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

Vital Capacity (VC) is defined as the maximum volume of air a person can breathe out (expire) after a forced inspiration. It is the sum of Tidal Volume (TV), Inspiratory Reserve Volume (IRV), and Expiratory Reserve Volume (ERV).

Formula: Vital Capacity (VC) = TV + IRV + ERV

Significance of Vital Capacity:

1. Indicator of Respiratory Health: A high vital capacity indicates strong respiratory muscles and healthy lung function. A decrease in VC can be a sign of lung diseases like pulmonary fibrosis or congestion.
2. Diagnostic Tool: It helps in diagnosing restrictive and obstructive lung diseases.
3. Athletic Performance: Athletes, swimmers, and mountain climbers often have a higher vital capacity, allowing for greater gaseous exchange to meet high energy demands.

The volume of air remaining in the lungs after a normal, passive expiration is the sum of the Expiratory Reserve Volume (ERV) and the Residual Volume (RV). This combined volume is also known as the Functional Residual Capacity (FRC).

• Expiratory Reserve Volume (ERV): Approx. 1000-1100 mL
• Residual Volume (RV): Approx. 1100-1200 mL

Therefore, Functional Residual Capacity (FRC) = ERV + RV ≈ 2100 - 2300 mL.

Diffusion of gases occurs effectively only in the alveoli due to the following specialized features:

1. Thin Respiratory Membrane: The barrier between the alveolar air and blood, known as the respiratory membrane, is extremely thin (less than a millimetre). It is made up of three layers:
   • The thin squamous epithelium of the alveoli.
   • The endothelium of the alveolar capillaries.
   • The basement substance between them.
2. Large Surface Area: The human lungs contain millions of alveoli, providing a massive surface area (about 80-100 m²) for gaseous exchange.
3. Rich Blood Supply: The alveoli are densely covered with a network of blood capillaries, ensuring a continuous flow of blood to pick up oxygen and release carbon dioxide.
4. Moist Surface: The inner surface of the alveoli is lined with a thin film of fluid, which allows oxygen to dissolve before diffusing across the membrane.

Other parts of the respiratory system, like the trachea and bronchi, have thick walls and are primarily designed for conducting air, not for diffusion.

Carbon dioxide (CO₂) is transported in the blood from the tissues to the lungs in three main ways:

1. As Bicarbonate Ions (HCO₃⁻) (Approx. 70%): This is the primary method.
   • CO₂ from tissues diffuses into Red Blood Cells (RBCs).
   • Inside the RBC, CO₂ reacts with water (H₂O) in the presence of the enzyme carbonic anhydrase to form carbonic acid (H₂CO₃).
   • Carbonic acid is unstable and quickly dissociates into a hydrogen ion (H⁺) and a bicarbonate ion (HCO₃⁻).
   • Most of the HCO₃⁻ ions diffuse out of the RBC into the blood plasma. To maintain electrical neutrality, chloride ions (Cl⁻) move from the plasma into the RBCs. This exchange is called the Chloride Shift.
2. As Carbamino-haemoglobin (Approx. 20-25%):
   • CO₂ binds directly to the amino groups of the globin part of haemoglobin (Hb) to form a reversible compound called carbamino-haemoglobin.
3. Dissolved in Blood Plasma (Approx. 7%):
   • A small amount of CO₂ dissolves directly in the blood plasma and is transported in its dissolved state.

The correct option is: (ii) pO₂ higher, pCO₂ lesser

Explanation:

Atmospheric air is the fresh air we inhale, while alveolar air is the air inside the lungs that has mixed with the residual air and has undergone gas exchange with the blood.

• Atmospheric Air: pO₂ is ~159 mm Hg and pCO₂ is ~0.3 mm Hg.
• Alveolar Air: pO₂ is ~104 mm Hg and pCO₂ is ~40 mm Hg.

Therefore, the partial pressure of oxygen (pO₂) is higher in atmospheric air, and the partial pressure of carbon dioxide (pCO₂) is lesser.

Breathing involves two stages: Inspiration (inhalation) and Expiration (exhalation). It is achieved by creating a pressure gradient between the lungs and the atmosphere.

1. Inspiration (Inhalation):
This is an active process where air is drawn into the lungs. It occurs when the pressure inside the lungs (intra-pulmonary pressure) is lower than the atmospheric pressure.

• Contraction of the Diaphragm: The dome-shaped diaphragm contracts and flattens, increasing the volume of the thoracic cavity in the antero-posterior axis.
• Contraction of External Intercostal Muscles: These muscles contract and lift the ribs and sternum upwards and outwards, increasing the volume of the thoracic cavity in the dorso-ventral axis.
• Increased Thoracic Volume: The overall increase in the volume of the thoracic cavity causes a similar increase in lung volume.
• Decreased Intra-pulmonary Pressure: An increase in volume leads to a decrease in pressure. The intra-pulmonary pressure drops below the atmospheric pressure.
• Air Rushes In: Air moves from the higher pressure (atmosphere) to the lower pressure (lungs).

2. Expiration (Exhalation):
This is typically a passive process where air is expelled from the lungs. It occurs when the intra-pulmonary pressure is higher than the atmospheric pressure.

• Relaxation of Diaphragm: The diaphragm relaxes and returns to its original dome shape, decreasing the thoracic volume.
• Relaxation of External Intercostal Muscles: These muscles relax, causing the ribs and sternum to return to their normal positions.
• Decreased Thoracic Volume: This leads to a corresponding decrease in lung volume due to the elastic recoil of the lungs.
• Increased Intra-pulmonary Pressure: The decrease in lung volume raises the intra-pulmonary pressure above the atmospheric pressure.
• Air is Expelled: Air flows out of the lungs.

The neural system maintains and moderates the respiratory rhythm. This is controlled by specialized centres in the brainstem.

1. Medulla Oblongata:
   • Respiratory Rhythm Centre (RRC): This is the primary centre responsible for generating the basic rhythm of breathing (inspiration and expiration).
2. Pons Varolii:
   • Pneumotaxic Centre: Moderates the RRC's function. It can reduce the duration of inspiration, altering the respiratory rate. It acts as a "switch-off" point for inspiration.
   • Apneustic Centre: Promotes inspiration by sending stimulatory signals to the RRC, leading to prolonged inspiration.
3. Chemosensitive Area:
   • Located adjacent to the rhythm centre, it is highly sensitive to changes in blood CO₂ and H⁺ ion concentration. An increase in these stimulates the centre to increase breathing rate and depth.
4. Chemoreceptors:
   • Receptors in the aortic arch and carotid artery also recognize changes in CO₂ and H⁺ concentration and send signals to the rhythm centre. They are also sensitive to pO₂ changes, though this role is less significant under normal conditions.

The partial pressure of carbon dioxide (pCO₂) has a significant effect on oxygen transport by influencing haemoglobin's affinity for oxygen. This is known as the Bohr's Effect.

• In Tissues (High pCO₂):
  • Actively metabolizing tissues produce a large amount of CO₂.
  • High pCO₂ increases H⁺ ion concentration (lowers pH).
  • This **decreases the affinity of haemoglobin for oxygen**.
  • The oxygen-haemoglobin dissociation curve shifts to the **right**.
  • Haemoglobin readily **unloads oxygen** to the tissues.
• In Lungs (Low pCO₂):
  • In the alveoli, pCO₂ is low.
  • This **increases the affinity of haemoglobin for oxygen**.
  • The oxygen-haemoglobin dissociation curve shifts to the **left**.
  • This facilitates the efficient **loading of oxygen** onto haemoglobin.

In summary, high pCO₂ promotes the unloading of O₂, while low pCO₂ promotes the loading of O₂.

When a person goes up a hill, the atmospheric pressure and partial pressure of oxygen (pO₂) decrease. This leads to several physiological adjustments:

1. Hypoxia: The lower pO₂ leads to oxygen deficiency at the tissue level, called hypoxia.
2. Increased Breathing Rate (Hyperventilation): Peripheral chemoreceptors detect the low pO₂ and signal the brain to increase the rate and depth of breathing to compensate.
3. Increased Heart Rate: The heart beats faster to deliver the available oxygen to tissues more quickly.
4. Acclimatization (Long-term):
   • Increased RBC Production: The kidneys release erythropoietin, stimulating bone marrow to produce more RBCs and haemoglobin.
   • Increased 2,3-BPG: The concentration of 2,3-BPG increases in RBCs, which decreases haemoglobin's affinity for O₂, promoting easier release of oxygen to the tissues.

Initial symptoms like breathlessness, headache, and nausea are known as Altitude Sickness.

In insects, the site of gaseous exchange is a network of fine tubes called tracheoles, which are part of their tracheal system.

Explanation: Insects have air-filled tubes called tracheae that open to the outside through pores called spiracles. These tracheae branch into finer tubes called tracheoles, which penetrate deep into the body tissues. Gaseous exchange occurs directly between the air in the tracheoles and the body cells via diffusion, without the use of blood for transport.

Definition:
The oxygen-haemoglobin dissociation curve is a graph that plots the percentage saturation of haemoglobin with oxygen against the partial pressure of oxygen (pO₂).

Reason for its Sigmoidal (S-shaped) Pattern:
The S-shape is due to positive cooperativity in the binding of oxygen to haemoglobin.

• Initial Binding is Slow: Binding of the *first* O₂ molecule to a heme group is difficult, causing the initial flat part of the curve.
• Cooperative Binding: This first binding changes the shape of the haemoglobin molecule, increasing the affinity of the remaining heme groups for oxygen.
• Rapid Saturation: The binding of the second and third O₂ molecules is progressively easier and faster, causing the steep middle portion of the curve.
• Saturation Point: As haemoglobin nears 100% saturation, it becomes harder for the fourth O₂ molecule to find an empty site, causing the curve to flatten at the top (plateau).

Hypoxia is a condition where the body or a region of the body is deprived of adequate oxygen supply at the tissue level.

Types and Causes of Hypoxia:

1. Hypoxic Hypoxia: Caused by low pO₂ in arterial blood (e.g., high altitude, lung disease).
2. Anemic Hypoxia: Reduced oxygen-carrying capacity of blood (e.g., anemia, carbon monoxide poisoning).
3. Stagnant (Ischemic) Hypoxia: Reduced blood flow to tissues (e.g., heart failure, thrombosis).
4. Histotoxic Hypoxia: Cells are unable to use the delivered oxygen (e.g., cyanide poisoning).

Symptoms: Shortness of breath, rapid heart rate, confusion, bluish skin (cyanosis), headache, and dizziness.

(a) Difference between IRV and ERV

(b) Difference between Inspiratory Capacity and Expiratory Capacity

(c) Difference between Vital Capacity and Total Lung Capacity

Respiratory disorders are diseases affecting the lungs and other parts of the respiratory system.

1. Asthma

• Cause: An allergic reaction to allergens (pollen, dust) leading to inflammation of bronchi and bronchioles.
• Symptoms: Wheezing, difficulty in breathing (dyspnea), coughing due to spasm of bronchial muscles.

2. Emphysema

• Cause: Major cause is cigarette smoking. Leads to damage of alveolar walls.
• Symptoms: Shortness of breath, chronic cough. The respiratory surface area for gas exchange is drastically reduced. The damage is irreversible.

The trachea, primary, secondary, and tertiary bronchi are lined with C-shaped incomplete cartilaginous rings. Their function is to prevent these air passages from collapsing during inspiration when the air pressure is low.

The regulation of respiration is primarily controlled by the concentration of CO₂ and H⁺ ions, as chemoreceptors are highly sensitive to them. In contrast, peripheral chemoreceptors respond to changes in pO₂ only when it drops to a very low, critical level (e.g., below 60 mm Hg), which does not happen under normal physiological conditions.

Chloride Shift (or Hamburger's Phenomenon) is the process where chloride ions (Cl⁻) diffuse from the blood plasma into the RBCs in exchange for bicarbonate ions (HCO₃⁻) that diffuse out. It is necessary to maintain the electrochemical or ionic balance across the RBC membrane.

Vital Capacity (VC) = TV + IRV + ERV
VC = 500 + 2800 + 1100 = 4400 mL

Total Lung Capacity (TLC) = VC + RV
TLC = 4400 + 1200 = 5600 mL

The human respiratory system is divided into two parts: the conducting part and the respiratory or exchange part.

The conducting part consists of the external nostrils, nasal passage, pharynx, larynx, trachea, bronchi, and bronchioles. Its functions are:

1. Air Transport: It transports atmospheric air to the alveoli.
2. Filtering: It clears the inhaled air of foreign particles using hair and mucus.
3. Humidification: It adds moisture to the inhaled air.
4. Temperature Regulation: It brings the temperature of inhaled air to body temperature.

Transport Mechanism:
About 97% of O₂ is transported by binding to haemoglobin (Hb) in RBCs to form oxyhaemoglobin. The remaining 3% is dissolved in plasma.

1. In Lungs: High pO₂ (~104 mm Hg) forces O₂ to bind with Hb.
2. In Tissues: Low pO₂ (~40 mm Hg) causes oxyhaemoglobin to dissociate and release O₂ to the cells.

Factors Affecting O₂ Binding (Shifts in Dissociation Curve):

• Partial Pressure of O₂ (pO₂): Higher pO₂ favours loading; lower pO₂ favours unloading.
• Partial Pressure of CO₂ (pCO₂): High pCO₂ decreases affinity, favouring unloading (right shift - Bohr's effect).
• Hydrogen Ion Concentration (pH): Low pH (acidic) decreases affinity, favouring unloading (right shift).
• Temperature: High temperature decreases affinity, favouring unloading (right shift).

1. (a) What is the medical term for the condition experienced by the trekkers?
   The condition is called Altitude Sickness or Acute Mountain Sickness (AMS), a form of hypoxia.
2. (b) Why did they experience breathlessness?
   At high altitudes, the partial pressure of atmospheric oxygen (pO₂) is low. To compensate, the respiratory centre increases the breathing rate, leading to breathlessness.
3. (c) What two long-term physiological changes will occur in their bodies if they stay at high altitude for a few weeks?
   Two changes are: 1. Increased Red Blood Cell (RBC) production. 2. Increased concentration of 2,3-BPG in RBCs to facilitate oxygen release to tissues.