Respiratory System, Ventilation & Evolution

• Respiratory systems facilitate the exchange of oxygen and carbon dioxide.

Introduction

• Passive diffusion relies on surface area, distance, resistance, and partial pressure differences.
• Diffusion alone is effective only over short distances (less than 0.5 mm) if unaided.
• Respiratory and circulatory systems are interlinked for effective gas exchange.
• Pumps create a partial pressure gradient to aid in diffusion.
• External respiration involves gas exchange between the environment and the organism, while internal respiration involves gas exchange at the cellular level.
• Ventilation is the movement of air or water across a respiratory surface, and perfusion is the flow of blood to the respiratory surface.

Respiratory Organs - Gills

• Gills can be external or internal.
• Some gills are pouched or opercular, like those found in some fish.
• Septal gills are another type of gill structure found in certain aquatic species.

Respiratory Organs - Gas Bladders

• Gas bladders function in buoyancy and respiration.
• Gas bladders present in teleosts, bowfins, gars, sturgeons, and bichirs as well as the lungs of lungfishes.
• Two main types of gas bladders are physostomous (connected to the digestive tract via a pneumatic duct) and physoclistous (gas bladder not directly connected to digestive tract).
• Primitive air-breathing involved a four-stroke buccal pump, while some actinopterygians and sarcopterygians use a two-stroke buccal pump system.
• Air-breathing also occurs through gill pouches in some species.

Respiratory Organs - Skin

• Some animals, like the Lake Titicaca frog and hairy frog, use skin as a respiratory organ.
• The skin facilitating gas exchange contains a network of blood vessels under the epidermis
• Fingerlike papillae increase surface area for respiration

Respiratory Organs - Accessory

• Accessory respiratory organs often involve gulping air and can have dual functions.

Respiratory Organs - Embryos

• Anamniote embryos respire through the skin.
• Birds and reptiles have porous shells with a chorioallantois membrane for respiration.

Ventilatory Mechanisms

• Cilia and surfactant aid ventilation.
• Muscular pumps push water or air across respiratory surfaces.
• Aquatic organisms are reliant on a dual pump and ram ventilation.
• Air-breathing organisms on land facilitate respiration via a buccal or aspiration pump.

Dual Pump

• The dual pump mechanism involves a continuous, unidirectional flow, often using a two-stroke system.
• Stages include an oral valve, opercular valve, buccal cavity, and opercular cavity.

Buccal Pump (Pulse Pump)

• Amphibians and some lungfish have a basic two-stroke buccal pump.
• Expansion and compression phases alternate with most amphibians, like the African lungfish using this respiration method.
• Aquatic amphibians and lungfish may use a four-stroke buccal pump.

Aspiration Pump

• Aspiration pumps do not push air; rather, they suck it in.
• Tidal flow exchanges air bidirectionally.
• Feeding does need to be decoupled from respiration in animals who aspirate.

Phylogeny

• Agnathans/Cyclostomes are jawless (ex: lamprey/hagfish).
• Cartilaginous fishes (Elasmobranchs) are sharks, skates, and rays.
• Teleosts are bony fishes.
• Lungfishes, amphibians, reptiles, mammals, and birds also feature in the phylogeny of respiratory systems.

Agnathans - Lamprey

• Lampreys use tidal muscle contraction and relaxation for gill ventilation.

Agnathans - Hagfish

• Hagfish use efferent gill ducts, radial arteries, and branchial pouches as part of their respiratory system.

Gill Structure

• Primary and secondary filaments make up gill structures.
• Structural and respiratory units consist of a holobranch, hemibranch, and gill rays.

Elasmobranchs

• Elasmobranchs have septal gills and use a dual pump, sometimes with ram ventilation.
• Gill structure includes spiracles, interbranchial septums, and parabranchial chambers.

Teleosts

• Teleosts use opercular gills and a dual pump.
• Gill structures include the hyoid arch, internal gill slits, and operculum.

Amphibians

• Amphibians use skin, gills, and lungs for respiration, often supported by a buccal pump using a two-stroke mechanism.

Reptiles

• Reptile embryos have pharyngeal gill slits, and adults use aspiration pumps.
• Lung shape change affects airflow
• Intercostal muscles move ribs -Inhalation involves ribs moving forward and outward -Exhalation involves ribs moving towards the back and inward

Snakes

• Snakes' lengthy bodies mean sometimes being restricted to only one lung.
• Ribs and muscles facilitate respiration, even during prey swallowing.

Crocodiles/Caimans

• Ventilation in crocodiles involves the liver, lungs, rib movement via intercostals, and diaphragmatic/abdominal muscles.

Turtles

• Changes in volume affect the lungs, and leg movement assists breathing, although function can be limited by shell.

Tortoise

• Limiting membranes and muscle contraction aid breathing by viscera changing the lung volume
• These include transverse abdominal and oblique abdominal muscles controlling the shoulders.

Mammals

• Mammals use aspiration pumps with bidirectional flow.
• Structural components include the diaphragm and rib cage.
• The diaphragm is anterior to the liver and acts directly on the pleural cavity
• Muscles such as the transverse abdominus, serratus, rectus abdominus, and external/internal intercostals facilitate breathing.

Mammals - Inhalation

• External intercostal muscles move the ribs and sternum forward and outward, increasing pleural cavity volume.
• The diaphragm contracts and flattens out to increase pleural cavity volume.
• Reduced pressure causes air to rush into the elastic lungs.

Mammals - Exhalation

• Internal intercostal muscles move the rib/sternum backwards
• Decreases the pleural cavity volume.
• The diaphragm relaxes and resumes a dome shape.
• Elastic forces and gravity helps airflow be forced out.

Mammals - Quiet vs. Vigorous Breathing

• Quiet breathing: Inhalation involves the diaphragm. Exhalation relies on elasticity and gravity.
• Vigorous breathing: Ribs, the diaphragm, and muscles are involved. Ground reactions assist, and the viscera moves.

Birds

• Birds' respiratory systems are specialized for flight.
• There's an aspiration pump with unidirectional flow.
• Bird’s parabronchi=tiny one-way passages, and small air capillaries perform gas exchange.
• Alveoli are not present. Mammalian alveoli are a point of contrast.

Birds - Air Sacs

• Birds have nine avascular air sacs among viscera and in long bones.

Birds - Structural Subdivision

• Subdivisions includes anterior air sac, trachea, primary bronchi, and posterior air sac.

Birds - Breathing Cycle

• Cycle 1 Inhalation: air goes to posterior whilst lungs perform gas exchange
• Cycle 1 Exhalation: posterior->mediodorsal
• Cycle 2 Inhalation: lungs to anterior
• Cycle 2 Exhalation: anterior-> outside

Gas Transfer: Patterns

• Respiratory and circulatory systems function together.
• Blood flow maintains a partial pressure gradient which maximizes efficiency of exchange.

Gas Transfer: Rates

• Ventilation and perfusion efficiency of respiration.
• Reduced medium, air or water, can lower ventilation speed. Normal, fast and slow rates of ventilation are a consideration.

Gas Transfer: Water vs. Air

• Partial pressure, extraction rate, density, and viscosity are factors.
• Unidirectional and tidal flow dynamics are considerations.

Evolution of Respiratory Organs

• Transition from water to land.
• In seasonal hypoxic environments, low oxygen favors air-breathing.
• A fish ancestor and selective pressure led to the development of lungs/gas bladders.

Evolution of Respiratory Organs - Mechanisms

• A dual pump moves water across gills in fish
• A buccal pump moves water across gills in amphibians
• Special cavity loss of opercular leads to aspiration/buccal exchange
• Buccal transition leads to the adaptation of respiration using an aspiration pump.
• Reptiles decouple ventilation from feeding

Respiratory Organs - Evolution

• Fish use a dual pump while amphibians use a modified buccal pump and the modified version is used by mammals, reptiles and birds too.
• Lung cavities are used during aspiration and buccal pumps are used when the ventilation is opercular assisted.

Explore key factors affecting passive diffusion of respiratory gases. Compare unidirectional and bidirectional ventilation. Understand the role of chorioallantois in respiration and the advantages of aspiration pumping. Learn about breathing muscles.