cells for catabolic reactions to release energy (Cellular Respiration).
• Respiratory Organs: Vary across the animal kingdom depending on habitat and level of organization.
  • Lower Invertebrates (Sponges, Coelenterates, Flatworms): Simple diffusion across the body surface.
  • Earthworms: Moist cuticle.
  • Insects: Network of tracheal tubes.
  • Aquatic Arthropods & Molluscs: Gills (Branchial respiration).
  • Terrestrial Vertebrates (Reptiles, Birds, Mammals): Lungs (Pulmonary respiration).
  • Amphibians (e.g., Frogs): Can respire through moist skin (Cutaneous respiration), buccal cavity, and lungs.

2. Human Respiratory System

• Pathway of Air:

  • External Nostrils: Entry point of air.
  • Nasal Cavity: Lined with mucous membrane and hair; filters, warms, and moistens incoming air.
  • Pharynx: Common passage for food and air. Nasopharynx (air only), Oropharynx, Laryngopharynx.
  • Larynx (Sound Box): Cartilaginous box containing vocal cords. Glottis is the opening into the larynx, covered by the epiglottis (a cartilaginous flap) during swallowing to prevent food entry.
  • Trachea (Windpipe): Straight tube extending up to the mid-thoracic cavity. Supported by C-shaped cartilaginous rings to prevent collapse. Lined with ciliated pseudostratified epithelium and goblet cells (mucus).
  • Bronchi: Trachea divides at the level of the 5th thoracic vertebra into right and left primary bronchi. Each enters a lung.
  • Bronchioles: Primary bronchi further divide into secondary, tertiary bronchi, and finally into terminal bronchioles. Lack cartilaginous rings but have smooth muscle.
  • Alveoli: Terminal bronchioles give rise to respiratory bronchioles, alveolar ducts, and finally clusters of thin-walled, vascularized bag-like structures called alveoli. These are the primary sites of gas exchange.

• Lungs:

  • Located in the thoracic cavity, resting on the diaphragm.
  • Covered by a double-layered membrane called the Pleura:
    • Parietal Pleura: Outer layer, lining the thoracic cavity.
    • Visceral Pleura: Inner layer, adhering to the lung surface.
    • Pleural Fluid: Present between the layers; reduces friction during breathing movements.
  • Right lung has 3 lobes; Left lung has 2 lobes (to accommodate the heart).
  • The conducting part (nostrils to terminal bronchioles) transports, filters, humidifies, and warms air.
  • The respiratory or exchange part (alveoli and their ducts) is the site of actual gas exchange.

3. Mechanism of Breathing

• Breathing involves creating a pressure gradient between the atmosphere and the alveoli.
• Inspiration (Inhalation):
  • An active process.
  • Contraction of the Diaphragm (flattens and moves down).
  • Contraction of the External Intercostal Muscles (lifts ribs and sternum upwards and outwards).
  • Increases the volume of the thoracic cavity anteroposteriorly and dorsoventrally.
  • Causes an increase in pulmonary volume.
  • Decreases intra-pulmonary pressure (becomes lower than atmospheric pressure).
  • Air rushes into the lungs.
• Expiration (Exhalation):
  • Generally a passive process (at rest).
  • Relaxation of the Diaphragm (returns to dome shape).
  • Relaxation of the External Intercostal Muscles (ribs and sternum return to original position).
  • Decreases the volume of the thoracic cavity.
  • Causes a decrease in pulmonary volume.
  • Increases intra-pulmonary pressure (becomes slightly higher than atmospheric pressure).
  • Air is forced out of the lungs.
  • Forced Expiration: Is an active process involving contraction of Internal Intercostal Muscles and Abdominal Muscles.

4. Respiratory Volumes and Capacities (Approximate values for a healthy adult)

• Tidal Volume (TV): Volume of air inspired or expired during normal, quiet breathing (~500 mL).
• Inspiratory Reserve Volume (IRV): Additional volume of air that can be forcibly inspired after a normal inspiration (~2500 - 3000 mL).
• Expiratory Reserve Volume (ERV): Additional volume of air that can be forcibly expired after a normal expiration (~1000 - 1100 mL).
• Residual Volume (RV): Volume of air remaining in the lungs even after a forcible expiration (~1100 - 1200 mL). Cannot be measured directly by spirometry.
• Inspiratory Capacity (IC): Total volume of air a person can inspire after a normal expiration (TV + IRV ≈ 3000 - 3500 mL).
• Expiratory Capacity (EC): Total volume of air a person can expire after a normal inspiration (TV + ERV ≈ 1500 - 1600 mL).
• Functional Residual Capacity (FRC): Volume of air remaining in the lungs after a normal expiration (ERV + RV ≈ 2100 - 2300 mL). This is the resting volume of the lungs.
• Vital Capacity (VC): Maximum volume of air a person can breathe in after a forced expiration, OR maximum volume of air a person can breathe out after a forced inspiration (IRV + TV + ERV ≈ 4000 - 4600 mL).
• Total Lung Capacity (TLC): Total volume of air accommodated in the lungs at the end of a forced inspiration (VC + RV or IRV + TV + ERV + RV ≈ 5100 - 5800 mL).
• Spirometer: Instrument used to measure respiratory volumes (except RV, FRC, TLC).

5. Exchange of Gases

• Occurs by simple diffusion based on partial pressure gradients.
• Partial Pressure: Pressure contributed by an individual gas in a mixture of gases. Denoted as pO2, pCO2.
• Sites of Exchange:
  • Alveoli to Blood: O2 diffuses from alveoli (pO2 ~104 mmHg) to pulmonary capillaries (pO2 ~40 mmHg). CO2 diffuses from pulmonary capillaries (pCO2 ~45 mmHg) to alveoli (pCO2 ~40 mmHg).
  • Blood to Tissues: O2 diffuses from systemic capillaries (pO2 ~95 mmHg) to tissues (pO2 ~40 mmHg). CO2 diffuses from tissues (pCO2 ~45 mmHg) to systemic capillaries (pCO2 ~40 mmHg).
• Factors Affecting Diffusion Rate:
  • Partial pressure gradient (higher gradient = faster diffusion).
  • Solubility of gases (CO2 is 20-25 times more soluble than O2).
  • Thickness of the diffusion membrane (thinner = faster diffusion).
  • Surface area of the respiratory membrane (larger = faster diffusion).
• Respiratory Membrane: Consists of:
  1. Thin squamous epithelium of alveoli.
  2. Endothelium of alveolar capillaries.
  3. Basement substance (thin layer) between them. (Total thickness << 1 mm).

Partial Pressures (mmHg) - Important Values:

6. Transport of Gases

• Transport of Oxygen (O2):
  • Dissolved in Plasma: ~3% (due to low solubility).
  • Bound to Hemoglobin (Hb): ~97%. Hb is a red-colored iron-containing protein in RBCs. Each Hb molecule can bind reversibly with up to 4 molecules of O2 to form Oxyhemoglobin (HbO2).
  • Binding depends on pO2 (major factor), pCO2, H+ concentration (pH), and temperature.
  • Oxygen Dissociation Curve: A sigmoid (S-shaped) curve obtained by plotting % saturation of Hb with O2 against pO2.
    • High pO2, Low pCO2, Low H+ (High pH), Lower Temp (in Alveoli): Favors binding of O2 to Hb (formation of oxyhemoglobin). Curve shifts LEFT.
    • Low pO2, High pCO2, High H+ (Low pH), Higher Temp (in Tissues): Favors dissociation of O2 from Hb (release of O2 to tissues). Curve shifts RIGHT (Bohr Effect - effect of H+/CO2 on O2 binding).
• Transport of Carbon Dioxide (CO2):
  • Dissolved in Plasma: ~7%.
  • Bound to Hemoglobin: ~20-25% as Carbaminohemoglobin (HbCO2). Binding occurs with the amino group on Hb (not the iron part) and is favored by low pO2 (in tissues).
  • As Bicarbonate Ions (HCO3-): ~70% (Most important method).
    • CO2 diffuses into RBCs in tissues.
    • Combines with water (H2O) to form Carbonic Acid (H2CO3). This reaction is greatly accelerated by the enzyme Carbonic Anhydrase (present in high concentration in RBCs, also in plasma but less).
    • CO2 + H2O <--Carbonic Anhydrase--> H2CO3
    • H2CO3 quickly dissociates into Bicarbonate ions (HCO3-) and Hydrogen ions (H+).
    • H2CO3 <--> H+ + HCO3-
    • HCO3- moves out of RBCs into the plasma. To maintain electrical neutrality, Chloride ions (Cl-) move from plasma into RBCs (Chloride Shift or Hamburger Phenomenon).
    • H+ ions are buffered by Hb within the RBCs.
    • In the alveoli (high pO2, low pCO2), the entire process reverses: HCO3- enters RBCs, Cl- leaves, H+ is released from Hb, H+ combines with HCO3- to form H2CO3, which splits into CO2 and H2O (catalyzed by carbonic anhydrase), and CO2 diffuses into the alveoli.

7. Regulation of Respiration

• Maintained by the neural system.
• Respiratory Rhythm Centre: Located in the Medulla Oblongata. Primarily responsible for generating the basic respiratory rhythm (inspiration/expiration).
• Pneumotaxic Centre: Located in the Pons. Can moderate the function of the respiratory rhythm centre, primarily by reducing the duration of inspiration, thus altering the respiratory rate.
• Chemosensitive Area: Adjacent to the rhythm centre in the Medulla. Highly sensitive to changes in blood CO2 concentration and H+ concentration. Increased CO2 or H+ stimulates this centre, leading to increased respiratory rate and depth to eliminate CO2.
• Peripheral Chemoreceptors: Located in the Aortic Arch and Carotid Artery. Also sensitive to changes in CO2 and H+ concentration, and significantly to low pO2. They send signals to the rhythm centre for corrective actions.
• Role of Oxygen: Has an insignificant direct role in regulating the respiratory rhythm centre under normal conditions. It becomes important only when pO2 levels fall significantly.

8. Disorders of the Respiratory System

• Asthma: Difficulty in breathing causing wheezing due to inflammation and spasm of bronchi and bronchioles, often triggered by allergens.
• Emphysema: Chronic disorder where alveolar walls are damaged, leading to decreased respiratory surface area. Major cause is cigarette smoking. Characterized by shortness of breath.
• Occupational Respiratory Disorders: Caused by long-term exposure to harmful dusts in certain industries.
  • Silicosis (silica dust) and Asbestosis (asbestos dust).
  • Lead to fibrosis (proliferation of fibrous connective tissues) causing serious lung damage. Protective masks are essential.

Multiple Choice Questions (MCQs)

1. The structure that prevents the entry of food into the windpipe during swallowing is:
   a) Glottis
   b) Larynx
   c) Epiglottis
   d) Pharynx

2. Which of the following muscles contract during normal, quiet inspiration?
   a) Internal intercostal muscles and Diaphragm
   b) External intercostal muscles and Diaphragm
   c) Abdominal muscles and Diaphragm
   d) Internal and External intercostal muscles only

3. The volume of air remaining in the lungs after a maximal forcible expiration is called:
   a) Tidal Volume (TV)
   b) Expiratory Reserve Volume (ERV)
   c) Residual Volume (RV)
   d) Vital Capacity (VC)

4. The partial pressure of oxygen (pO2) in the alveoli is approximately:
   a) 40 mmHg
   b) 95 mmHg
   c) 104 mmHg
   d) 159 mmHg

5. The majority of carbon dioxide (CO2) produced by body tissues is transported to the lungs as:
   a) Dissolved CO2 in plasma
   b) Carbaminohemoglobin
   c) Bicarbonate ions (HCO3-)
   d) Carbonic acid (H2CO3)

6. The enzyme crucial for the rapid conversion of CO2 and water into carbonic acid within RBCs is:
   a) Carboxypeptidase
   b) Carbonic Anhydrase
   c) ATPase
   d) Succinate dehydrogenase

7. A shift of the oxygen dissociation curve to the right indicates:
   a) Increased affinity of Hb for O2
   b) Decreased dissociation of O2 from Hb
   c) Facilitated uptake of O2 in the lungs
   d) Decreased affinity of Hb for O2, facilitating O2 release in tissues

8. The primary respiratory rhythm centre is located in the:
   a) Pons
   b) Medulla Oblongata
   c) Cerebellum
   d) Cerebrum

9. Which respiratory disorder is characterized by damage to the alveolar walls, often caused by smoking?
   a) Asthma
   b) Bronchitis
   c) Emphysema
   d) Silicosis

10. The exchange of Cl- ions from plasma into RBCs and HCO3- ions from RBCs into plasma is known as:
    a) Bohr Effect
    b) Haldane Effect
    c) Chloride Shift (Hamburger Phenomenon)
    d) Simple Diffusion

Answer Key:

1. c) Epiglottis
2. b) External intercostal muscles and Diaphragm
3. c) Residual Volume (RV)
4. c) 104 mmHg
5. c) Bicarbonate ions (HCO3-)
6. b) Carbonic Anhydrase
7. d) Decreased affinity of Hb for O2, facilitating O2 release in tissues
8. b) Medulla Oblongata
9. c) Emphysema
10. c) Chloride Shift (Hamburger Phenomenon)

Study these notes thoroughly. Remember the specific values for volumes and partial pressures, understand the mechanisms of breathing and transport, and know the locations and functions of the regulatory centres. Good luck with your preparation!