Gaeseous exchange

Alveoli- Tiny folds of the lung epithilium to increase the surface area.

Bronchi + Bronchiloles - Smaller airways leading to the lungs.

Diaphragm- A layer of muscle beneath the lungs

Trachea- The main airway leading from the back of the mouth to the lungs

Ventilation- The refreshing of air in the lungs, so that there is a higher oxygen concentration than in the blood, and a lower carbon dioxide concentration.

• The gaseous exchange system in mammals consists of lungs and associated airways that carry air into and out of the lungs.

• The lungs are a pair of inflatable sacs lying in the chest cavity.

• Air can pass into the lungs through the nose and along the trachea (wind pipe), bronchi and bronchioles.

• Finally, it reaches tiny air-filled sacs called alveoli. These are the surfaces where the exchange of gases takes place.

• The lungs are protected by the ribcage. The ribs are held together by intercostal muscles.

• The action of these muscles and the diaphragm helps to produce breathing movements (ventilation)

• Gas passes through the thin walls of the alveoli.

• Oxygen passes from the air in the alveoli to the blood in the capillaries.

• CO2 passes from the blood to the air in the alveoli.

• The lungs must maintain a steep concentration gradient in each direction in order to ensure that the diffusion can continue.

• The diaphragm contracts to move down and become flatter - this displaces the digestive organs downwards.

• The external intercostal muscles contract to raise the ribs.

• The volume in the chest cavity is increased.

• The pressure in the chest cavity drops below the atmospheric pressure.

• Air is moved into the lungs.

• The diaphragm relaxes and it is pushed up by the displaced organs underneath.

• The external intercostal muscles relax and the ribs fall; the internal intercostal muscles can contract to help puch air out more forcefully - this usually only happens during exercise or coughing or sneezing.

• The volume of the chest cavity is decreased.

• The pressure in the lungs increases and rises above pressure in the surrounding atmosphere.

• Air is moved out of the lungs.

• The lung consists of large numbers of tiny air-filled sacs called alveoli.

• These are comprised of squamous epithelium and are surrounded by blood capillaries, so that the distance they diffuse is very short.

• The alveolus walls contain elastic fibres that stretch during inspiration but then recoil to help push air out during expiration.

To be effective airways must:

• Be large enough to allow sufficient air to flow without obstruction.
• Be supported to prevent collapse when the air pressure inside is low during inspiration.
• Be flexible in order to allow movement.

• The airways are lined by cilliated epithelium, which contributes to keeping the lungs healthy.

• Goblet cells in the epithelium release mucus, which traps pathogens.

• The cilia then move the mucus to the top of the airway, where it is swallowed.

• The glandular tissue in the loose tissue also produces mucus.

• The trachea and bronchus walls have a similar structure. However, the brochi are narrower than the trachea.

• These airways are supported by rings of cartilage which prevent collapse during inspiration.

• The rings of cartilage in the trachea are C-shaped which allows flexibility and space for food to pass down the oesophagus.

• The bronchioles are much narrower than the bronci.

• The larger bronchioles may have some cartilage, but smaller ones have no cartilage.

• The wall is comprised mostly of smooth muscle and elastic fibres.

• The smallest bronchioles end in clusters of alveoli.

• The smooth muscle can contract.

• The action of the smooth muscle will constrict the airway, making the airway narrower.

• Constriction of the lumen can restrict flow of air to and from the alveoli.

• This is important if there are harmful substances in the air.

• The contraction of the smooth muscle and airflow is not a voluntary act and may occur as a result from an allergic reaction.

• Once the smooth muscle has contracted, it cannot be reversed on its own.

• The smooth muscle is elongated again by the elastic fibres.

• When the smooth muscle contracts it deforms the elastic fibres; relaxing the smooth muscle returns it to its original shape.

• This acts to dialate the airway.

• Lung volumes can be measured using a spirometer.

• This is a device that measures the movement of air in and out of the lungs as a person breathes.

• A float-chamber spirometer consists of a chamber of air or medical grade oxygen floating on a tank of water.

• During inspiration, air is drawn from the chamber so that the lid moves down.

• During expiration, the air returns to the chamber raising the lid.

• These movments may be recorded on a datalogger.

• The subject should be healthy and not have asthema.

• The soda lime should be fresh and functioning.

• There should be no air leaks in the apparatus, as this would give invalid or inaccurate results.

• The mouthpiece should be sterilised.

• The water chamber must not be overfilled.

Vital capacity

• The maximum volume of air that can be moved by the lungs in one breath.
• This is measured by taking a deep breath and expiring all the air possible from the lungs.
• It is affected by the size, age, gender and exercise levels of the person.

Residual Volume

• The volume of air that remains in the lungs even after forced expiration.

Tidal Volume

• The volume of air inhaled or exhaled in one breath, usually at rest.

• Bony fish must exchange gases with the water in which they live.

• They can use gills in order to absorb oxygen dissolved in water and release carbon dioxide into the water.

• The oxygen concentration will typically be lower than is found in air.

• Most bony fish have 5 pairs of gills which are covered by a bony plate called the operculum.

• Each gill consists of two gill filaments (primary lamallae) attached to a bony arch.

• The filaments are very thin, and their surface is folded into many secondary lamalae, providing a very large surface area.

• Blood capillaries carry deoxygenated blood close to the surface of the secondary lamallae where exchange takes place.

• Blood flows along the gill arch and out along the gill filaments to the secondary lamellae.

• The blood then flows through the capillaries in the opposite direction to the flow of water over the lamallae.

• This arrangement creates a countercurrent flow that absorbs the maximum amount of oxygen from the water.

• Bony fish can keep water flowing over the gills by using a buccal-opercular pump.

• The bucal cavity (mouth) can change volume.

• The floor of the mouth moves downward, drawing water into the buccal cavity.

• The mouth closes and the floor is raised again pushing water through the gills.

• As water is pushed from the buccal cavity, the operculum moves outwards.

• This movement reduces the pressure in the opercular cavity (the space under the operculum), helping water to flow through the gills.

• Insects don't transport oxygen in blood.

• Insects have an open circulatory system in which the body fluid acts as both blood and tissue fluid.

• Circulation is slow and can be affected by body movements.

• Insects possess an air-filled tracheal system which supplies air directly to all the respiring tissues.

• Air enters the system via a pore in each segment, called a spiracle.

• The air is transported through a series of tubes called tracheae. These divide into smaller and smaller tubes, called tracheoles.

• The ends of the tracheoles are open and filled with tracheal fluid.

• Gaseous exchange occurs between the air in the tracheole and the tracheal fluid.

• Some exchange can also occur across the thin walls of the tracheoles.

• Many insects are very active and need a good supply of oxygen.

• When insects are active, the tracheal fluid can be withdrawn into the body fluid in order to increase the surface area of the tracheole wall exposed to air.

• This means that more oxygen can be absorbed when the insect is active. 

Larger insects can also ventilate their tracheal system by movements of the body.

This can be acheived in a number of ways:

• In many insects, sections of the tracheal system are expanded and have flexible walls. These act as air sacs which can be squeezed by the action of the flight muscles.Repetitive expansion and contraction of these sacs ventilate the tracheal system.

• In some insects, movements of the wings alter the volume of the thorax. As the thorax volume decreases, air in the tracheal system is put under pressure and is pushed out of the tracheal system. When the thorax increases in volume, the pressure inside drops and air is pushed into the tracheal system from outside.

• Some insects have developed this ventilation even further.Locusts can alter the volume of their abdoman by specialised breathing movements. These are coordinated with opening and closing valves in the spiracles. As the abdomen expands, spiracles at the front end of the body open and air enters the tracheal system. As the abdomen reduces in volume, the spiracles at the rear end of the body open and air can leave the tracheal system.

Buccal cavity - The mouth.

Countercurrent flow- Where two fluids flow in opposite directions

Filaments- Slender branches of tissue that make up the gill. They are often called primary lamallae.

Secondary lamallae-  Folds of filament to increase surface area. They are also called gill plates.

Operculum- A bony flap that covers and protects the gills.

Spiracle- An external opening or pore that allows air in or out of the tracheae.

Tracheal fluid- The fluid found at the ends of tracheoles in the tracheal system.

Tracheal system- A system of air-filled tubes.