L4 - gaseous exchange in other animals

• Gas exchange in fish:

  -more difficult for fish than mammals because oxygen concentration in water is less than 1% compared to 20% in air

  -specialised exchange surface helps maintain high diffusion rate

  -need molecular oxygen and cannot break down water for oxygen

  -bony fish have small surface area to diffusion has to occur through a specialised exchange surface

  -developed specialised organs called gills
• Advantages of an exchange system always being exposed to an aquatic environment:

  -cell membranes of gas exchange surface kept moist due to contact with water

  -no need to prevent water loss from gas exchange
• Disadvantages of an exchange surface always being exposed to water:

  -oxygen concentration in water is low, especially in warmer and/or saltier water

  -gas exchange system must therefore be very efficient to absorb enough oxygen for aerobic respiration
• Gills structure:

  -fish possess several gills located behind and to the side of their buccal cavity (mouth cavity)

  -the gills of bony fish are covered by an operculum (a hard plate like flap made of bone)

  -the operculum protects the gills and helps the fish to breathe
• Internal gill structure:

  -the gills are composed of three main parts; filaments, rakers, and arches

  -the arches are the structural component of the gill

  -rakers prevent debris from entering the fish through the gills

  -filaments absorb oxygen

  
• In fish, gill lamellae are used to increase the surface area in contact with the environment to maximise gas exchange (both to attain oxygen and to expel carbon dioxide) between the water and the blood
• Countercurrent flow:

  -a rich supply of blood flows through the gill filaments in the opposite direction from the water passing the gills

  -this countercurrent mechanism increase the concentration gradient and efficiency of gas exchange

  -across the whole gill lamella, blood meets water with a higher concentration of oxygen than its own

  -this is important to get all the available oxygen out of the water and into the blood by diffusion
• Countercurrent flow:

  -in countercurrent flow, the concentration of oxygen is always higher in the water than in the blood

  -if the blood flowed in the same direction as the water, then the blood would only be able to get 50% of the oxygen from the water

  -the blood and water would reach an equilibrium in oxygen content and diffusion would no longer take place
• Ventilation in fish:

  -ventilation mechanism creates the pressure differences needed to provide a constant stream of water over the gills

  -water is drawn through the mouth into the buccal cavity using muscles

  -water passes over gills and leaves through operculum
• Fish ventilation:

  -mouth opens and volume inside buccal cavity increases

  -this decreases the pressure and water flows in

  -mouth closes and squeezes water out of buccal cavity, over gills and out of operculum on each side
• Fish ventilation:

  -mouth opens

  -operculum closes

  -buccal floor lowers

  -buccal cavity volume increases

  -pressure decreases

  -water flows in
• Primitive cartilaginous fish:

  -do not contain a sophisticated gas exchange system

  -they simply keep moving to force water through the gills

  -this is called ram ventilation

  -also use a parallel system so only utilise around 50% of the oxygen from the water (concentration gradient is not as steep)
• Insect gaseous exchange:

  -have a segmented body with a hard exoskeleton made of chitin which is impermeable to gases

  -the second segment (thorax) has small holes called spiracles on either side

  -more spiracles are arranged in a line on each side of the abdomen (third section)

  -spiracles are open to the inside through air filled tubes called trachea which penetrate inside the body

  -trachea held open by chitin rings

  -oxygen and carbon dioxide exchanged via spiracles and through the tracheal system
• Insect parts

  
• Insect exoskeleton: too thick for gas exchange
• Insect tracheoles:

  -branch off of the tracheae

  -elongated cells

  -no chitin

  -freely permeable to gas

  -run between the cells of the insect

  -where most of gaseous exchange occurs
• Insect tracheae:

  -tubes leading from spiracles

  -largest tubes in an insect’s respiratory system (1mm)

  -carry air into and along the body

  -lined with chitin (prevents collapsing)

  -chitin is impermeable to gas (little gas exchange)

  -contains tracheal fluid
• Insect spiracles:

  -air can enter and leave here

  -water also lost from here

  -small bristles prevent unwanted particles from entering
• Spiracles:

  -can be seen as the openings at the body surface that lead to tracheae and eventually, tracheoles

  -tracheoles must reach every cell as insects don’t have a circulatory system to do this

  -body movements or muscle contractions speed up the rate of diffusion of gases from tracheae into body cells
• Spiracles:

  -guarded by valves which are controlled by muscles enabling insects to open and close their spiracles

  -when they need lots of oxygen or the environment is moist, the spiracles will be open

  -when they don’t need as much oxygen or the environment is dry, the spiracles will be closed or only slightly open to reduce water loss

  -bristles in the spiracles filter unwanted particles out of he air so they don’t damage the gas exchange surface

  gas exchange system is inside the insects body where it is humid to help prevent desiccation (removal of moisture)
• Structure of the tracheal system:

  -respiratory gases ,ove in and out of the tracheal system along a diffusion gradient, by mass transport, and dissolving into the ends of the tracheoles that are filled with water (tracheal fluid)

  -gaseous exchange occurs between the air in the tracheoles and the tracheal fluid

  -when an insect is particularly active, the tracheal fluid can be withdrawn to provide a larger surface area for exchange between the tracheoles and the tissues

  
• Insect ventilation:

  -insects forcibly ventilate their trachea by using expanded sections of the tracheal system with flexible walls

  -the expanded sections act as air sacs reduce the abdomen volume with the contraction of muscles as air is forced out of the trachea

  -as muscles relax, the abdomen springs back to its normal volume as air is drawn in

  -wing movement can also alter the volume in the thorax; as the volume decreases, air in the tracheal system is pushed out; as the volume increases, the pressure inside drops and air is pushed into the tracheal system
• Insect air sacs:

  -attached to the trachea

  -store extra air (oxygen) until it’s needed for respiration
• High energy demand insect adaptations:

  -mechanical ventilation; air is actively pumped into the system by muscular pumping movements of the thorax and abdomen

  -additional air sacs
• One way flow of ventilation in insects:

  -muscle movements can be coordinated with opening and closing of spiracle valves to provide a one way flow of air through the insect

  -the one way flow increases the efficiency of gas exchange as carbon dioxide rich air can be expelled without mixing with incoming fresh air
• One way ventilation:

  -thorax expands and spiracles at the front end of the body open

  -air enters the thorax

  -thorax reduces in volume

  -spiracles in the abdomen open

  -air leaves the tracheal system via the abdomen
• Branched tracheoles increase the surface area for diffusion
• Tracheal fluid being withdrawn allows air to move closer to the cells so there is a shorter distance for diffusing
• Continuous air supply is important to maintain a large concentration gradient
• Thin tracheole walls increase permeability