Gaseous Exchange

Gaseous exchange in animals
The majority of animals need oxygen in order to oxidize the organic materials and produce energy for cellular activities.
The oxidation of the food not only yields energy but also carbon dioxide which must be constantly removed from the body.
The process of moving oxygen into the body and carbon dioxide out of the body is called breathing in or ventilation. Gaseous exchange involves the passage of carbon dioxide through a respiratory surface. Diffusion is the main transport process involved in gaseous exchange.
Characteristics of the respiratory surfaces
1. They have a large surface area in order to increase the rate of diffusion
2. They are usually thin and permeable in order to reduce the resistance to diffusion
3. They are moist to dissolve the gases
4. They are well supplied with blood.
Types of respiratory surfaces in animals
Small animals such as amoeba use their entire body surface for gaseous exchange. They have a high surface area /volume ratio. As organisms increase in size, the surface area/volume ratio decreases, hence there is need to have special respiratory system or organs.
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.
insect gas exchange

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


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
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
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.

Gaseous exchange in mammals e.g. man
The breathing system of a mammal consists of a pair of lungs which are thin walled elastic sacs lying in the thoracic cavity. The walls of the thorax consists of the ribs and the intercostal muscles while the floor consists of the diaphragm, a muscular flap of tissue between the thorax and the abdomen
main parts of the breathing system in man


Air enters the lungs through the trachea which is devided into two brochi, one to each lung. The trachea and bronchi have walls made up of rings of cartilage. Inside the lungs , each bronchus is divided into smaller tubes called bronchioles. The bronchioles terminate in saclike atria giving rise to numerous air sacs or alveoli. Each alveolus is a thin walled sac covered by numerous blood capillaries
Exchange of air between the lungs and the outside is made possible by changes in the volume of the thoracic cavity. This volume is altered by the movements of the intercostal muscles and the diaphragm.

The following events happen during inspiration
• The diaphragm contracts and moves downwards
• The ribs are raised upwards and outxwards by the contraction of the external intercostals muscles
• The volume of the thoracic cavity increases, thus reducing the pressure. Air then rushes into the lungs from outside through the nostrils.
• The diaphragm relaxes and is pushed upwards by the abdominal organs. It thus assumes a dome shape
• The internal intercostals muscles contract and the ribs move downwards and inwards
• The volume of the thoracic cavity decreases, thus increasing the pressure. Air is then forced to out of the lungs

Gaseous exchange between the alveoli and the capillaries
 The walls of the alveoli and the capillaries are very thin and closely attached to each other. This makes diffusion of gases very efficient because the distance between the inside of the capillary and the inside of the alveolus is very small.
 Furthermore, the lungs have over 700 million alveoli offering a large surface area for gaseous exchange
 The walls of the alveoli are also moist, this makes oxygen dissolve easily
Blood from the tissues has a high concentration of carbondioxide and very little oxygen compared to alveolar air. The concentration gradient favours diffusion of carbondioxide into the alveolus and oxygen into the blood plasma in the capillaries. The oxygen is then picked by the haemoglobin of red blood cells and transported in combination with it as oxyhaemoglobin.
Carbondioxide which is at a higher concentration in the blood is normally carried as bicarbonate ions in the plasma. This breaks down and releases carbondioxide which then diffuses into the alveolus.



Percentage composition of inspired and expired air (% by volume)
Component Inspired air Expired air
Oxygen 21 16
Carbon dioxide 0.04 4
Nitrogen 79 79
Moisture Variable saturated

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