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Learning Objectives (LOs)

  • Read about the buccal pump and illustrate the pressure status inside the oral cavity on animal diagrams:
    • Use ‘-’ for negative pressure and ‘+’ for positive pressure.
  • Familiarize with tetrapod lung diversity.
    • Contrast and define faveoli vs. alveoli.
  • Contrast parabronchi in avian lungs with alveoli in mammalian lungs.
  • Compare unidirectional ventilation and bidirectional (tidal) ventilation.
    • Discuss the advantages of both ventilation types.
  • Define dead space to better understand tidal ventilation.
  • Identify types of vertebrates that use ram ventilation.
  • Explain the three types of active pumping, giving examples.
  • Trace a draft of air through biphasic breathing in birds to understand unidirectional and continuous airflow in avian lungs.

Respiratory Surfaces/Organs

  • Lungs (bidirectional/tidal ventilation):
    • Elastic endodermal outpocketings from the pharynx; lung volume expands and decreases.
    • Paired structures, located ventral to the digestive tract and connected via trachea.
    • Increased compartmentalization is a result of evolutionary development.

Tetrapod Lungs: Diversity

  • Amphibians:

    • Respiratory surface developed anteriorly.
    • Faveoli: Internal subdivisions in the lung wall opening into a common central chamber.

  • Reptiles:

    • Generally have a single central air chamber.
    • Faveoli with smooth muscles, may be subdivided.
    • Posterior non-exchange region (e.g., saccular portion in snakes) functions as a bellows, not vascularized.

  • Birds:

    • Lungs connected to air sacs create a parabronchial system with one-way passages.
    • Faveoli surround these passages, allowing air conduction with 6 to 12 avascular air sacs for lung irrigation.

  • Mammals:

    • Respiratory tree (trachea, bronchi, bronchioles).
    • Alveoli are blind-ended compartments connected to bronchioles, offering a total alveolar area that is 10 times that of similarly sized amphibians, supporting a higher gas exchange rate.

Ventilation Types

  1. Unidirectional Ventilation:

    • Water enters the buccal cavity and flows across gills, exiting through slits.

  2. Tidal Ventilation:

    • Air enters the trachea, moves to lungs, and exits via the same path.
    • Mixing of fresh and old air occurs; roughly 30% of inhaled air (about 150 mL in humans) stays in the respiratory dead space (airways not involved in gas exchange).

Muscular Aids to Ventilation

  • Gill-based ventilation is powered by branchiomeric muscles.
  • Lung-based ventilation is powered by axial musculature.
Types of Active Pumping

  1. Dual Pump (e.g., sharks):

    • Suction Phase: Expansion of buccal and opercular cavities draws water in.
    • Force Phase: Compression expels water out across gills.

  2. Buccal Pump:

    • Involves expansion and compression of the mouth cavity to ventilate lungs (e.g., amphibians).
    • Two-stroke system:
    1. Expand;
    2. Compress airflow out through nares.
    • Four-stroke system (e.g., aquatic salamanders): includes multiple phases for inhalation and exhalation.

  3. Aspiration Pump (e.g., amniotes):

    • Air is sucked into lungs by negative pressure created by rib cage movements.
    • Inhalation involves diaphragm and rib cage expanding, while exhalation involves contraction.
Biphasic Breathing in Birds
  • Air flows through trachea and splits into lungs and air sacs during inhalation.
  • This system ensures unidirectional and continuous airflow within lungs.
Inhalation and Exhalation Cycles:
  1. Inhalation 1: Air into trachea and primary bronchi, into posterior air sacs.
  2. Exhalation 1: Air moves from posterior sacs into lungs, and lung air exits via trachea.
  3. Inhalation 2: Air again divides, refilling posterior sacs while pushing spent air into anterior sacs.
  4. Exhalation 2: Air flows from anterior sacs and lung air exits through the trachea.