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

Detailed Study Notes – Chapter 14: Breathing and Exchange of Gases (Class 11 Biology, Notes, PDFs, Quizzes, MCQs)

1. The Purpose of Respiration

Respiration is a vital process for all organisms. Its primary functions are:

• Energy Production: Oxygen (O₂) is utilised to indirectly break down simple molecules like glucose, amino acids, and fatty acids through catabolic reactions. This process releases energy required for various life activities.
• Waste Removal: Carbon dioxide (CO₂), a harmful byproduct of these catabolic reactions, is released from the cells and expelled from the body.
• Continuous Gas Exchange: The process ensures a continuous supply of O₂ to the cells and the removal of CO₂ produced by them. This exchange of O₂ from the atmosphere with CO₂ from the cells is known as breathing, or more commonly, respiration.

2. Respiratory Organs in the Animal Kingdom

The mechanism of breathing varies across different animal groups, primarily influenced by their habitat and level of organisation.

Animal Group	Respiratory Mechanism/Organ	Description
Lower Invertebrates (Sponges, Coelenterates, Flatworms)	Simple Diffusion	Gases are exchanged across the entire body surface.
Earthworms	Moist Cuticle (Cutaneous Respiration)	Gas exchange occurs through their moist skin.
Insects	Tracheal Tubes	A network of tubes transports atmospheric air directly within the body.
Aquatic Arthropods & Molluscs	Gills (Branchial Respiration)	Special vascularized structures used for gas exchange in water.
Terrestrial Forms	Lungs (Pulmonary Respiration)	Vascularized bags used for gas exchange with air.
Vertebrates	Gills (Fishes) or Lungs (Amphibians, Reptiles, Birds, Mammals)	Fishes use gills. Most other vertebrates use lungs.
Amphibians (e.g., Frogs)	Lungs and Moist Skin (Cutaneous Respiration)	Capable of respiring through both lungs and their skin.

3. The Human Respiratory System

The human respiratory system is a complex network of organs and passages designed for efficient gas exchange. It is divided into two main parts: the conducting part and the exchange part.

3.1. The Conducting Part

This part transports atmospheric air to the alveoli. It also filters the air, removes foreign particles, humidifies it, and brings it to body temperature. The pathway is as follows:

1. External Nostrils: A pair of openings above the upper lips.
2. Nasal Passage & Chamber: Leads from the nostrils.
3. Pharynx: A common passage for both food and air.
4. Larynx (Sound Box): A cartilaginous box that opens into the trachea and is involved in sound production. The epiglottis, a thin elastic flap, covers the glottis during swallowing to prevent food from entering the larynx.
5. Trachea: A straight tube extending to the mid-thoracic cavity, supported by incomplete cartilaginous rings.
6. Bronchi: The trachea divides at the 5th thoracic vertebra into right and left primary bronchi. These further divide into secondary and tertiary bronchi.
7. Bronchioles: Repeated divisions of bronchi lead to very thin terminal bronchioles. The trachea, bronchi, and initial bronchioles are all supported by incomplete cartilaginous rings.

3.2. The Respiratory or Exchange Part

This is the site where the actual diffusion of O₂ and CO₂ between blood and atmospheric air occurs.

• Alveoli: Each terminal bronchiole gives rise to numerous thin, irregular-walled, vascularized bag-like structures called alveoli. The branching network of bronchi, bronchioles, and alveoli comprises the lungs.

3.3. The Lungs and Thoracic Chamber

• Lungs: Humans have two lungs, which are covered by a double-layered membrane called the pleura.
  • Pleural Fluid: The fluid between the pleural layers reduces friction on the lung surface during breathing movements.
  • The outer pleural membrane is in contact with the thoracic lining, while the inner pleural membrane is in contact with the lung surface.
• Thoracic Chamber: The lungs are situated in the anatomically air-tight thoracic chamber.
  • Boundaries: Formed dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs, and on the lower side by the dome-shaped diaphragm.
  • Function: The setup is such that any change in the volume of the thoracic cavity is reflected in the lung (pulmonary) cavity, which is essential for breathing as pulmonary volume cannot be altered directly.

4. The Mechanism of Breathing

Breathing involves two stages: inspiration (inhalation) and expiration (exhalation), driven by the creation of a pressure gradient between the lungs and the atmosphere. A healthy human breathes 12-16 times per minute on average.

• Inspiration (Active Process):
  1. Initiation: The diaphragm contracts, increasing thoracic volume in the antero-posterior axis. The external intercostal muscles contract, lifting the ribs and sternum, increasing volume in the dorso-ventral axis.
  2. Volume Change: The overall increase in thoracic volume causes a corresponding increase in pulmonary volume.
  3. Pressure Change: The increase in pulmonary volume decreases the intra-pulmonary pressure to below atmospheric pressure.
  4. Air Movement: This negative pressure gradient forces air from the outside to move into the lungs.
• Expiration (Passive Process):
  1. Initiation: The diaphragm and external intercostal muscles relax, returning to their normal positions.
  2. Volume Change: The thoracic and pulmonary volumes decrease.
  3. Pressure Change: The decrease in pulmonary volume increases the intra-pulmonary pressure to slightly above atmospheric pressure.
  4. Air Movement: This positive pressure gradient causes the expulsion of air from the lungs.
• Forced Breathing: The strength of inspiration and expiration can be increased with the help of additional abdominal muscles.

5. Respiratory Volumes and Capacities

The volume of air involved in breathing can be measured using a spirometer. These measurements are crucial for the clinical assessment of pulmonary functions.

Volume/Capacity	Abbreviation	Description	Typical Value	Formula
Tidal Volume	TV	Volume of air inspired or expired during a normal respiration.	~500 mL	N/A
Inspiratory Reserve Volume	IRV	Additional volume of air a person can inspire by a forcible inspiration.	2500 – 3000 mL	N/A
Expiratory Reserve Volume	ERV	Additional volume of air a person can expire by a forcible expiration.	1000 – 1100 mL	N/A
Residual Volume	RV	Volume of air remaining in the lungs even after a forcible expiration.	1100 – 1200 mL	N/A
Inspiratory Capacity	IC	Total volume of air a person can inspire after a normal expiration.	3000 – 3500 mL	TV + IRV
Expiratory Capacity	EC	Total volume of air a person can expire after a normal inspiration.	1500 – 1600 mL	TV + ERV
Functional Residual Capacity	FRC	Volume of air that will remain in the lungs after a normal expiration.	2100 – 2300 mL	ERV + RV
Vital Capacity	VC	The maximum volume of air a person can breathe in after a forced expiration (or breathe out after a forced inspiration).	4000 – 4600 mL	ERV + TV + IRV
Total Lung Capacity	TLC	Total volume of air accommodated in the lungs at the end of a forced inspiration.	5100 – 5800 mL	VC + RV

6. Exchange of Gases

Gas exchange occurs in the alveoli and between blood and tissues via simple diffusion, driven by a pressure/concentration gradient.

• Partial Pressure: The pressure contributed by an individual gas in a mixture of gases (e.g., pO₂ and pCO₂).
• Diffusion Gradient: Gases move from an area of higher partial pressure to an area of lower partial pressure.
• Solubility: The solubility of CO₂ is 20-25 times higher than that of O₂, allowing it to diffuse much more rapidly per unit difference in partial pressure.
• Diffusion Membrane: This membrane, through which gases exchange in the alveoli, is less than a millimetre thick and consists of three layers:
  1. The thin squamous epithelium of the alveoli.
  2. The endothelium of the alveolar capillaries.
  3. The basement substance in between them.

Partial Pressures of Respiratory Gases (in mm Hg)

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

7. Transport of Gases in the Blood

Blood is the primary medium for transporting O₂ and CO₂.

7.1. Transport of Oxygen

• ~97% is transported by Red Blood Cells (RBCs), bound to haemoglobin, a red-colored, iron-containing pigment.
• ~3% is carried in a dissolved state in the plasma.
• Oxyhaemoglobin: O₂ binds reversibly with haemoglobin to form oxyhaemoglobin. Each haemoglobin molecule can carry a maximum of four O₂ molecules.
• Oxygen Dissociation Curve: A sigmoid (S-shaped) curve is obtained when the percentage saturation of haemoglobin with O₂ is plotted against the pO₂. This curve shows the relationship between pO₂ and O₂ binding.
  • In the Alveoli: High pO₂, low pCO₂, lower H⁺ concentration, and lower temperature all favour the formation of oxyhaemoglobin (loading of O₂).
  • In the Tissues: Low pO₂, high pCO₂, high H⁺ concentration, and higher temperature all favour the dissociation of O₂ from oxyhaemoglobin (unloading of O₂).
• Delivery: Every 100 ml of oxygenated blood delivers approximately 5 ml of O₂ to the tissues under normal conditions.

7.2. Transport of Carbon Dioxide

• ~70% is transported as bicarbonate (HCO₃⁻). This process is facilitated by the enzyme carbonic anhydrase, which is highly concentrated in RBCs.
  • Reaction: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
  • In Tissues: High pCO₂ drives the reaction to the right, forming bicarbonate.
  • In Alveoli: Low pCO₂ drives the reaction to the left, releasing CO₂ to be exhaled.
• ~20-25% is transported by RBCs, bound to haemoglobin as carbamino-haemoglobin. This binding is high when pCO₂ is high and pO₂ is low (in tissues).
• ~7% is carried in a dissolved state in the plasma.
• Delivery: Every 100 ml of deoxygenated blood delivers approximately 4 ml of CO₂ to the alveoli.

8. Regulation of Respiration

The body can maintain and moderate the respiratory rhythm to meet tissue demands, a process controlled by the neural system.

• Respiratory Rhythm Centre: A specialised centre in the medulla region of the brain that is primarily responsible for regulating the respiratory rhythm.
• Pneumotaxic Centre: Located in the pons region of the brain, it can moderate the function of the rhythm centre. Signals from this centre can reduce the duration of inspiration, thereby altering the respiratory rate.
• Chemosensitive Area: Situated adjacent to the rhythm centre, it is highly sensitive to changes in CO₂ and hydrogen ion (H⁺) concentrations. An increase in these substances activates the centre, which signals the rhythm centre to adjust the respiratory process to eliminate them.
• Peripheral Receptors: Receptors in the aortic arch and carotid artery can also detect changes in CO₂ and H⁺ concentrations and send signals to the rhythm centre for corrective action.
• Role of Oxygen: The role of oxygen in the regulation of respiratory rhythm is considered quite insignificant.

9. Disorders of the Respiratory System

• Asthma: Characterised by difficulty in breathing and wheezing, caused by inflammation of the bronchi and bronchioles.
• Emphysema: A chronic disorder where alveolar walls are damaged, leading to a decreased respiratory surface area. A major cause is cigarette smoking.
• Occupational Respiratory Disorders: In industries involving grinding or stone-breaking, long-term exposure to dust can overwhelm the body’s defence mechanisms. This can lead to inflammation and fibrosis (proliferation of fibrous tissues), causing serious lung damage. Protective masks are recommended for workers in such industries.

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Q&A Section

Short-Answer Questions (25 Questions)

1. What is the primary purpose of breathing, as described in the text?
2. How do lower invertebrates like sponges and coelenterates perform gas exchange?
3. Describe the function of the epiglottis in the human respiratory system.
4. What is the role of the incomplete cartilaginous rings in the trachea and bronchi?
5. Distinguish between the conducting part and the exchange part of the respiratory system.
6. What is the function of the pleural fluid found between the pleural membranes?
7. Explain how the thoracic chamber is an “air-tight” chamber by describing its boundaries.
8. Describe the role of the diaphragm and external intercostal muscles during normal inspiration.
9. Why is normal expiration considered a passive process?
10. What is Tidal Volume (TV) and what is its approximate value in a healthy adult?
11. Define Vital Capacity (VC). Which respiratory volumes does it encompass?
12. What is Residual Volume (RV) and why is it significant?
13. Name the primary sites of gas exchange in the human body.
14. What is partial pressure, and how does it drive the process of diffusion?
15. Why does CO₂ diffuse much faster than O₂ across the diffusion membrane?
16. Name the three layers that make up the diffusion membrane in the alveoli.
17. What percentage of oxygen is transported by haemoglobin, and what is the resulting compound called?
18. What is the oxygen dissociation curve, and what is its characteristic shape?
19. List the conditions in the tissues that favour the dissociation of oxygen from oxyhaemoglobin.
20. What is the primary form in which carbon dioxide is transported in the blood?
21. What is the role of the enzyme carbonic anhydrase in CO₂ transport?
22. Name the two primary centres in the brain that regulate respiration and state their locations.
23. How does the chemosensitive area in the medulla respond to an increase in blood CO₂ and H⁺ levels?
24. Describe the physiological effects of emphysema.
25. What is fibrosis in the context of occupational respiratory disorders?

Multiple-Choice Questions (20 Questions)

1. Which of the following organisms uses a moist cuticle for respiration? a) Insect b) Fish c) Earthworm d) Flatworm
2. The cartilaginous box that produces sound in humans is called the: a) Pharynx b) Trachea c) Epiglottis d) Larynx
3. The trachea divides into a right and left primary bronchi at the level of the: a) 2nd thoracic vertebra b) 5th thoracic vertebra c) 7th cervical vertebra d) Diaphragm
4. During inspiration, the intra-pulmonary pressure: a) Is higher than the atmospheric pressure b) Is equal to the atmospheric pressure c) Is less than the atmospheric pressure d) Does not change
5. What is the average breathing rate for a healthy human? a) 8-10 breaths/minute b) 12-16 breaths/minute c) 20-25 breaths/minute d) 30-35 breaths/minute
6. The volume of air remaining in the lungs after a forcible expiration is the: a) Tidal Volume (TV) b) Expiratory Reserve Volume (ERV) c) Vital Capacity (VC) d) Residual Volume (RV)
7. The maximum volume of air a person can breathe in after a forced expiration is known as: a) Total Lung Capacity b) Vital Capacity c) Inspiratory Capacity d) Functional Residual Capacity
8. The partial pressure of oxygen (pO₂) in the alveoli is approximately: a) 40 mm Hg b) 95 mm Hg c) 104 mm Hg d) 159 mm Hg
9. The solubility of CO₂ is how many times higher than that of O₂? a) 5-10 times b) 10-15 times c) 20-25 times d) 50-55 times
10. What percentage of O₂ is transported in a dissolved state through the plasma? a) 3% b) 25% c) 70% d) 97%
11. Each haemoglobin molecule can carry a maximum of how many molecules of O₂? a) One b) Two c) Three d) Four
12. Which of the following factors is favourable for the formation of oxyhaemoglobin in the alveoli? a) High pCO₂ b) High H⁺ concentration c) High temperature d) High pO₂
13. The majority (70%) of CO₂ is transported in the blood as: a) Carbamino-haemoglobin b) Dissolved CO₂ c) Bicarbonate ions d) Carbonic acid
14. Every 100 ml of deoxygenated blood delivers approximately how much CO₂ to the alveoli? a) 2 ml b) 4 ml c) 5 ml d) 7 ml
15. The respiratory rhythm centre is located in the: a) Pons b) Medulla c) Aortic arch d) Cerebrum
16. Which centre can moderate the function of the respiratory rhythm centre by reducing the duration of inspiration? a) Chemosensitive area b) Carotid body c) Pneumotaxic centre d) Aortic arch receptors
17. The regulation of respiratory rhythm is primarily driven by the concentration of: a) O₂ b) CO₂ and H⁺ ions c) Nitrogen d) Bicarbonate ions
18. Asthma is a disorder caused by: a) Damage to the alveolar walls b) Inflammation of bronchi and bronchioles c) Proliferation of fibrous tissues in the lungs d) Fluid accumulation in the pleura
19. A major cause of emphysema is: a) Cigarette smoking b) Bacterial infection c) Dust inhalation d) A genetic defect
20. Fibrosis is a characteristic of which category of respiratory disorders? a) Asthma b) Emphysema c) Hypoxia d) Occupational Respiratory Disorders

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Essay Questions (10 Questions with Answers)

1. Describe the complete pathway of air from the external nostrils to the alveoli, detailing the structures involved and their functions.
   • Answer: The pathway begins at the external nostrils, which lead into the nasal passage and then the nasal chamber. The air then passes into the pharynx, which serves as a common passage for food and air. From the pharynx, air enters the larynx (or sound box) through the glottis. The larynx is a cartilaginous structure that also prevents food entry via the epiglottis, a flap that covers the glottis during swallowing. The air then moves into the trachea, a straight tube supported by incomplete cartilaginous rings to prevent collapse. At the 5th thoracic vertebra, the trachea divides into the right and left primary bronchi, which also contain cartilaginous rings. The bronchi undergo repeated divisions to form secondary and tertiary bronchi, followed by bronchioles, which finally terminate in thin-walled, vascularized, bag-like structures called alveoli, the site of gas exchange.
2. Explain the mechanical process of breathing, covering both inspiration and expiration under normal conditions.
   • Answer: Breathing operates by creating a pressure gradient between the lungs and the atmosphere. Inspiration is an active process initiated by the contraction of the diaphragm, which increases the volume of the thoracic chamber in the antero-posterior axis. Simultaneously, the external intercostal muscles contract, lifting the ribs and sternum, which increases the thoracic volume in the dorso-ventral axis. This overall increase in thoracic volume leads to an increase in pulmonary volume, which in turn decreases the intra-pulmonary pressure below atmospheric pressure, causing air to flow into the lungs. Expiration is a passive process where the diaphragm and intercostal muscles relax, causing the thoracic and pulmonary volumes to decrease. This increases the intra-pulmonary pressure above atmospheric pressure, forcing air out of the lungs.
3. Define and differentiate between Vital Capacity (VC) and Total Lung Capacity (TLC).
   • Answer: Vital Capacity (VC) is defined as the maximum volume of air a person can breathe in after a forced expiration, or alternatively, the maximum volume of air a person can breathe out after a forced inspiration. It represents the total exchangeable air in the lungs and is the sum of Expiratory Reserve Volume (ERV), Tidal Volume (TV), and Inspiratory Reserve Volume (IRV). Total Lung Capacity (TLC) is the total volume of air accommodated in the lungs at the end of a forced inspiration. It includes all lung volumes: Residual Volume (RV), ERV, TV, and IRV. The key difference is that TLC includes the Residual Volume (RV)—the air that cannot be expelled—while VC does not. Therefore, TLC is equal to Vital Capacity plus Residual Volume (TLC = VC + RV).
4. Discuss the factors that influence the diffusion of gases across the alveolar membrane.
   • Answer: The exchange of gases across the alveolar membrane occurs by simple diffusion, which is influenced by three main factors. First is the pressure or concentration gradient, specifically the partial pressure gradient of O₂ and CO₂. Gases always diffuse from an area of higher partial pressure to an area of lower partial pressure. Second is the solubility of the gases. CO₂ is 20-25 times more soluble in the diffusion membrane than O₂, which allows it to diffuse much more efficiently. The third factor is the thickness of the diffusion membrane itself. The membrane is extremely thin (much less than a millimetre), consisting of only three layers, which presents a very short distance for gases to travel, thus facilitating rapid diffusion.
5. Elaborate on the transport of oxygen in the blood, including the role of haemoglobin and the factors affecting its binding with oxygen.
   • Answer: Oxygen is transported in the blood primarily by binding to haemoglobin (Hb), an iron-containing pigment in red blood cells. About 97% of O₂ is transported as oxyhaemoglobin (HbO₂), while the remaining 3% is dissolved in the plasma. The binding of O₂ to Hb is reversible and is primarily influenced by the partial pressure of oxygen (pO₂). Other factors that affect this binding include the partial pressure of carbon dioxide (pCO₂), hydrogen ion concentration (H⁺), and temperature. In the alveoli, where pO₂ is high and pCO₂, H⁺, and temperature are low, conditions are favourable for the formation of oxyhaemoglobin. Conversely, in the tissues, where pO₂ is low and pCO₂, H⁺, and temperature are high, conditions favour the dissociation of O₂ from haemoglobin, allowing it to be delivered to the cells.
6. Detail the three main mechanisms for carbon dioxide transport in the blood.
   • Answer: Carbon dioxide is transported from the tissues to the lungs via three mechanisms. The primary mechanism, accounting for about 70% of CO₂ transport, is as bicarbonate ions (HCO₃⁻). In the RBCs at the tissue level, CO₂ combines with water to form carbonic acid, a reaction catalysed by the enzyme carbonic anhydrase. Carbonic acid then dissociates into H⁺ and HCO₃⁻. The second mechanism, accounting for 20-25% of transport, is by binding to haemoglobin to form carbamino-haemoglobin. This binding is favoured by high pCO₂ and low pO₂ in the tissues. The third and least significant mechanism is transport in a dissolved state in the blood plasma, which accounts for about 7% of the total CO₂.
7. Explain the neural regulation of respiration, mentioning the key brain centres involved and their specific roles.
   • Answer: Respiration is regulated by the neural system to match the body’s metabolic demands. The primary control centre is the respiratory rhythm centre, located in the medulla region of the brain, which establishes the basic rhythm of breathing. Its function can be moderated by the pneumotaxic centre, located in the pons region. The pneumotaxic centre can send signals to reduce the duration of inspiration, thereby altering the respiratory rate. Additionally, a chemosensitive area adjacent to the rhythm centre in the medulla is highly sensitive to changes in blood CO₂ and H⁺ concentrations. An increase in these substances activates this area, which then signals the rhythm centre to make adjustments to increase ventilation and eliminate the excess CO₂ and H⁺.
8. Describe the diffusion gradients for O₂ and CO₂ at both the alveolar and tissue levels, using the partial pressure values provided in the text.
   • Answer: At the alveolar level, the pO₂ is 104 mm Hg, while in the deoxygenated blood arriving from the tissues, it is 40 mm Hg. This steep gradient drives O₂ from the alveoli into the blood. For CO₂, the pCO₂ in deoxygenated blood is 45 mm Hg, while in the alveoli, it is 40 mm Hg. This gradient drives CO₂ from the blood into the alveoli to be exhaled. At the tissue level, the gradients are reversed. Oxygenated blood arrives with a pO₂ of 95 mm Hg, while the tissues have a pO₂ of 40 mm Hg, causing O₂ to diffuse from the blood into the tissues. Meanwhile, the tissues have a high pCO₂ of 45 mm Hg due to metabolism, while the oxygenated blood has a pCO₂ of 40 mm Hg. This gradient drives CO₂ from the tissues into the blood.
9. What is the significance of the oxygen dissociation curve? Explain why conditions in the tissues cause a “shift” that favours oxygen unloading.
   • Answer: The oxygen dissociation curve is a sigmoid-shaped graph that plots the percentage saturation of haemoglobin with oxygen against the partial pressure of oxygen (pO₂). Its significance lies in illustrating how haemoglobin’s affinity for oxygen changes under different physiological conditions. In the tissues, metabolic activity produces CO₂ and acids (increasing H⁺ concentration), and also generates heat. According to the curve, high pCO₂, high H⁺ concentration, and higher temperature all decrease haemoglobin’s affinity for oxygen. This causes the curve to “shift to the right,” meaning that for any given pO₂, haemoglobin will be less saturated. This is highly adaptive, as it facilitates the dissociation of oxygen from oxyhaemoglobin, ensuring that more oxygen is released or “unloaded” precisely where it is needed most—in the metabolically active tissues.
10. Compare and contrast the respiratory disorders Asthma and Emphysema as described in the source.
    • Answer: Asthma and Emphysema are both serious respiratory disorders, but they affect the respiratory system in different ways. Asthma is an inflammatory condition affecting the airways—specifically the bronchi and bronchioles. This inflammation causes the airways to narrow, leading to symptoms like difficulty breathing and wheezing. It is often characterised by acute episodes. Emphysema, on the other hand, is a chronic disorder that involves structural damage to the lungs themselves. In emphysema, the walls of the alveoli are damaged and break down, which leads to a significant decrease in the total surface area available for gas exchange. While asthma affects air conduction, emphysema primarily damages the site of gas exchange. A major cause cited for emphysema is cigarette smoking.

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Answer Keys

Answer Key: Short-Answer Questions

1. The primary purpose is to continuously provide oxygen (O₂) to cells for catabolic reactions to derive energy, and to release the harmful carbon dioxide (CO₂) produced during these reactions.
2. Lower invertebrates perform gas exchange by simple diffusion over their entire body surface.
3. The epiglottis is a thin, elastic cartilaginous flap that covers the glottis (the opening of the larynx) during swallowing to prevent the entry of food into the airway.
4. The incomplete cartilaginous rings provide structural support to the trachea and bronchi, ensuring they remain open and do not collapse, thus maintaining a clear passage for air.
5. The conducting part (from nostrils to terminal bronchioles) transports, filters, humidifies, and warms the air. The exchange part (alveoli and their ducts) is the site of actual gas diffusion between blood and air.
6. The pleural fluid, located between the two pleural layers surrounding the lungs, reduces friction on the lung surface as they expand and contract during breathing.
7. The thoracic chamber is bounded dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs, and inferiorly by the dome-shaped diaphragm, forming an enclosed, air-tight cavity.
8. During inspiration, the diaphragm contracts and flattens, and the external intercostal muscles contract to lift the ribs and sternum. Both actions increase the volume of the thoracic cavity, causing the lungs to expand.
9. Normal expiration is passive because it relies on the simple relaxation of the diaphragm and external intercostal muscles. This causes the thoracic cavity to return to its original, smaller volume without active muscle contraction.
10. Tidal Volume (TV) is the volume of air inspired or expired during a normal, quiet respiration. Its approximate value is 500 mL.
11. Vital Capacity (VC) is the maximum volume of air a person can breathe in after a forced expiration. It includes the sum of Inspiratory Reserve Volume (IRV), Tidal Volume (TV), and Expiratory Reserve Volume (ERV).
12. Residual Volume (RV) is the volume of air that remains in the lungs even after a maximum forcible expiration (approx. 1100-1200 mL). It is significant because it prevents the complete collapse of the alveoli.
13. The primary sites of gas exchange are the alveoli. Exchange also occurs between the blood and the body tissues.
14. Partial pressure is the pressure contributed by an individual gas within a mixture of gases. Diffusion is driven by a partial pressure gradient, as gases naturally move from an area of higher partial pressure to an area of lower partial pressure.
15. CO₂ diffuses much faster than O₂ because its solubility in the diffusion membrane is 20-25 times higher than that of oxygen, which compensates for its smaller partial pressure gradient.
16. The three layers are: (1) the thin squamous epithelium of the alveoli, (2) the endothelium of the alveolar capillaries, and (3) the basement membrane between them.
17. About 97% of oxygen is transported by haemoglobin. The resulting compound formed when oxygen binds to haemoglobin is called oxyhaemoglobin.
18. The oxygen dissociation curve is a graph that shows the percentage saturation of haemoglobin with oxygen at various partial pressures of oxygen. It has a characteristic sigmoid or ‘S’ shape.
19. The conditions in tissues favouring dissociation are low pO₂, high pCO₂, high hydrogen ion (H⁺) concentration, and higher temperature.
20. The primary form is as bicarbonate ions (HCO₃⁻), which accounts for about 70% of CO₂ transport.
21. Carbonic anhydrase is an enzyme highly concentrated in RBCs that greatly accelerates the conversion of CO₂ and water into carbonic acid, and the reverse reaction. This facilitates the rapid formation and breakdown of bicarbonate for CO₂ transport.
22. The respiratory rhythm centre is located in the medulla region of the brain, and the pneumotaxic centre is located in the pons region of the brain.
23. An increase in CO₂ and H⁺ activates the chemosensitive area, which in turn signals the rhythm centre to make necessary adjustments in the respiratory process to increase ventilation and eliminate these substances.
24. Emphysema is a chronic disorder in which the alveolar walls are damaged. This leads to a significant decrease in the respiratory surface area available for gas exchange.
25. Fibrosis is the proliferation of fibrous tissue in the lungs. It is a result of long-term inflammation caused by exposure to dust in certain occupational settings, leading to serious lung damage.

Answer Key: Multiple-Choice Questions

1. c) Earthworm
2. d) Larynx
3. b) 5th thoracic vertebra
4. c) Is less than the atmospheric pressure
5. b) 12-16 breaths/minute
6. d) Residual Volume (RV)
7. b) Vital Capacity
8. c) 104 mm Hg
9. c) 20-25 times
10. a) 3%
11. d) Four
12. d) High pO₂
13. c) Bicarbonate ions
14. b) 4 ml
15. b) Medulla
16. c) Pneumotaxic centre
17. b) CO₂ and H⁺ ions
18. b) Inflammation of bronchi and bronchioles
19. a) Cigarette smoking
20. d) Occupational Respiratory Disorders

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Glossary of Key Terms

Term	Definition
Alveoli	Thin, irregular-walled, vascularized bag-like structures at the end of terminal bronchioles where gas exchange occurs.
Asthma	An enzyme, highly concentrated in RBCs, that catalyses the conversion of CO₂ and water to carbonic acid.
Branchial Respiration	Respiration through gills, as seen in most aquatic arthropods, molluscs, and fishes.
Breathing	The process of exchanging O₂ from the atmosphere with CO₂ produced by the cells. Involves inspiration and expiration.
Bronchi	The two main air passages that branch off from the trachea and lead to the lungs.
Carbamino-haemoglobin	The compound formed when carbon dioxide binds to haemoglobin. Accounts for 20-25% of CO₂ transport.
Carbonic Anhydrase	The thin, three-layered barrier in the alveoli (squamous epithelium, endothelium, basement membrane) across which gases are exchanged.
Cutaneous Respiration	Respiration through a moist skin or cuticle, as seen in earthworms and frogs.
Diaphragm	A dome-shaped muscle at the base of the thoracic cavity that plays a major role in breathing.
Diffusion Membrane	A neural centre in the pons region of the brain that can moderate the respiratory rhythm centre.
Emphysema	A chronic disorder in which alveolar walls are damaged, leading to a decreased respiratory surface area.
Epiglottis	A thin, elastic cartilaginous flap that covers the glottis during swallowing to prevent food from entering the larynx.
Expiration	The stage of breathing where alveolar air is released out of the lungs.
Expiratory Capacity (EC)	Total volume of air a person can expire after a normal inspiration (TV + ERV).
Expiratory Reserve Volume (ERV)	The additional volume of air that can be forcibly expired after a normal expiration (1000-1100 mL).
Fibrosis	The proliferation of fibrous tissues, which can cause serious lung damage in occupational respiratory disorders.
Functional Residual Capacity (FRC)	The volume of air remaining in the lungs after a normal expiration (ERV + RV).
Gills	Special vascularized structures used for branchial respiration in aquatic animals.
Haemoglobin	A red-colored, iron-containing pigment in RBCs that binds reversibly with oxygen.
Inspiration	The stage of breathing during which atmospheric air is drawn into the lungs.
Inspiratory Capacity (IC)	Total volume of air a person can inspire after a normal expiration (TV + IRV).
Inspiratory Reserve Volume (IRV)	The additional volume of air that can be forcibly inspired after a normal inspiration (2500-3000 mL).
Intercostal Muscles	Muscles located between the ribs (external and internal) that assist in changing the volume of the thoracic cavity during breathing.
Larynx	A cartilaginous box in the throat, also known as the sound box, which helps in sound production.
Lungs	Vascularized bag-like organs used for pulmonary respiration by terrestrial vertebrates.
Oxyhaemoglobin	The compound formed when oxygen binds to haemoglobin in the RBCs.
Oxygen Dissociation Curve	A sigmoid (S-shaped) curve that shows the percentage saturation of haemoglobin with oxygen at different partial pressures of oxygen.
Partial Pressure	The pressure contributed by an individual gas in a mixture of gases (e.g., pO₂ or pCO₂).
Pharynx	A portion of the throat that serves as a common passage for both food and air.
Pleura	The double-layered membrane that covers the lungs. The space between contains pleural fluid.
Pneumotaxic Centre	Respiration through the lungs.
Pulmonary Respiration	A specialised neural center in the medulla region of the brain primarily responsible for regulating respiration.
Residual Volume (RV)	The volume of air remaining in the lungs even after a forcible expiration (1100-1200 mL).
Respiratory Rhythm Centre	A specialised neural centre in the medulla region of the brain primarily responsible for regulating respiration.
Spirometer	An instrument used to estimate the volume of air involved in breathing movements for clinical assessment.
Tidal Volume (TV)	The volume of air inspired or expired during a normal respiration (approx. 500 mL).
Total Lung Capacity (TLC)	The total volume of air accommodated in the lungs at the end of a forced inspiration (VC + RV).
Trachea	The windpipe; a straight tube extending from the larynx to the mid-thoracic cavity, where it divides into bronchi.
Vital Capacity (VC)	The maximum volume of air a person can breathe in after a forced expiration (ERV + TV + IRV).