Exchange Flashcards
What are the features of specialised exchange surfaces?
A large surface area relative to the volume of the organism which increases the rate of exchange.
Very thin so that the diffusion distance is short and therefore materials cross the exchange surface rapidly.
Selectively permeable to allow selected materials to cross.
Movement of the environment medium, for example, air, to maintain a diffusion gradient.
A transport system to ensure the movement of the internal medium, for example, blood, in order to maintain a diffusion gradient.
What is the relationship between diffusion?
Diffusion ∝ surface area x difference in concentration / length of diffusion pathway.
Why are exchange surfaces within the organism?
Being thin, specialised exchange surfaces are easily damaged and dehydrated.
They are therefore often located inside an organism.
Where an exchange surface is located inside the body, the organism needs to have a means of moving the external medium over the surface, e.g. a means of ventilating the lungs in a mammal.
Why do fish have specialised exchange surfaces?
Fish have a waterproof, and therefore a gas-tight, outer covering.
Being relatively large they also have a small surface area to volume ratio.
Their body surface is therefore not adequate to supply and remove their respiratory gases and so have evolved - the gills.
What is the structure of the gills?
The gills are located within the body of the fish, behind the head.
They are made up of gill filaments, stacked in a pile.
At right angles to the filaments are gill lamellae, which increase the surface area of the gills.
Water is taken through the mouth and forced over the gills and out through the opening on each side of the body.
What is the counter current flow?
The flow of water over the gill lamellae and the flow of blood within them are opposite directions.
It is important that the maximum possible gas exchange is achieved.
If the water and blood flowed in the same direction, far less gas exchange would take place.
What is the counter current exchange principle?
The blood and water flow over the gill lamellae in opposite directions.
This arrangement means that:
Blood that is already well loaded with oxygen meets water, which has its maximum concentration of oxygen. Therefore diffusion of oxygen from the water to the blood takes place.
Blood with little oxygen in it meets water which has had most, but not all, its oxygen removed. Again, diffusion of oxygen from the water to blood takes place.
Why is the counter current exchange needed in fish?
A diffusion gradient for oxygen uptake is maintained across the entire width of the gill lamellae.
In this way, about 80% of the oxygen available in the water is absorbed into the blood of the fish.
If the flow of water and blood had been in the same direction (parallel flow), the diffusion gradient would only be maintained across part of the length of the gill lamellae and only 50% of the available oxygen would be absorbed by the blood.
What is ventilation?
To maintain diffusion of gases across the alveolar epithelium, air is constantly moved in and out of the lungs - breathing/ventilation.
What is inspiration?
When the air pressure of the atmosphere is greater than the air pressure inside the lungs, air is forced into the lungs.
What is expiration?
When the air pressure in the lungs is greater than that of the atmosphere, air is forced out of the lungs.
How do the pressure changes in the lungs occur?
By movement of three sets of muscles:
The diaphragm, which is a sheet of muscle that separates the thorax from the abdomen.
The intercostal muscles, which lie between the ribs, two sets:
The internal intercostal muscles, whose contraction leads to expiration.
The external intercostal muscles, whose contraction leads to inspiration.
What is the process of inspiration?
The external intercostal muscles contract, while the internal intercostal muscles relax.
The ribs are pulled upwards and outwards, increasing the volume of the thorax.
The diaphragm muscles contract, causing it to flatten, which also increases the volume of the thorax.
The increased volume of the thorax results in the reduction of pressure in the lungs.
Atmospheric pressure is now greater than pulmonary pressure, so air is forced into the lungs.
What is the process of expiration?
The internal intercostal muscles contract, while the external intercostal muscles relax.
The ribs are pulled downwards and inwards, decreasing the volume of the thorax.
The diaphragm muscles relax and so it is pushed up again by the contents of the abdomen that were compressed during inspiration. The volume of the thorax is further decreased.
The decreased volume of the thorax increases pressure in the lungs.
Pulmonary pressure is now greater than that of the atmosphere, so air is forced out of the lungs.
How does exercise affect breathing?
During normal quiet breathing, the recoil of elastic tissue in the lungs is the main cause of air being forced out.
Only under more strenuous conditions such as exercise do the various muscles play a major part.
How does gas exchange take place in single-celled organisms?
They are small and therefore have a large surface area : volume ratio.
Oxygen is absorbed by diffusion across their body surface, which is covered only by a cell-surface membrane.
Carbon dioxide from respiration diffuses out across their body surface similarly.
Where a living cell is surrounded by a cell wall, this is no additional barrier to the diffusion of gases.
How do respiratory gases move in and out of the tracheal system?
Along a diffusion gradient.
Mass transport
The ends of the tracheoles are filled with water.
How do gases move along diffusion gradient?
When cells are respiring, oxygen is used up and so its concentration towards the ends of the tracheoles falls.
This creates a diffusion gradient that causes gaseous oxygen to diffuse from the atmosphere along the tracheae and tracheoles to the cells.
Carbon dioxide is produced by cells during respiration.
This creates a diffusion gradient in the opposite direction.
This causes gaseous carbon dioxide to diffuse along the tracheoles and trachea from the cells to the atmosphere.
As diffusion in air is much more rapid than in water, respiratory gases are exchanged quicker by this method.
How do gases move by mass transport?
The contraction of muscles in insects can squeeze the trachea enabling mass movements of air in and out.
This further speeds up the exchange of respiratory gases.
How do gases move - the ends of the tracheoles are filled with water?
During periods of major activity, the muscle cells around the tracheoles respire and carry out some anaerobic respiration.
This produces lactate, which is soluble and lowers the water potential of the muscle cells.
Water therefore moves into the cells from the tracheoles by osmosis.
The water in the ends of the tracheoles decreases in volume and in doing so draws air further into them.
This means the final diffusion pathway is in a gas rather than a liquid phase, and therefore diffusion is more rapid.
This increases the rate at which air is moved in the tracheoles but leads to greater water evaportation.
What is the structure of the gas exchange system in insects?
For gas exchange, insects have evolved an internal network of tubes called trachea.
The tracheae are supported by strengthened rings to prevent them from collapsing.
The tracheae divide into smaller dead-end tubes called tracheoles, that extend throughout all the body tissues of the insect.
In this way, atmospheric air, with the oxygen it contains, is brought directly to the respiring tissues, as there is a short diffusion pathway from a tracheole to any body cell.
