gaseous exchange in bony fish,frog and insects

Gaseous exchange in insects
The respiratory system consists of a network of tubes forming the tracheal system. The tubes open to the outside through pores called spiracles located on the sides of the thorax and the abdomen. The tubes called the trachea are lined with cuticle and have spiral rings which prevent the walls from collapsing inwards.
The trachea is divided into smaller tubes called tracheoles which are closely associated with the tissues. Some insects have air sacs connected to the trachea. These air sacs can be inflated or deflated in order to facilitate gaseous exchange
Ventilation is brought about by the contraction and relaxation of the abdominal muscles. In locusts, air is drawn into the body through the thoracic spiracles and expelled through the abdominal spiracles.
Diagram

Gaseous exchange in amphibians e.g. a frog
Amphibians live in two environments air and water and are therefore adapted to gaseous exchange in land and in water. They also show change of respiratory surfaces and organs as they develop from gills in tadpoles to lungs, skin and mouth in adults.
A tadpole lives in water all the time and carries out gaseous exchange with water by means of gills (external gills in young tadpoles and internal gills in older tadpoles). Gaseous exchange occurs at the gill filaments.
When the frog develops into an adult it begins to exchange gases with air and it uses three different respiratory surfaces. These are
1. Skin this is thin, making oxygen diffuse easily into the blood and carbondioxide out, moist by secretions from the mucous glands in it therefore oxygen can dissolve easily. It is also well supplied with blood
2. Lining of the mouth cavity- this is also moist and well supplied with blood
3. Lungs- these are thin walled, with internal folding
Diagram

The lining of the mouth cavity and the lungs are used for Gaseous exchange when the frog is out of the water. Lungs are not very efficient since some of the oxygen in the air reaching them has already been taken up by the lining of the mouth cavity. When a frog is in water , it relies almost entirely on the skin for Gaseous exchange
Ventilation
The floor of the mouth is lowered and air is drawn in through the nostrils. When the nostrilsare closed and the floor of the mouth is raised, air is forced into the lungs (inspiration). When the floor of the mouth is lowered again, the pressure of the abdominal contents forces air out of the lungs through the nostrils (expiration)
Gaseous exchange in bony fish (e.g. tilapia)
Gaseous exchange in fish takes place between the gills and the surrounding water. The gills are located in the opercular cavity covered by a flap of skin called the operculum. Each gill consists of a number of thin leaf like lamellae projecting from a skeletal base (brachial arch) situated in the wall of the pharynx.
Each gill is supported by a gill bar through which blood vessels send branches to the filaments.
Diagram of the gill
Functions of parts of the gill
1. Gill rakers. These filter large particles in the water before they reach and damage the gill filaments
2. Gill bar. These provide attachment and support for the gill filaments
3. Gill filaments. These are the sites of gas exchange
Ventilation
As the mouth opens, the floor of the mouth is lowered. Pressure inside the mouth is lowered and this causes water to be drawn into the bucal cavity. Meanwhile the operculum is closed, preventing water from entering or leaving through the opening.
As the mouth closes and the floor of the mouth is raised, pressure in the bucal cavity increases. Water is forced over the gills as the opercula are forced to open. As water passes over the gills, oxygen is absorbed and carbondioxide from the gills dissolves in the water.