insects and fish gas exchange

• respiratory gases move in and out of insect tracheal system in three ways:

  • along a diffusion gradient
  • mass transport
  • the ends of the tracheoles are filled with water
• along a diffusion gradient (insects)

  • oxygen is used up in cell respiration and so its concentration towards the ends of the tracheoles falls. this creates a diffusion gradient that causes oxygen to diffused from the atmosphere along the trachea and tracheoles to the cells.
  • carbon dioxide is produced in respiration, which creates a diffusion gradient in the opposite direction. this causes carbob dioxide to diffuse along the tracheoles and trachea from cells to the atmosphere.
  • as diffusion in air is much more rapid that in water, respiratory gases are exchanged quickly
• mass transport (insects)


  the contraction of muscles in insects can squeeze the trachea, enabling mass movements of air in and out. this further speeds up the exchange of respiratory gases
• the ends of the tracheoles are filled with water (insects)

  during major activity, the muscle cells around the tracheoles carry out anaerobic respiration. this produces lactate, which is soluble and lowers the water potential of muscle cells. so water moves into cells from the tracheoles by osmosis. the water in the ends of the tracheoles decreases in volume and so draws air further into them. this means the final diffusion pathway is in a gas rather than liquid phase. so diffusion is more rapid. this increases the rate that air is moved into the tracheoles but leads to more water evaporation
• spiracles (insects) 


  these are tiny pores where gases enter and leave the trachea. they may be opened and closed by a valve. when they are open, water vapour can evaporate from the insect. for much of the time insects keep their spiracles closed to prevent this water loss. periodically they open their spiracles to allow gas exchange
• Structure of the gills

  • They are made up of gill filaments stacked in a pile
  • At right angles to the filaments are gill lamellae, which increase the surface area
  • Water is taken in through the mouth and forced over the gills and out through an opening on each side of the body
• Countercurrent flow 


  The flow of water over the gill lamellae and the flow of blood within them are in opposite directions
• The countercurrent exchange principle 


  The countercurrent flow means that:

  • Blood that is already loaded with oxygen meets water, which has its max concentration of water. so diffusion of oxygen from the water to the blood takes place
  • Blood with little oxygen meets water with most of its oxygen removed, so diffusion of oxygen from the water to the blood takes place
• Result of countercurrent


  A diffusion gradient for oxygen uptake is maintained across the entire width of the gill lamellae. about 80% of oxygen in water is absorbed by blood.

  If flow in blood and water was in the same direction, gradient would only be maintained across part of gill lamellae and only 50% of oxygen would be absorbed
• Limiting water loss in insects

  • Small surface area to volume ratio to minimise the area that water is lost
  • Waterproof coverings over their body surfaces. this is a rigid outer skeleton of chitin that is covered with a waterproof cuticle
  • Spiracles are the openings of trachea at the body surface which can be closed to reduce water loss. this conflicts with the need for oxygen so occurs mainly when the insect is at rest
• gas exchange in insects 


  .

  A) spiracle

  B) body surface

  C) fluid-filled ends of tracheoles

  D) trachea

  E) tracheoles

  F) muscle fibre

  
• structure of a leaf


  .

  A) upper epidermis

  B) mesophyll cells

  C) lower epidermis

  D) chloroplast

  E) vacuole

  F) nucleus

  G) air space

  H) guard cell

  I) stomatal pore

  
• human respiratory system

  .

  A) nasal cavity

  B) nostril

  C) trachea

  D) bronchus

  E) lung

  
• digestive system 


  .

  A) tongue

  B) salivary glands

  C) oesophagus

  D) liver

  E) stomach

  F) pancreas

  G) large intestine

  H) small intestine

  I) rectum

  J) anus