RESPIRATORY ORGANS IN ANIMALS: Diverse Strategies for Gas Exchange

I. Overview of Respiratory Mechanisms:

• Adaptations Across Animal Groups:
  • Varied Strategies:
    • Different animal groups employ diverse mechanisms for gas exchange based on their habitats and organizational levels.

II. Lower Invertebrates:

• Diffusion-Based Respiration:
  • Examples:
    • Sponges, coelenterates, flatworms, etc.
  • Mechanism:
    • Exchange of O₂ and CO₂ occurs through simple diffusion over the entire body surface.

III. Earthworms and Insects:

• Moist Cuticle and Tracheal Tubes:
  • Earthworms:
    • Utilize their moist cuticle.
  • Insects:
    • Employ a network of tubes (tracheal tubes) to transport atmospheric air within the body.

IV. Aquatic Arthropods and Molluscs:

• Gill-Based Respiration:
  • Mechanism:
    • Utilization of vascularized structures called gills for gas exchange.
  • Examples:
    • Most aquatic arthropods and molluscs.

V. Terrestrial Forms:

• Lung-Based Respiration:
  • Mechanism:
    • Employ vascularized bags called lungs for the exchange of gases.
  • Examples:
    • Terrestrial vertebrates like amphibians, reptiles, birds, and mammals.

VI. Amphibians:

• Cutaneous Respiration:
  • Additional Mechanism:
    • Moist-skinned amphibians like frogs can respire through their skin (cutaneous respiration).

VII. Vertebrates:

• Fish:
  • Respiratory Organ:
    • Gills.
• Amphibians, Reptiles, Birds, Mammals:
  • Respiratory Organ:
    • Lungs.

VIII. Adaptation to Environment:

• Tailored to Habitat:
  • Animals exhibit respiratory adaptations in response to their specific environmental niches, ensuring efficient gas exchange.

HUMAN RESPIRATORY SYSTEM: An In-Depth Overview

I. External Nostrils to Nasal Chamber:

• Nostrils:
  • Pair of external nostrils above the upper lips.
• Nasal Passage and Chamber:
  • Nostrils lead to the nasal passage, opening into the nasal chamber.

II. Nasal Chamber to Pharynx:

• Common Passage for Food and Air:
  • Nasal chamber opens into the pharynx.
  • Pharynx is a common passage for both food and air.

III. Pharynx to Larynx and Trachea:

• Larynx (Sound Box):
  • Cartilaginous box aiding in sound production.
  • Glottis can be covered by the epiglottis during swallowing to prevent food entry.
• Trachea:
  • Straight tube extending to mid-thoracic cavity.
  • Divides into right and left primary bronchi at the 5th thoracic vertebra.

IV. Bronchi and Bronchioles:

• Bronchi Division:
  • Primary bronchi divide into secondary and tertiary bronchi.
  • Further divisions lead to bronchioles, ending in terminal bronchioles.
• Cartilaginous Support:
  • Incomplete cartilaginous rings support tracheae, primary, secondary, and tertiary bronchi.

V. Alveoli:

• Structure:
  • Very thin, irregular-walled, vascularized bag-like structures.
• Location:
  • Terminal bronchioles give rise to alveoli.
• Respiratory or Exchange Part:
  • Site for diffusion of O₂ and CO₂ between blood and atmospheric air.

VI. Lungs and Pleura:

• Double-Layered Pleura:
  • Covers two lungs.
  • Pleural fluid reduces friction on lung surface.
• Thoracic Chamber:
  • Formed dorsally by vertebral column, ventrally by sternum, laterally by ribs, and on the lower side by diaphragm.
  • Thoracic chamber is anatomically air-tight.

VII. Respiratory Process:

1. Breathing or Pulmonary Ventilation:
   • Drawing in atmospheric air and releasing CO₂-rich alveolar air.
2. Diffusion of Gases:
   • O₂ and CO₂ diffusion across the alveolar membrane.
3. Transport of Gases:
   • Gases transported by blood.
4. Diffusion in Tissues:
   • O₂ and CO₂ diffusion between blood and tissues.
5. Utilization in Cells:
   • Cells utilize O₂ for catabolic reactions, releasing CO₂.

VIII. Thoracic Cavity and Lung Volume:

• Anatomical Setup:
  • Any change in thoracic cavity volume reflects in the pulmonary cavity.
• Essential for Breathing:
  • Enables breathing as pulmonary volume cannot be directly altered.

IX. Key Respiratory Steps:

• Breathing
• Gas Diffusion
• Gas Transport
• Tissue Gas Diffusion
• Cellular Utilization

MECHANISM OF BREATHING: Understanding Inspiration and Expiration

I. Breathing Stages:

• Two Stages:
  • Inspiration: Drawing in atmospheric air.
  • Expiration: Releasing alveolar air.

II. Pressure Gradient:

• Pressure Difference:
  • Movement of air results from a pressure gradient between lungs and atmosphere.
  • Inspiration: Intra-pulmonary pressure < Atmospheric pressure.
  • Expiration: Intra-pulmonary pressure > Atmospheric pressure.

III. Muscular Involvement:

• Muscles Involved:
  • Diaphragm:
    • Initiates inspiration by contracting.
    • Increases thoracic chamber volume antero-posteriorly.
  • External and Internal Intercostal Muscles:
    • Contribute to volume increase dorso-ventrally.

IV. Mechanism of Inspiration:

• Diaphragm Contraction:
  • Increases thoracic chamber volume.
• External Intercostal Muscles:
  • Lift up ribs and sternum.
  • Increases thoracic volume dorso-ventrally.
• Volume Increase:
  • Thoracic and pulmonary volume rise.
• Intra-pulmonary Pressure:
  • Decreases below atmospheric pressure.
• Air Movement:
  • Atmospheric air flows into lungs.

V. Mechanism of Expiration:

• Diaphragm and Muscles Relax:
  • Return to normal positions.
• Volume Decrease:
  • Thoracic and pulmonary volume decrease.
• Intra-pulmonary Pressure:
  • Increases above atmospheric pressure.
• Air Expulsion:
  • Air expelled from lungs.

VI. Additional Muscles:

• Abdominal Muscles:
  • Assist in enhancing inspiration and expiration strength.

VII. Breathing Rate:

• Average Rate:
  • A healthy human breathes 12-16 times/minute.
• Spirometer Usage:
  • Estimates air volume involved in breathing movements.
  • Assists in clinical pulmonary function assessment.

RESPIRATORY VOLUMES AND CAPACITIES: Understanding Lung Function

I. Respiratory Volumes:

1. Tidal Volume (TV):
   • The volume of air inhaled/exhaled during normal respiration.
   • Approx. 500 mL.
2. Inspiratory Reserve Volume (IRV):
   • Additional air volume inspired by forceful inhalation.
   • Averages 2500 mL to 3000 mL.
3. Expiratory Reserve Volume (ERV):
   • Additional air volume expired by forceful exhalation.
   • Averages 1000 mL to 1100 mL.
4. Residual Volume (RV):
   • Air volume remaining in lungs after forceful exhalation.
   • Averages 1100 mL to 1200 mL.

II. Respiratory Capacities:

1. Inspiratory Capacity (IC):
   • Total air volume a person can inhale after normal exhalation.
   • Includes TV and IRV (TV + IRV).
2. Expiratory Capacity (EC):
   • Total air volume a person can exhale after normal inhalation.
   • Includes TV and ERV (TV + ERV).
3. Functional Residual Capacity (FRC):
   • Air volume remaining in lungs after normal exhalation.
   • Includes ERV and RV (ERV + RV).
4. Vital Capacity (VC):
   • Maximum air volume a person can inhale after forced exhalation.
   • Includes ERV, TV, and IRV, or the maximum volume exhaled after forced inhalation.
5. Total Lung Capacity (TLC):
   • Total air volume in lungs after forced inhalation.
   • Includes RV, ERV, TV, and IRV, or VC + RV.

EXCHANGE OF GASES IN THE RESPIRATORY SYSTEM: Alveolar Dynamics

I. Alveolar Exchange:

1. Primary Sites for Gas Exchange:
   • Alveoli: Main sites for gas exchange.
   • The exchange also occurs between blood and tissues.
2. Mechanism:
   • Exchange is based on simple diffusion.
   • Relies on pressure/concentration gradients.
   • Solubility of gases and membrane thickness affect diffusion rates.
3. Partial Pressure:
   • The pressure of an individual gas in a gas mixture.
   • Represented as pO₂ (oxygen) and pCO₂ (carbon dioxide).

II. Gas Exchange Data:

• Oxygen (O₂):
  • Concentration gradient from alveoli to blood and blood to tissues.
  • Facilitates diffusion in both directions.
• Carbon Dioxide (CO₂):
  • Concentration gradient from tissues to blood and blood to alveoli.
  • Higher solubility (20-25 times more than O₂) enhances diffusion.

III. Diffusion Membrane:

• Components:
  • Thin squamous epithelium of alveoli.
  • Endothelium of alveolar capillaries.
  • Basement substance (thin basement membrane) between them.
• Thickness:
  • The total thickness is less than a millimeter.
  • Favors efficient diffusion of O₂ from alveoli to tissues and CO₂ from tissues to alveoli.

TRANSPORT OF GASES IN THE BLOOD: Oxygen and Carbon Dioxide Dynamics

I. Oxygen Transport:

1. Blood as the Medium:
   • RBCs carry about 97% of O₂.
   • The remaining 3% is dissolved in plasma.
2. Haemoglobin Interaction:
   • Haemoglobin (iron-containing pigment in RBCs) reversibly binds with O₂.
   • Each hemoglobin molecule can carry up to four O₂ molecules.
   • Binding is influenced by pO₂, pCO₂, H+ concentration, and temperature.
   • The oxygen Dissociation Curve (sigmoid) illustrates saturation with O₂.
3. Dissociation at Tissues:
   • O₂ binds in the lungs (high pO₂, low pCO₂, lower temperature).
   • Dissociates at tissues (low pO₂, high pCO₂, higher temperature).
   • Facilitates O₂ delivery to tissues.

II. Carbon Dioxide Transport:

1. Haemoglobin Interaction:
   • About 20-25% of CO₂ is carried by hemoglobin as carbamino-hemoglobin.
   • Binding influenced by pCO₂.
2. Enzyme Facilitation:
   • Carbonic anhydrase enzyme is present in high concentrations in RBCs.
   • Catalyzes conversion of CO₂ to bicarbonate (HCO₃–) and H⁺.
   • Reaction occurs at tissue and alveolar sites.
3. Release at Alveoli:
   • CO₂ is trapped as bicarbonate in tissues.
   • Released as CO₂ at alveoli (low pCO₂).
   • Facilitates CO₂ elimination.

REGULATION OF RESPIRATION: Neural and Chemical Control

I. Neural Regulation:

1. Respiratory Rhythm Centre:
   • Located in the medulla region of the brain.
   • Primary regulator of respiratory rhythm.
2. Pneumotaxic Centre:
   • Located in the pons region of the brain.
   • Modifies functions of the respiratory rhythm center.
   • Regulates duration of inspiration, influencing respiratory rate.
3. Chemosensitive Area:
   • Adjacent to the rhythm center.
   • Highly sensitive to CO₂ and hydrogen ions.
   • Activated by increased CO₂ and H⁺, signaling adjustments in respiration.

II. Chemical Regulation:

1. Chemosensitivity:
   • The chemosensitive area responds to changes in CO₂ and H⁺ concentration.
   • Activation leads to adjustments in the respiratory process.
2. Receptor Recognition:
   • Receptors in the aortic arch and carotid artery.
   • Recognize changes in CO₂ and H⁺ concentration.
   • Transmit signals to the rhythm center for corrective actions.
3. Limited Role of Oxygen:
   • Oxygen’s role in respiratory rhythm regulation is insignificant.
   • Regulation primarily centered around CO₂ and H⁺ levels.

DISORDERS OF THE RESPIRATORY SYSTEM

1. Asthma:

• Definition: Difficulty in breathing, accompanied by wheezing.
• Cause: Inflammation of bronchi and bronchioles.
• Symptoms:
  • Wheezing sound during breathing.
  • Constriction of airways.
• Management:
  • Anti-inflammatory medications.
  • Bronchodilators to relieve symptoms.

2. Emphysema:

• Definition: Chronic disorder with damage to alveolar walls.
• Cause: Mainly linked to cigarette smoking.
• Symptoms:
  • Reduced respiratory surface area.
  • Difficulty in exhaling.
• Management:
  • Smoking cessation.
  • Medications to ease symptoms.

3. Occupational Respiratory Disorders:

• Definition: Disorders resulting from occupational exposure.
• Cause: Prolonged exposure to dust in certain industries.
• Consequences:
  • Dust exposure overwhelms the body’s defense mechanisms.
  • Inflammation leads to fibrosis (fibrous tissue proliferation).
  • Serious lung damage.
• Prevention:
  • Workers should wear protective masks.
  • Adequate ventilation in workplaces.