Ventilation in fish and insects

In a nutshell

Ventilation is crucial for the survival of fish and insects. Both of these types of organisms have different gas exchange and ventilation methods. This summary details the counter-current system and the use of tracheae in gas exchange.

Gas exchange in fish

The method of gas exchange in fish is known as the counter-current system. Fish have various adaptations to facilitate this exchange.

Biology; Exchange and transport; KS5 Year 12; Ventilation in fish and insects

1.	Water enters the fish through its mouth.
2.	The water passes back out of the fish through its gills.
3.	Each gill has branches known as gill filaments (or primary lamellae). These increase the surface area for gas exchange.
4.	The gill filaments are covered with gill plates (or secondary lamellae). These provide a further increase in the surface area for gas exchange.
5.	The gill is then supported by a gill arch.
6.	Gill plates are formed of thin cells and they have a good capillary network. This increases the rate of diffusion.
7.	Fish have a counter-current system. This means that the water will flow in one direction and blood will flow in the other direction. The benefit of this is that it increases the concentration gradient. This is because the concentration of oxygen is higher than in the blood.

Ventilation in fish

Bony fish, including salmon, are ventilated. This is described below.

Biology; Exchange and transport; KS5 Year 12; Ventilation in fish and insects

The space inside the fish mouth is called the buccal cavity. When the fish opens its mouth (1.), the floor of the buccal cavity will be lowered (2.). This increases the volume and decreases the pressure of the buccal cavity. As a result, water will be drawn in.

The fish will close its mouth and the floor of the buccal cavity will be raised (3.). This decreases the volume and increases the pressure in the buccal cavity. As a result, water will be pushed out over the gill filaments for gas exchange (4.). Each gill has a protective bony cover called the operculum (5.). The pressure increase that occurs in the buccal cavity will cause the operculum to open and water will leave the gills (6.)

Dissecting fish gills

Fish gills can be dissected to provide further insight into how they are ventilated.

1.	Move the operculum and remove the gills.
2.	Cut each gill arch through the bone at the top and bottom. You should be able to see the gill filaments.
3.	Draw and label the gill.

Note: This is a messy process so wear an apron, lab coat, gloves and goggles.

Gas exchange in insects

Unlike fish, insects use tracheae for gas exchange.

Note: Tracheae is the plural of trachea.

Biology; Exchange and transport; KS5 Year 12; Ventilation in fish and insects — 1. Thorax, 2. Abdomen, 3. Tracheae and tracheoles, 4. Spiracle, 5. Trachea, 6. Carbon dioxide diffusing into the tracheoles, 7. Tracheole is close to the cells to reduce the diffusion distance, 8. Oxygen diffuses into the respiring cells.

A.	Spiracles are pores on the insect's body and air will enter the tracheae through spiracles.
B.	Oxygen will travel down its concentration gradient towards the cells.
C.	Carbon dioxide travels down its concentration gradient towards the spiracles so it can be released from the insect and into the air.
D.	Tracheae branch into tracheoles which increases the surface area for gas exchange. These have thin and permeable walls and contain fluid for oxygen to dissolve into.
E.	Oxygen will diffuse out of the fluid and into the insect's body cells.
F.	Insects use rhythmic abdominal movements to change their body volume and move air in and out of the spiracles.

Dissecting the tracheae in insects

The tracheae can also be viewed in insects by following the method below.

1.	Pin an insect to a dissecting board.
2.	Remove a piece of the exoskeleton of the insect from the abdomen.
3.	Fill the abdomen with saline solution through a syringe.
4.	Silver tubes should be visible. These are the tracheae. They should appear silver as they are filled with air.
5.	Use a wet mount slide to view the tracheae under an optical microscope. You should be able to see rings of chitin in the tracheal walls that support the tracheae.

Note: It is best to use bigger insects like cockroaches as these are easier to work on.