Gas Exchange in Fish & Insects (OCR A Level Biology): Revision Note

Gas exchange in fish & insects

The tracheal system in insects

• All insects possess a rigid exoskeleton with a waxy coating that is impermeable to gases

• Spiracles are openings in the exoskeleton of an insect which allow air to flow into the internal system of tubes known as the tracheal system

• Tracheae (singular trachea) are tubes within the insect respiratory system which lead to narrower tubes known as tracheoles

  • Rigid rings of chitin keep the tracheae open

• Many tracheoles carry oxygen into the muscle fibres of the insect, where gas exchange takes place

  • The ends of the tracheoles are filled with tracheal fluid; gases can dissolve in this fluid before diffusing to the cells for gas exchange

  • The large number of tracheoles that are in contact with the muscle cells provides a large surface area for gas exchange

Ventilation mechanism in insects

• When insects are at rest their energy requirements are low and diffusion alone is fast enough to supply oxygen to the cells

  • In this state insects may close their spiracles to reduce water loss by evaporation

• Active insects need a rapid supply of oxygen; this can be achieved as follows:

  • contracting and relaxing the muscles of the thorax and abdomen alters the volume, and therefore pressure, inside the tracheae, drawing air in and out

  • during flight the tracheal fluid at the narrow ends of the tracheoles is drawn into the respiring muscle; removing fluid from the tracheoles reduces the diffusion distance between the air and the muscle cells, speeding up diffusion

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