Class 11 Biology Notes Chapter 17 (Chapter 17) – Examplar Problems (English) Book

Detailed Notes with MCQs of Chapter 17: Breathing and Exchange of Gases. This is a crucial chapter for understanding human physiology and frequently appears in government exams. We'll cover the key concepts based on the NCERT Exemplar perspective, focusing on points vital for competitive exams.

Chapter 17: Breathing and Exchange of Gases - Detailed Notes

1. Introduction:

• Breathing (Ventilation): The mechanical process of moving air into (inspiration) and out of (expiration) the lungs. It facilitates the exchange of O2 and CO2.
• Respiration: A broader term encompassing:
  • Breathing (External Respiration stage 1)
  • Exchange of gases across the alveolar membrane (External Respiration stage 2)
  • Transport of gases by blood.
  • Exchange of gases between blood and tissues (Internal Respiration).
  • Utilization of O2 by cells for catabolic reactions releasing energy (Cellular Respiration).
• Need for Gas Exchange: Organisms need a continuous supply of O2 for cellular respiration (energy production) and need to remove CO2, a metabolic waste product.

2. Human Respiratory System:

• Conducting Part: Transports atmospheric air to the alveoli, clears it from foreign particles, humidifies it, and brings it to body temperature.
  • External Nostrils -> Nasal Chamber -> Nasopharynx -> Larynx (Sound Box) -> Trachea (Windpipe) -> Primary Bronchi -> Secondary Bronchi -> Tertiary Bronchi -> Bronchioles -> Terminal Bronchioles.
  • Larynx: Cartilaginous box (contains vocal cords). Glottis is the opening, covered by epiglottis (cartilaginous flap) during swallowing to prevent food entry.
  • Trachea: Straight tube extending up to mid-thoracic cavity, divides at the level of the 5th thoracic vertebra. Supported by incomplete C-shaped cartilaginous rings (prevent collapse).
• Respiratory or Exchange Part: Site of actual diffusion of O2 and CO2 between blood and atmospheric air.
  • Alveoli and their ducts.
  • Alveoli: Very thin-walled, irregular, vascularised bag-like structures. Form the primary site for gas exchange. Wall is composed of thin squamous epithelium.
• Lungs:
  • Paired, located in the airtight thoracic chamber.
  • Covered by double-layered pleura: Outer parietal pleura (lines thoracic cavity) and inner visceral pleura (adheres to lung surface). Pleural fluid between layers reduces friction.
  • Right lung has 3 lobes; Left lung has 2 lobes (provides space for the heart - cardiac notch).
• Thoracic Chamber: Formed dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs, and on the lower side by the dome-shaped diaphragm. Any change in thoracic volume reflects in the pulmonary (lung) volume.

3. Mechanism of Breathing:

• Breathing involves Inspiration (active) and Expiration (passive, can be forceful/active).
• Based on creating a pressure gradient between the lungs and the atmosphere.
  • Intra-pulmonary pressure: Pressure inside the lungs.
  • Atmospheric pressure: Pressure outside the body.
• Inspiration:
  • Diaphragm contracts (flattens).
  • External intercostal muscles contract (lift ribs and sternum upwards and outwards).
  • Increases volume of the thoracic cavity -> Increases pulmonary volume.
  • Decreases intra-pulmonary pressure (becomes less than atmospheric pressure).
  • Air moves from higher pressure (atmosphere) to lower pressure (lungs).
• Expiration:
  • Diaphragm relaxes (returns to dome shape).
  • External intercostal muscles relax (ribs and sternum return to original position).
  • Decreases volume of the thoracic cavity -> Decreases pulmonary volume.
  • Increases intra-pulmonary pressure (becomes slightly higher than atmospheric pressure).
  • Air moves from higher pressure (lungs) to lower pressure (atmosphere).
• Forced Expiration: Involves contraction of internal intercostal muscles and abdominal muscles.

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

• Tidal Volume (TV): Volume of air inspired or expired during normal respiration (~500 mL).
• Inspiratory Reserve Volume (IRV): Additional volume of air a person can inspire by forceful inspiration (~2500 - 3000 mL).
• Expiratory Reserve Volume (ERV): Additional volume of air a person can expire by forceful expiration (~1000 - 1100 mL).
• Residual Volume (RV): Volume of air remaining in the lungs even after forceful expiration (~1100 - 1200 mL). Prevents lung collapse.
• Inspiratory Capacity (IC): Total volume of air inspired after normal expiration (TV + IRV) (~3000 - 3500 mL).
• Expiratory Capacity (EC): Total volume of air expired after normal inspiration (TV + ERV) (~1500 - 1600 mL).
• Functional Residual Capacity (FRC): Volume of air remaining in lungs after normal expiration (ERV + RV) (~2100 - 2300 mL).
• Vital Capacity (VC): Maximum volume of air a person can breathe in after forced expiration OR maximum volume of air a person can breathe out after forced inspiration (ERV + TV + IRV) (~4000 - 4600 mL).
• Total Lung Capacity (TLC): Total volume of air accommodated in the lungs at the end of forced inspiration (RV + ERV + TV + IRV or VC + RV) (~5100 - 5800 mL).
• Spirometer: Instrument used to measure respiratory volumes (except RV, FRC, TLC which require indirect methods).

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 (e.g., pO2, pCO2).

• Factors affecting diffusion rate:

  • 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).
  • Surface area of the respiratory membrane (larger = faster).

• Diffusion Membrane (Alveolar-Capillary Membrane): Very thin (< 1 mm), consists of:

  1. Thin squamous epithelium of alveoli.
  2. Endothelium of alveolar capillaries.
  3. Basement substance (thin layer) between them.

• Partial Pressure Gradients (Approximate values in mmHg):

  Gas	Atmospheric Air	Alveoli	Deoxygenated Blood	Oxygenated Blood	Tissues
  O2	159	104	40	95	<40
  CO2	0.3	40	45	40	>45

• Exchange Sequence:

  • Alveoli to Blood: O2 diffuses from alveoli (pO2 104) to blood (pO2 40). CO2 diffuses from blood (pCO2 45) to alveoli (pCO2 40).
  • Blood to Tissues: O2 diffuses from blood (pO2 95) to tissues (pO2 <40). CO2 diffuses from tissues (pCO2 >45) to blood (pCO2 40).

6. Transport of Gases:

• Oxygen Transport:
  • ~3% dissolved in plasma.
  • ~97% bound to Hemoglobin (Hb) in RBCs, forming Oxyhemoglobin (HbO2).
  • Hb + 4O2 <=> Hb(O2)4 (Reversible reaction)
  • Binding depends mainly on pO2. Also affected by pCO2, H+ concentration (pH), and temperature.
  • Oxygen-Hemoglobin Dissociation Curve: Sigmoid (S-shaped) curve showing the relationship between pO2 and % saturation of Hb with O2.
    • Factors shifting the curve to the RIGHT (decreased affinity, favors O2 unloading in tissues):
      • Increased pCO2
      • Increased H+ concentration (decreased pH) - Bohr Effect
      • Increased temperature
      • Increased 2,3-Diphosphoglycerate (2,3-DPG)
    • Factors shifting the curve to the LEFT (increased affinity, favors O2 loading in lungs): Opposite conditions.
• Carbon Dioxide Transport:
  • ~7% dissolved in plasma.
  • ~20-25% bound to Hb as Carbaminohemoglobin (HbCO2) (binds to amino group of globin part, influenced by pCO2, pO2 is a major factor).
  • ~70% transported as Bicarbonate ions (HCO3-) in plasma.
    • CO2 diffuses into RBCs.
    • Combines with water: CO2 + H2O <--[Carbonic Anhydrase]--> H2CO3 (Carbonic acid)
    • H2CO3 dissociates: H2CO3 <=> H+ + HCO3-
    • Carbonic Anhydrase: Enzyme present in high concentration in RBCs (also small amount in plasma), greatly accelerates the reaction.
    • HCO3- moves out of RBCs into plasma. To maintain electrical neutrality, Cl- ions move from plasma into RBCs (Chloride Shift / Hamburger Phenomenon).
    • In the lungs (low pCO2), the entire process reverses, releasing CO2 into alveoli.
• Summary: Every 100 ml of deoxygenated blood delivers approx. 4 ml of CO2 to the alveoli. Every 100 ml of oxygenated blood delivers approx. 5 ml of O2 to the tissues under normal conditions.

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. Its signals can reduce the duration of inspiration, thereby altering the respiratory rate.
• Apneustic Centre: Located in the lower Pons (often considered part of the medullary complex interaction). Promotes inspiration. Its activity is modulated by the pneumotaxic centre.
• Chemosensitive Area: Situated adjacent to the rhythm centre in the medulla. Highly sensitive to changes in blood pCO2 and H+ concentration. Increased CO2 or H+ activates this centre, which signals the rhythm centre to make adjustments (increase rate and depth) to eliminate CO2.
• Peripheral Chemoreceptors: Located in the Aortic Arch and Carotid Artery. Also sensitive to changes in pCO2 and H+ concentration, but also respond to significant decreases in pO2. Send signals to the rhythm centre for corrective actions.
• Role of O2: The role of oxygen in regulating respiratory rhythm is quite insignificant under normal conditions; pCO2 and H+ are the primary drivers.
• Hering-Breuer Reflex: Stretch receptors in the walls of bronchi and bronchioles, activated during over-inflation of lungs, send inhibitory signals via the Vagus nerve to the inspiratory centre to terminate inspiration and start expiration. Prevents lung over-distension.

8. Disorders of 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 substances in certain industries.
  • Silicosis (silica dust - mining, stone-breaking)
  • Asbestosis (asbestos dust - construction, insulation)
  • Characterized by fibrosis (proliferation of fibrous connective tissues) causing serious lung damage. Protective masks are essential.
• Bronchitis: Inflammation of the bronchi, often causing coughing with mucus. Can be acute (often viral) or chronic (often related to smoking or pollution).
• Pneumonia: Inflammation of the alveoli, which may fill with fluid or pus, caused by bacterial, viral, or fungal infections. Impairs gas exchange.

Multiple Choice Questions (MCQs):

1. Which of the following correctly represents the sequence of structures air passes through during inhalation?
   (a) Nasopharynx -> Larynx -> Trachea -> Bronchioles -> Bronchi -> Alveoli
   (b) Nasal Chamber -> Larynx -> Pharynx -> Trachea -> Bronchi -> Alveoli
   (c) Nostrils -> Nasal Chamber -> Nasopharynx -> Larynx -> Trachea -> Bronchi -> Bronchioles -> Alveoli
   (d) Nostrils -> Pharynx -> Larynx -> Trachea -> Bronchioles -> Alveoli

2. During normal quiet breathing, the contraction of which muscles initiates inspiration?
   (a) Internal intercostal muscles only
   (b) Diaphragm only
   (c) External intercostal muscles and Diaphragm
   (d) Abdominal muscles and Diaphragm

3. What is the approximate value of Residual Volume (RV) in a healthy human adult?
   (a) 500 mL
   (b) 1100 - 1200 mL
   (c) 2500 - 3000 mL
   (d) 4000 - 4600 mL

4. The diffusion membrane for gas exchange in the lungs is composed of all the following EXCEPT:
   (a) Squamous epithelium of alveoli
   (b) Endothelium of alveolar capillaries
   (c) Basement substance between alveoli and capillary
   (d) Smooth muscle layer of the bronchiole

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

6. A shift of the oxygen-hemoglobin dissociation curve to the right indicates:
   (a) Increased affinity of Hb for O2
   (b) Decreased pCO2 in the blood
   (c) Decreased pH (increased H+ concentration)
   (d) Decreased body temperature

7. The respiratory rhythm centre responsible for generating the basic breathing pattern is located in the:
   (a) Pons
   (b) Cerebellum
   (c) Medulla Oblongata
   (d) Hypothalamus

8. The enzyme crucial for the rapid formation of carbonic acid within RBCs during CO2 transport is:
   (a) Carbonic dehydrogenase
   (b) Carbonic anhydrase
   (c) Carboxykinase
   (d) ATPase

9. Which respiratory disorder is characterized by damage to alveolar walls, leading to a decrease in respiratory surface area, often linked to smoking?
   (a) Asthma
   (b) Bronchitis
   (c) Emphysema
   (d) Silicosis

10. The primary stimulus for the chemosensitive area located near the respiratory rhythm centre is an increase in:
    (a) Blood pO2
    (b) Blood pCO2 and H+ concentration
    (c) Blood pH
    (d) Cerebrospinal fluid pO2

Answer Key for MCQs:

1. (c)
2. (c)
3. (b)
4. (d)
5. (c)
6. (c)
7. (c)
8. (b)
9. (c)
10. (b)

Remember to correlate these notes with the diagrams in your NCERT book and practice the Exemplar problems for this chapter thoroughly. Understanding the pressure gradients and the factors affecting gas transport, especially the Oxygen Dissociation Curve, is key. Good luck with your preparation!