Chapter 2.5
Respiratory System in Vertebrates (Fishes to Mammals)

A. Respiration and Respiratory System

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Introduction

Definition of Respiration
Respiration is the biochemical process by which organisms exchange gases (O₂ and CO₂) with their environment to support cellular metabolism.

 

I. Functions of the Respiratory System

1. Gas Exchange: Oxygen uptake and carbon dioxide removal.
2. pH Regulation: Maintains acid-base balance via CO₂ expulsion.
3. Thermoregulation: Panting in mammals and birds.
4. Vocalization: Laryngeal structures in mammals and birds.

 

II. Evolutionary Context
  • Early vertebrates relied on cutaneous respiration (skin).
  • Aquatic vertebrates evolved gills for efficient oxygen extraction from water.
  • Terrestrial vertebrates developed lungs for aerial respiration.

 

III. Types of Respiratory Systems in Vertebrates

A. Cutaneous Respiration

  • Definition: Gas exchange through the skin.
  • Examples: 
    • Amphibians (frogs, salamanders).
    • Some fish (e.g., lungfish during estivation).
  • Limitations: Requires moist skin; inefficient for large animals.

B. Gills (Aquatic Respiration)

  • Found in: Fishes, larval amphibians, some aquatic reptiles.
  • Structure: Filamentous projections with lamellae for increased surface area.
  • Mechanism: Countercurrent exchange system maximizes O₂ uptake.

C. Lungs (Aerial Respiration)

  • Found in: Tetrapods (amphibians, reptiles, birds, mammals).
  • Evolution: Derived from swim bladders in primitive fish.
  • Types:
    • Simple sac-like lungs (amphibians, reptiles).
    • Alveolar lungs (mammals).
    • Parabronchial lungs (birds, unidirectional airflow).

D. Accessory Respiratory Structures

1. Swim Bladders:

  • Derived from lungs in some fish (e.g., teleosts).
  • Functions in buoyancy, occasionally respiration.

2. Buccopharyngeal Respiration:

  • Frogs use mouth lining for gas exchange.

3. Cloacal Respiration:

  • Some turtles use cloaca for underwater breathing.
Close-up of mudskippers on a sandy beach shore displaying unique amphibious behavior.

B. Gills, Lungs, and Other Respiratory Structures

I. Detailed Structure and Function of Respiratory Organs

A. Gills in Fishes and Amphibians

Structure:

  • Gill Arches: Bony or cartilaginous support.
  • Gill Filaments: Primary sites of gas exchange.
  • Lamellae: Thin, vascularized folds increasing surface area.

Mechanism:

  1. Ventilation: Water flows over gills via buccal pumping or ram ventilation (sharks).
  2. Countercurrent Exchange: Blood flows opposite to water, maximizing O₂ diffusion.

Adaptations:

  • Active Fish (e.g., Tuna): High gill surface area for rapid gas exchange.
  • Air-Breathing Fish (e.g., Lungfish): Modified gills for bimodal respiration.

 

B. Lungs in Tetrapods

1) Amphibians

  • Simple Sacs: Poorly subdivided; rely on cutaneous respiration.
  • Positive Pressure Breathing: Gulping air via buccal pumping.

2) Reptiles

  • More Complex: Septa increase surface area.
  • Negative Pressure Breathing: Rib expansion draws air in.

3) Birds

  • Parabronchial Lungs: Unidirectional airflow via air sacs.
  • Cross-Current Exchange: Highly efficient O₂ uptake for flight.

4) Mammals

  • Alveolar Lungs: Millions of alveoli for gas exchange.
  • Diaphragm: Enhances negative pressure breathing.

 

C. Swim Bladders and Other Structures

  • Evolutionary Link: Swim bladders in ray-finned fish became primitive lungs.
  • Bimodal Breathing: Some fish (e.g., gar) use both gills and lungs.

 

II. Comparative Respiratory Physiology Across Vertebrates

 

III. Major Evolutionary Milestones in Vertebrate Respiration

1) Origin of Gills (~500 MYA)

  • Early chordates (e.g., lancelets) used pharyngeal slits for filter-feeding.
  • Jawless fish (agnathans) adapted slits for gas exchange.

2) Transition to Lungs (~400 MYA)

  • Lobe-finned fish (e.g., Eusthenopteron ) developed primitive lungs.
  • Swim Bladder Divergence: Ray-finned fish modified lungs for buoyancy and primitive lung-based respiration.

3) Terrestrial Adaptations (~375 MYA)

  • Early tetrapods (e.g., Acanthostega) retained gills but had rudimentary lungs.
  • Amphibians relied on cutaneous respiration due to inefficient lungs.

4) Amniote Innovations (~320 MYA)

  • Reptiles: Developed septate lungs for better O₂ uptake.
  • Birds: Evolved air sacs for unidirectional flow (flight adaptation).
  • Mammals: Alveolar lungs and diaphragm for high metabolic demands.
 
Conclusion

The vertebrate respiratory system showcases remarkable evolutionary innovation, from gills in fish to the highly efficient lungs of birds and mammals. Key milestones include: i) Gill-to-Lung Transition** for terrestrial life, ii) Avian Air Sacs  enabling endurance flight, and iii) Mammalian Alveoli supporting endothermy.

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