3.1.6 Gas Exchange in Fish & Insects

Tracheal System of an Insect

• All insects possess a rigid exoskeleton with a waxy coating that is impermeable to gases
• Insects have evolved a breathing system that delivers oxygen directly to all the organs and tissues of their bodies
• A spiracle is an opening in the exoskeleton of an insect which has valves
  • It allows air to enter the insect and flow into the system of tracheae

• Tracheae are tubes within the insect respiratory system which lead to tracheoles (narrower tubes)
  • There are rigid rings of chitin that keep the tracheae open

• A large number of tracheoles run between cells and into the muscle fibres - the site of gas exchange
• For smaller insects, this system provides sufficient oxygen via diffusion

Image showing the structure of the tracheal system of an insect. The tracheoles are the site of gas exchange.

Ventilation mechanism in insects

• Very active, flying insects need a more rapid supply/intake of oxygen. They create a mass flow of air into the tracheal system by:
  • Closing the spiracles
  • Using abdominal muscles to create a pumping movement for ventilation

• Also, during flight the fluid found at the narrow ends of the tracheoles is drawn into the respiring muscle so gas diffuses across quicker (due to the diffusion distance being shorter)

Gills of Fish

• Oxygen dissolves less readily in water
  • A given volume of air contains 30 times more oxygen than the same volume of water

• Fish are adapted to directly extract oxygen from water
• Structure of fish gills in bony fish:
  • Series of gills on each side of the head
  • Each gill arch is attached to two stacks of filaments
  • On the surface of each filament, there are rows of lamellae
  • The lamellae surface consists of a single layer of flattened cells that cover a vast network of capillaries

• Mechanism:
  • The capillary system within the lamellae ensures that the blood flow is in the opposite direction to the flow of water - it is a counter-current system
  • The counter-current system ensures the concentration gradient is maintained along the whole length of the capillary
  • The water with the lowest oxygen concentration is found adjacent to the most deoxygenated blood

Image showing the structure of fish gills and the counter-current system within gills

Ventilation mechanism in fish

• The ventilation mechanism in fish constantly pushes water over the surface of the gills and ensures they are constantly supplied with water rich in oxygen (maintaining the concentration gradient)
• When the fish open their mouth they lower the floor of the buccal cavity. This causes the volume inside the buccal cavity to increase, which causes a decrease in pressure within the cavity
• The pressure is higher outside the mouth of the fish and so water flows into the buccal cavity
• The fish then raises the floor of the buccal cavity to close its mouth, increasing the pressure within the buccal cavity
• Water flows from the buccal cavity (high pressure) into the gill cavity (low pressure)
• As water enters pressure begins to build up in the gill cavity and causes the operculum (a flap of tissue covering the gills) to be forced open and water to exit the fish
• The operculum is pulled shut when the floor of the buccal cavity is lowered at the start of the next cycle

The pressure differences created by the opening of the mouth causes water to be constantly pushed across the surface of the gills