Lecture 15 - Breathing and Gas Exchange
I. Introduction to Breathing
• Animals breathe to obtain oxygen (O2 ) for cellular respiration and to expel carbon
dioxide (CO2 ).
• Key Concept:
– Oxygen is required for ATP production in mitochondria.
– Carbon dioxide (CO2 ) is a byproduct of the Krebs cycle and must be removed
to prevent acidification of tissues.
• Breathing involves both external respiration (exchange between environment and
lungs/gills) and internal respiration (exchange between blood and tissues).
II. Differences Between Air and Water Breathers
• Comparison of Oxygen Availability:
– Air has approximately 21% oxygen content, while water typically has only
0.7% dissolved oxygen.
– Oxygen solubility in water decreases with increasing temperature and salinity.
• Energy Expenditure:
– Water breathers (e.g., fish) must move large volumes of water across gills,
leading to high energy costs.
– Air breathers (e.g., mammals, birds) use less energy for ventilation due to
lower density and viscosity of air.
• Evolutionary Adaptations:
– Fish developed highly efficient gills with a counter-current exchange mechanism.
– Mammals evolved alveoli for increased surface area and efficient gas exchange.
1 III. General Respiratory Structures
• Cutaneous Gas Exchange:
– Occurs in amphibians and some reptiles.
– Limited by surface area and skin permeability.
• Lungs, Gills, and Tracheal Systems:
– Lungs (e.g., mammals, birds): Composed of alveoli for high surface area.
– Gills (e.g., fish): Utilize counter-current flow to maximize oxygen extraction.
– Tracheal System (insects): Gases are transported directly to tissues through
spiracles.
IV. Animal Ventilation Patterns
• Passive Ventilation:
– Utilized by smaller aquatic animals that rely on water currents.
• Active Ventilation:
– Requires muscular effort to move air or water over respiratory surfaces.
• Types of Active Ventilation:
– Unidirectional (e.g., birds, fish): Continuous flow in one direction.
– Tidal (e.g., mammals): Air moves in and out through the same pathway.
V. Gas Exchange Mechanisms
• Tidal Exchange (Mammals):
– Airflow is bidirectional, reducing overall efficiency.
• Cross-Current Exchange (Birds):
– Air flows perpendicularly to blood flow, optimizing oxygen uptake.
• Counter-Current Exchange (Fish):
– Water and blood flow in opposite directions, creating a steep oxygen gradient.
2 VI. Factors Influencing Gas Exchange
• Surface Area: Larger surface areas facilitate more efficient gas exchange.
• Diffusion Distance: Shorter distances between air/water and blood vessels increase efficiency.
• Partial Pressure of Oxygen (PO2 ): High PO2 levels favor oxygen binding to
hemoglobin.
• Environmental Conditions: Temperature, pH, and humidity affect gas solubility
and diffusion.
VII. Respiratory Pigments
• Hemoglobin (Hb):
– Has a sigmoidal oxygen dissociation curve due to cooperative binding.
– Bohr Effect: Oxygen affinity decreases with lower pH and higher carbon dioxide (CO2 ) levels.
• Myoglobin (Mb): Stores oxygen in muscle tissues.
• Hemocyanins (invertebrates): Use copper instead of iron for oxygen binding.
VIII. Circulatory Systems
• Open vs. Closed Systems:
– Open systems: Hemolymph bathes tissues directly.
– Closed systems: Blood is confined to vessels, allowing higher pressure.
• Heart Structures:
– Fish: 2 chambers (1 atrium, 1 ventricle).
– Amphibians: 3 chambers (2 atria, 1 ventricle).
– Mammals/Birds: 4 chambers (2 atria, 2 ventricles).
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Lecture 15 - Breathing and Gas Exchange
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