📝 Chapter Notes & Revision

Breathing and Exchange of Gases

🏫 MP BoardClass 11Biology

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Quick Revision Notes

Class 11 Biology: Breathing and Exchange of Gases

MP Board Examination

### Concept 1: Respiratory Organs in Animals

Different organisms use different organs for respiration based on their habitats and levels of organization:

  • General Body Surface (Plasma membrane): Sponges, Coelenterates, Flatworms (Diffusion).
  • Skin / Cuticle: Earthworms, Leeches, Amphibians (Cutaneous respiration).
  • Tracheal Tubes: Insects (Cockroaches).
  • Gills (Branchial respiration): Aquatic arthropods, Fishes, Molluscs, Tadpoles.
  • Lungs (Pulmonary respiration): Reptiles, Birds, Mammals (including Humans).

### Concept 2: Human Respiratory System

The human respiratory system consists of:

  1. Conducting Zone: Transport air to alveoli, clears dust, humidifies, and brings air to body temperature.
    • External nostrils $\rightarrow$ Nasal cavity $\rightarrow$ Pharynx $\rightarrow$ Larynx (Sound box / Voice box) $\rightarrow$ Trachea $\rightarrow$ Primary, Secondary, and Tertiary Bronchi $\rightarrow$ Bronchioles.
  2. Respiratory / Exchange Zone: Actual site of diffusion of $O_2$ and $CO_2$.
    • Terminal bronchioles $\rightarrow$ Alveoli (Vascularized bag-like structures).

### Concept 3: Mechanism of Breathing (Pulmonary Ventilation)

Breathing involves two stages: Inspiration (Inhalation) and Expiration (Exhalation). It is driven by pressure gradients between the lungs and atmosphere.

  • Inspiration (Inhalation):

    • Air enters the lungs.
    • Contraction of diaphragm $\rightarrow$ Volume of thoracic chamber increases in the antero-posterior axis.
    • Contraction of external intercostal muscles $\rightarrow$ Ribs and sternum lift up $\rightarrow$ Volume increases in the dorso-ventral axis.
    • Overall increase in thoracic volume $\rightarrow$ Intra-pulmonary pressure decreases below atmospheric pressure $\rightarrow$ Air rushes into the lungs.

  • Expiration (Exhalation):

    • Air is expelled from the lungs.
    • Diaphragm and intercostal muscles relax $\rightarrow$ Diaphragm and sternum return to normal positions.
    • Decrease in thoracic volume $\rightarrow$ Intra-pulmonary pressure increases above atmospheric pressure $\rightarrow$ Air is forced out.
    • Note: Forceful expiration utilizes additional muscles (internal intercostal muscles and abdominal muscles).

### Concept 4: Respiratory Volumes and Capacities (Spirometry)

Important terms and standard values for a healthy human adult:

  • Tidal Volume (TV): Volume of air inspired or expired during normal breathing.
    • Value: Approx. $500$ mL per breath ($6000$ to $8000$ mL per minute).
  • Inspiratory Reserve Volume (IRV): Maximum extra air a person can inspire by a forcible inspiration.
    • Value: $2500$ mL to $3000$ mL.
  • Expiratory Reserve Volume (ERV): Maximum extra air a person can expire by a forcible expiration.
    • Value: $1000$ mL to $1100$ mL.
  • Residual Volume (RV): Volume of air remaining in the lungs even after a maximum expiration.
    • Value: $1100$ mL to $1200$ mL. (Cannot be measured by spirometer).

Important Formulae for Capacities:

  • Inspiratory Capacity (IC) = TV + IRV
  • Expiratory Capacity (EC) = TV + ERV
  • Functional Residual Capacity (FRC) = ERV + RV
  • Vital Capacity (VC) = TV + IRV + ERV (or maximum air one can breathe out after forced inspiration)
  • Total Lung Capacity (TLC) = VC + RV (or TV + IRV + ERV + RV)

### Concept 5: Exchange of Gases

  • Exchange takes place primarily at the Alveoli and Tissues by Simple Diffusion.
  • Driven by pressure/concentration gradient (Partial Pressure, represented as $pO_2$ for oxygen and $pCO_2$ for carbon dioxide).

Partial Pressures (in mm Hg):

Respiratory GasAtmospheric AirAlveoliBlood (Deoxygenated)Blood (Oxygenated)Tissues
$O_2$$159$$104$$40$$95$$40$
$CO_2$$0.3$$40$$45$$40$$45$
  • Diffusion Membrane Thickness: Extremely thin ($\text{less than } 1 \mu m$), made of three layers:
    1. Squamous epithelium of alveoli.
    2. Endothelium of alveolar capillaries.
    3. Basement substance in between them.

### Concept 6: Transport of Gases

A. Transport of Oxygen ($O_2$):

  • $3%$ is dissolved in plasma.
  • $97%$ is bound to Hemoglobin (Hb) in RBCs.
  • Oxygen-Hemoglobin Dissociation Curve: A sigmoid curve obtained when percentage saturation of hemoglobin is plotted against $pO_2$.
    • Favorable conditions for $O_2$ binding at alveoli: High $pO_2$, low $pCO_2$, lower temperature, lesser $H^+$ concentration (high pH).
    • Favorable conditions for dissociation at tissues: Low $pO_2$, high $pCO_2$, higher temperature, higher $H^+$ concentration (low pH).
  • Bohr Effect: Shift of the oxyhemoglobin dissociation curve to the right due to increased $pCO_2$ or decreased pH, facilitating $O_2$ release in tissues.

B. Transport of Carbon Dioxide ($CO_2$):

  • $7%$ is dissolved in plasma.
  • $20-25%$ is carried as Carbaminohemoglobin (bound to Hb).
  • $70%$ is carried as Bicarbonate ions ($HCO_3^-$).
    • Reaction in RBCs: CO2 + H2O <---> H2CO3 <---> HCO3(-) + H(+) (Catalyzed by enzyme Carbonic Anhydrase).
    • Chloride Shift (Hamburger's Phenomenon): Movement of chloride ions ($Cl^-$) into RBCs from plasma to maintain ionic balance as $HCO_3^-$ moves out.

### Concept 7: Regulation of Respiration

  • Respiratory Rhythm Center: Located in the Medulla oblongata of the brain; primarily responsible for regulation.
  • Pneumotaxic Center: Located in the Pons region of the brain; can moderate the functions of the respiratory rhythm center by reducing the duration of inspiration.
  • Chemosensitive Area: Situated adjacent to the rhythm center, highly sensitive to $CO_2$ and $H^+$ concentration. Increase in these substances activates the center, which signals the respiratory rhythm center to make adjustments to eliminate excess gases.

### Concept 8: Disorders of the Respiratory System

  • Asthma: Spasm of bronchial muscles causing difficulty in breathing, accompanied by wheezing.
  • Emphysema: Chronic disorder where alveolar walls are damaged, decreasing the respiratory surface area (often caused by cigarette smoking).
  • Occupational Respiratory Disorders: Long-term exposure to dust (in stone-breaking or mining industries) causes inflammation leading to fibrosis (proliferation of fibrous tissues), e.g., Silicosis, Asbestosis.