Lecture 4: Respiration and Circulation

• Only dissolved oxygen is breathable by fishes
• Flowing water has a more even distribution
• Still water has more oxygen at the surface
• Salinity and dissolved oxygen in water are inversely proportional
• Heat and dissolved oxygen in water are inversely proportional
• Normoxia: When water and air are at equilibrium
• Hypoxia: When dissolved oxygen content in water is between 1% and 30%
• Anoxia: Lack of dissolved oxygen in water
• Most fish cannot live in water where dissolved oxygen is below 30%
• What are some causes of hypoxia?

  Nitrogen caused algal blooms and the resulting decomposition by bacteria that use O2

  The addition of higher salinity water with incomplete mixing
• What are some effects of hypoxia on fish?

  Fish kills

  Behavioral changes

  Physiology changes

  Drive the evolution of gills, other structures, and metabolism
• Fish breathe using unidirectional movement using external gills
• What are the two mechanisms fish use to breathe?

  Branchial pumping

  Ram ventilation
• Branchial pumping is the manual pumping of water over the gills
• Ram ventilation is the passive movement of water over the gills and saves energy
• Gills are the main site of gas exchange and are supported by boney gill arches and gill filament cartilage
• What artery runs to the dorsal aorta?

  Efferent artery
• What artery runs to the ventral aorta?

  Afferent artery
• The efferent artery carries oxygenated blood
• The afferent artery carries deoxygenated blood
• Gas is exchanged between blood and water via counter current exchange
• Why is counter-current exchange so useful for fish in respiration?

  It ensures a steady oxygen gradient

  It allows for the continual absorption of oxygen

  It leads to <10% efficiency of O2 transfer from water to blood
• Oxygen diffusion depends on surface area of the gills and the thickness of the epithelium
• Highly active fishes have larger gill surface areas and thin epithelia as well as more tightly packed lamellae
• Fish vary the ventilation volume or frequency to adjust the rate of gas exchange as represented by their switch from active to passive ventilation at high swimming velocities
• What are some alternative breathing modalities?

  Aquatic Skin Breathing

  Air Breathing through Gills

  Air Breathing through mouth

  Air Breathing through swim bladder

  Air Breathing through lungs
• Larval fish use their skin entirely for gas exchange
• Air breathing developed in certain fish so they could deal with hypoxia
• What is the cost of air-breathing in fish?

  Survival lower in the presence of surface predators
• What is a benefit of air-breathing in fish?

  Survival higher in extreme hypoxic conditions
• Gills modified to have thick, widely spaced lamellae and with respiratory organs like a gill fan or dendritic organ are useful in air-breathing
• Why do closely spaced lamellae prove insufficient for air-breathing?

  They stick together in air
• Some species of fish have a well-vascularized buccal cavity so they can breathe air
• Some fish have specialized guts allowing them to uptake air through swallowing it
• Chondrostei and gar have modified swim bladder allowing for some gas exchange
• Lungfish have developed heavily vascularized lungs and are obligate air breathers
• Why did lungfish develop lungs?

  It allowed them to survive extreme droughts
• Lungfish have vestigial gills for CO2 exchange
• What are the 6 factors affecting O2 consumption in fish?

  Life stage

  Body weight

  Activity level

  Temperature

  Dissolved oxygen

  Feeding
• Life Stage - Eggs use little oxygen

  Body weight - larger fish need more O2

  Activity Level - Active fish need more O2

  Temperature - Warmer waters require more O2

  Dissolved Oxygen - Low PO2 decreases O2 consumption

  Feeding - O2 consumption doubles after feeding