Breathing & Exchange of Gases

NCERT Class 11 Biology • Chapter 14 Solutions

Volumes, Capacities & Sites

1. Define Vital Capacity. What is its significance?

Definition: The maximum volume of air a person can breathe in after a forced expiration. It includes ERV + TV + IRV.

Significance:

  • It represents the maximum amount of air a person can renew in the respiratory system in a single breath.
  • A higher vital capacity allows for greater oxygen intake, essential for strenuous physical activities.
  • It is an indicator of lung health; decreased VC is seen in lung diseases.
2. State the volume of air remaining in the lungs after a normal breathing.

This is called the Functional Residual Capacity (FRC).

$$FRC = ERV + RV$$
Value $\approx 1000 \text{ ml (ERV)} + 1100 \text{ ml (RV)} = \mathbf{2100 \text{ to } 2300 \text{ ml}}$.
3. Why does diffusion occur only in the alveolar region?

Diffusion occurs in the alveoli because:

  1. Thin Membrane: The diffusion membrane is extremely thin (< 1mm), consisting of the squamous epithelium of alveoli, the endothelium of alveolar capillaries, and the basement substance.
  2. Surface Area: Alveoli provide a massive surface area for exchange.
  3. Vascularity: They are highly vascularized to maintain concentration gradients.

Other parts (trachea, bronchi) have thick walls (cartilage, smooth muscle) meant for conduction, not diffusion.

Gas Transport & Regulation

4. Major transport mechanisms for CO₂.

$CO_2$ is transported in three forms:

  1. As Bicarbonate (approx. 70%): RBCs contain carbonic anhydrase which converts $CO_2$ and $H_2O$ to $H_2CO_3$, which dissociates into $HCO_3^-$ and $H^+$. The bicarbonate diffuses into the plasma.
  2. As Carbamino-haemoglobin (approx. 20-25%): $CO_2$ binds to the amine group of haemoglobin. Binding is related to partial pressure of $CO_2$.
  3. Dissolved state (approx. 7%): Dissolved physically in blood plasma.
5. Comparison of pO₂ and pCO₂ in Atmosphere vs Alveoli.

(ii) pO₂ higher, pCO₂ lesser

Explanation: Atmospheric air is rich in oxygen ($pO_2 \approx 159$ mmHg) and low in carbon dioxide ($pCO_2 \approx 0.3$ mmHg). Alveolar air has lower $pO_2$ (104 mmHg) and higher $pCO_2$ (40 mmHg) due to mixing with residual air and diffusion from blood.

6. Explain the process of Inspiration.
[Image of Mechanism of Breathing Inspiration]

Inspiration is an active process initiated by muscle contraction:

  1. Diaphragm: Contracts and flattens, increasing the volume of the thoracic chamber in the antero-posterior axis.
  2. External Intercostal Muscles: Contract to lift up the ribs and sternum, increasing volume in the dorso-ventral axis.
  3. Result: Overall increase in thoracic volume $\rightarrow$ Decrease in intra-pulmonary pressure (below atmospheric pressure) $\rightarrow$ Air rushes into the lungs.
7. How is respiration regulated?

Regulation is primarily neural and chemical:

  • Respiratory Rhythm Centre (Medulla): Primarily responsible for regulation.
  • Pneumotaxic Centre (Pons): Moderates the functions of the rhythm centre (can reduce duration of inspiration).
  • Chemosensitive Area: Located near the rhythm centre, highly sensitive to $CO_2$ and $H^+$ ions (not Oxygen). Increase in these substances signals the rhythm centre to make adjustments.
8. What is the effect of pCO₂ on oxygen transport?

Bohr Effect: High $pCO_2$ facilitates the dissociation of oxygen from oxyhaemoglobin.

  • In Tissues (High $pCO_2$): The curve shifts to the right, affinity decreases, and $O_2$ is released to tissues.
  • In Alveoli (Low $pCO_2$): The affinity increases, favoring the binding of $O_2$ to haemoglobin.

Altitude, Insects & Disorders

9. What happens to the respiratory process up a hill?

As altitude increases, atmospheric pressure and $pO_2$ decrease. This leads to:

  • Hypoxia: Insufficient oxygen reaching tissues.
  • Symptoms: Nausea, fatigue, heart palpitations (Mountain Sickness).
  • Acclimatization (Body’s Response): To compensate, the body increases breathing rate, increases RBC production (polycythemia), and decreases binding affinity of hemoglobin.
10. Site of gaseous exchange in an insect?

Insects have a network of tubes called Tracheal tubes. Exchange occurs at the fine endings called Tracheoles, which are in direct contact with body cells. Air enters through openings called Spiracles.

11. Oxygen Dissociation Curve & Sigmoidal Pattern.
[Image of Oxygen Dissociation Curve]

Definition: A graph plotted between $pO_2$ (x-axis) and Percentage Saturation of Haemoglobin with Oxygen (y-axis).

Reason for Sigmoid Pattern: It is due to the cooperative binding of oxygen to the 4 haem units. Binding of the first $O_2$ molecule facilitates the binding of the subsequent molecules, causing a steep rise, which eventually plateaus as saturation is reached.

12. What is Hypoxia?
Hypoxia: A condition characterized by an inadequate supply of oxygen to the tissues or cells of the body.

It can be caused by:

  • Low atmospheric oxygen (High altitude).
  • Anemia (Low Hb).
  • Poor blood circulation (Ischemia).
  • Histotoxic causes (Cyanide poisoning preventing cells from using oxygen).

Distinctions & Tidal Volume

13. Distinguish between Respiratory Volumes/Capacities.
(a) IRV(a) ERV
Inspiratory Reserve Volume.Expiratory Reserve Volume.
Extra air inspired forcibly.Extra air expired forcibly.
2500 – 3000 ml.1000 – 1100 ml.

(b) Inspiratory Capacity(b) Expiratory Capacity
Total air inspired after normal expiration.Total air expired after normal inspiration.
$TV + IRV$.$TV + ERV$.

(c) Vital Capacity(c) Total Lung Capacity
Max air breathed in after forced expiration.Total air in lungs after forced inspiration.
$ERV + TV + IRV$.$RV + ERV + TV + IRV$ (includes Residual Volume).
14. What is Tidal Volume? Calculate for an hour.

Tidal Volume (TV): Volume of air inspired or expired during a normal respiration. It is approx 500 ml.

Calculation:
Normal Breathing Rate: 12-16 times/minute.
Minute Volume: $500 \times 12 = 6000$ to $500 \times 16 = 8000$ ml/min.
Hourly Volume: $6000 \times 60$ to $8000 \times 60$.
Result: Approx 360,000 ml to 480,000 ml per hour.
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