Respiratory System

Respiration vs. Circulation

• Driven by diffusion: molecules move from high → low partial pressure.

• Efficiency determined by:

  • Ventilation: airflow or water flow

  • Surface area

  • Tissue thickness

🔑 Key Terms

• Respiration (anatomical): gas exchange with environment

• Unidirectional: one-way flow (e.g., fish gills)

• Bidirectional (tidal): in and out (e.g., mammal lungs)

• Hypoxia: insufficient oxygen

🐟 Gills

• Internal: in pharyngeal pouches, covered by operculum (bony/cartilaginous fish)

• External: protruding, used by larvae and some adults (e.g., Necturus)

• Supported by: branchial arches

• Countercurrent exchange: maximizes O₂ absorption

🫧 Gas Bladders & Lungs

• Lungs: endodermal outpockets of pharynx

• Swim bladders:

  • Physostomous: connected to gut (primitive teleosts)

  • Physoclistous: sealed, uses rete mirabile and gas glands

🦶 Cutaneous Respiration

• Skin-based gas exchange

• Significant in amphibians (e.g., hairy frog, Telmatobius)

• Often supplements lung or gill function

🌬 Air Ventilation Mechanisms

🐟 Fish Respiration

• Gills with primary and secondary lamellae

• Covered by operculum in bony fish

• Ram ventilation in fast swimmers

• Some use air-filled sacs for respiration (via pneumatic duct)

🐸 Amphibians

• Rely on lungs, gills, skin

• Buccal pump for ventilation

• Faveoli: internal lung subdivisions for gas exchange

• Larvae often have external gills; lost during metamorphosis

🐢 Reptiles

• Lungs with septae

• Use aspiration pump via rib movement

• Specialized mechanisms:

  • Snakes: body wall muscles aspirate lungs

  • Crocs: liver acts as piston

  • Turtles: rely on limb movement, cloacal exchange, modified plastron

🐴 Mammals

• Diaphragm + intercostal/abdominal muscles drive ventilation

• Respiratory tree: trachea → bronchi → bronchioles → alveoli

• Alveoli: high surface area, thin-walled sacs for efficient exchange

• Viscera aid breathing during locomotion

🐦 Birds

• Most efficient respiratory system in vertebrates

• Parabronchi: tiny one-way tubes

• Air sacs: avascular, help push air in four-step cycle

• Use cross-current exchange

• Lightened bones + air sacs aid flight and breathing

🔄 Gas Transfer Patterns

Lung Structures

• Alveolar: sac-like (mammals)

• Faveolar (faviform): honeycombed (reptiles, amphibians)

• Parabronchial: tubes with capillaries (birds)

📊 Ventilation:Perfusion Ratios

• Reflects oxygen content of medium (air vs. water)

• Water has much less dissolved O₂—requires higher ventilation