NCERT Class 11 Biology: Chapter 14 — Breathing and Exchange of Gases

14.1 Respiratory Organs Across Animal Phyla & Human Respiratory System Anatomy #

Respiration is the biological process of exchanging oxygen (O₂) from the atmosphere with carbon dioxide (CO₂) produced by the cells. The mechanism of breathing varies across different animal groups depending mainly on their habitats and levels of structural organization:

• Simple diffusion across body surface: Sponges, coelenterates, and flatworms exchange O₂ and CO₂ by simple diffusion over their entire body surface.
• Cutaneous respiration: Earthworms use their moist skin (cuticle), and amphibians (frogs) can respire through their moist skin.
• Tracheal system: A network of branched tubes (tracheae and tracheoles) transporting atmospheric air directly to body cells in insects (cockroaches).
• Branchial respiration: Specialized vascularized gills used by most aquatic arthropods, molluscs, and fishes.
• Pulmonary respiration: Vascularized bags called lungs used by terrestrial vertebrates (amphibians, reptiles, birds, and mammals).

Human Respiratory System: Begins with external nostrils opening into the nasal cavity lined with mucus and fine hair. The nasal chamber opens into the pharynx (the common passage for food and air). The pharynx opens through the glottis into the larynx (a cartilaginous voice box producing sound). During swallowing, the glottis is covered by a thin, elastic cartilaginous flap called the epiglottis to prevent food entry into the windpipe.

The trachea is a straight tube extending down to the mid-thoracic cavity, where it divides at the level of the 5th thoracic vertebra (T5) into right and left primary bronchi. The trachea, primary, secondary, and tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous C-shaped rings to prevent airway collapse. Terminal bronchioles terminate into very thin, irregular-walled, highly vascularized bag-like structures called alveoli (the functional respiratory units where gas exchange occurs). The human lungs contain roughly 300 million alveoli, providing a massive surface area (~70–100 m²).

14.2 Mechanism of Breathing: Inspiration (Diaphragm & External Intercostals Contract) vs Expiration #

Breathing (pulmonary ventilation) involves two major physical phases: Inspiration, during which atmospheric air is drawn into the lungs, and Expiration, by which alveolar air is expelled out.

Air moves into and out of the lungs by generating a pressure gradient between the lungs and the atmosphere:

• Inspiration (Active Process):

  Occurs when intra-pulmonary pressure is lower than atmospheric pressure (negative pressure in the lungs). It is initiated by the contraction of the diaphragm, which flattens downward, increasing the volume of the thoracic chamber in the antero-posterior axis.

  Simultaneously, the contraction of external intercostal muscles lifts the ribs and the sternum upward and outward, causing an increase in the volume of the thoracic chamber in the dorso-ventral axis. The overall increase in thoracic volume increases pulmonary volume, dropping intra-pulmonary pressure below atmospheric pressure. Atmospheric air rushes into the lungs.

• Expiration (Passive Process under resting conditions):

  Occurs when intra-pulmonary pressure is higher than atmospheric pressure. The diaphragm and external intercostal muscles relax, returning the diaphragm to its dome shape and the sternum/ribs to their original positions. This reduces thoracic and pulmonary volume, raising intra-pulmonary pressure slightly above atmospheric pressure (+1 to +2 mmHg), forcing air out of the lungs.

  During forceful expiration (e.g., coughing, playing a trumpet), contraction of internal intercostal muscles and abdominal muscles increases expiratory force.

A healthy human breathes 12 to 16 times per minute. Pulmonary ventilation volumes are clinically assessed using a spirometer.

14.3 Respiratory Volumes & Capacities: TV (500 mL), IRV, ERV, RV, IC, FRC, VC & TLC #

Spirometry measures specific pulmonary volumes and diagnostic capacities vital for evaluating respiratory health:

14.4 Exchange of Gases: Partial Pressures (pO2 & pCO2 in Alveoli, Blood & Tissues) & Diffusion Membrane #

Alveoli are the primary sites of gas exchange. Gas exchange also occurs between blood and tissues. O₂ and CO₂ are exchanged across respiratory membranes by simple diffusion driven by partial pressure gradients.

Partial Pressure (pO₂ and pCO₂): The pressure contributed by an individual gas in a mixture of gases, expressed in mmHg. NCERT benchmark values across compartments:

Because the pO₂ in alveoli (104 mmHg) is much higher than in deoxygenated pulmonary blood (40 mmHg), oxygen diffuses rapidly from alveoli into blood. Conversely, pCO₂ is higher in blood (45 mmHg) than in alveoli (40 mmHg), driving CO₂ diffusion into alveoli. Crucial point: The solubility of CO₂ is 20 to 25 times higher than that of O₂; hence, the amount of CO₂ that diffuses across the diffusion membrane per unit difference in partial pressure is much higher.

The diffusion membrane is made of three microscopic layers: (1) thin squamous epithelium of alveoli, (2) endothelium of alveolar capillaries, and (3) intervening basement membrane. Its total thickness is much less than a millimeter (0.2–0.5 µm), facilitating rapid diffusion.

14.5 Transport of Oxygen: Oxyhemoglobin Dissociation Curve & Factors Shifting Curve (Bohr Effect) #

Blood is the medium of transport for O₂ and CO₂. About 97% of oxygen is transported by Red Blood Cells (RBCs) in the form of oxyhemoglobin, while the remaining 3% is dissolved in plasma.

Hemoglobin is a red-colored iron-containing pigment present in RBCs. Each hemoglobin molecule contains 4 heme groups and can carry a maximum of 4 molecules of O₂. Binding of oxygen is reversible:

A plot of percentage saturation of hemoglobin against pO₂ produces a characteristic Sigmoid (S-shaped) curve called the Oxygen-Hemoglobin Dissociation Curve:

• In Alveoli (Favorable for Oxyhemoglobin Formation): High pO₂, low pCO₂, low H⁺ concentration (higher pH), and lower temperature promote oxygen loading onto hemoglobin.
• In Tissues (Favorable for Oxygen Dissociation / The Bohr Effect): Low pO₂, high pCO₂, high H⁺ concentration (acidic pH), and higher temperature shift the dissociation curve to the right, facilitating the release of O₂ from oxyhemoglobin into metabolizing tissues.

Under normal physiological conditions, every 100 mL of oxygenated blood delivers approximately 5 mL of O₂ to tissues.

14.6 Transport of Carbon Dioxide: As Bicarbonate (70% via Carbonic Anhydrase), Carbaminohemoglobin (20-25%) & Dissolved in Plasma (7%) #

Carbon dioxide is carried by blood in three forms:

1. As Bicarbonate Ions (HCO₃⁻) (~70%): RBCs contain an extremely high concentration of the zinc-containing enzyme Carbonic Anhydrase (minute quantities also exist in plasma). Carbonic anhydrase catalyzes the reversible hydration of CO₂:
   CO₂ + H₂O  ⇌  H₂CO₃  ⇌  HCO₃⁻ + H⁺
   At the tissue site where pCO₂ is high, CO₂ diffuses into RBCs and forms HCO₃⁻ and H⁺. HCO₃⁻ diffuses out into plasma in exchange for chloride ions (Chloride Shift / Hamburger phenomenon). At the alveolar site where pCO₂ is low, the reaction proceeds in the opposite direction, releasing gaseous CO₂ into alveoli.
2. As Carbaminohemoglobin (~20 to 25%): CO₂ binds directly to the free amino groups of hemoglobin. Binding is favored by high pCO₂ and low pO₂ in tissues, and reversed in alveoli.
3. Dissolved in Plasma (~7%): Carried in physical solution in blood plasma.

Every 100 mL of deoxygenated blood delivers approximately 4 mL of CO₂ to the alveoli.

14.7 Regulation of Respiration: Medullary Respiratory Rhythm Center, Pons Pneumotaxic Center & Aortic/Carotid Chemoreceptors #

Human beings possess a significant ability to maintain and moderate the respiratory rhythm to suit the demands of the body tissues, mediated by the neural system:

• Respiratory Rhythm Center: Located in the medulla oblongata of the brain; the primary center responsible for regulating respiratory rhythm.
• Pneumotaxic Center: Located in the pons varolii of the brain; moderates the functions of the respiratory rhythm center. Neural signals from this center reduce the duration of inspiration, thereby altering the respiratory rate.
• Chemosensitive Area: Situated adjacent to the rhythm center in the medulla; highly sensitive to increases in CO₂ and hydrogen ions (H⁺). Increase in these substances activates this center, which in turn signals the rhythm center to make necessary adjustments to eliminate these substances.
• Peripheral Receptors: Receptors associated with the aortic arch and carotid artery recognize changes in CO₂ and H⁺ concentration and send corrective signals to the rhythm center. Crucial NCERT fact: The role of Oxygen (pO₂) in the regulation of respiratory rhythm is quite insignificant!

14.8 Respiratory Disorders: Asthma (Allergic Wheezing), Emphysema (Alveolar Wall Breakdown / Smoking) & Occupational Lung Diseases #

Respiratory disorders detailed in NCERT include:

• Asthma: A chronic allergic inflammatory condition of the airways characterized by spasms of bronchial smooth muscles, causing difficulty in breathing with characteristic wheezing (a whistling sound during expiration).
• Emphysema: A chronic obstructive pulmonary disease in which alveolar walls are permanently damaged and broken down, abnormally expanding air spaces and severely decreasing the respiratory surface area for gas exchange. One of the major causative factors is cigarette smoking.
• Occupational Respiratory Disorders: In industries involving grinding or stone-breaking (e.g., quartz, silica, asbestos mining), so much dust is generated that the body’s defense mechanisms cannot cope. Long exposure gives rise to chronic inflammation leading to fibrosis (proliferation of fibrous connective tissue) and serious lung damage. Workers in such industries must wear protective masks.

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