Insects, being larger and having a hard, chitinous and therefore impermeable exoskeleton, have a more
specialised gas exchange system.
Insects have no transport system so gases need to be transported directly to the respiring tissues.
There are tiny holes called spiracles along the side of the insect.
The spiracles are openings of small tubes running into the insect's body, the larger ones being called
tracheae and the smaller ones being called tracheoles.
The ends of these tubes, which are in contact with individual cells, contain a small amount of fluid in
which the gases are dissolved. The fluid is drawn into the muscle tissue during exercise. This increases the
surface area of air in contact with the cells. Gases diffuse in through the spiracles and down the tracheae
and tracheoles.
Ventilation movements of the body during exercise may help this diffusion.
The spiracles can be closed by valves and may be surrounded by tiny hairs. These help keep humidity
around the opening, ensure there is a lower concentration gradient of water vapour, and so less is lost from
the insect by evaporation.
Fish use gills for gas exchange.
Gills have numerous folds that give them a very large surface area.
The rows of gill filaments have many protrusions called gill lamellae. The folds are kept supported and
moist by the water that is continually pumped through the mouth and over the gills.
Fish also have an efficient transport system within the lamellae which maintains the concentration gradient
across the lamellae.
The arrangement of water flowing past the gills in the opposite direction to the blood
(called countercurrent flow) means that they can extract oxygen at 3 times the rate a human can.
Concurrent flow
To understand countercurrent flow, it is easiest to start by looking at concurrent flow where water and
blood flow over and through the lamellae in the same direction.
When the blood first comes close to the water, the water is fully saturated with oxygen and the blood has
very little.
There is therefore a very large concentration gradient and oxygen diffuses out of the water and into the
blood.
As you move along the lamella, the water is slightly less saturated and blood slightly more but the water
still has more oxygen in it so it diffuses from water to blood.
This continues until the water and the blood have reached equal saturation.
After this the blood can pick up no more oxygen from the water because there is no more concentration
gradient. The maximum saturation of the water is 100% so the maximum saturation of the blood is 50%.
Countercurrent flow
As the blood flows in the opposite direction to the water, it always flows next to water that has given up
less of its oxygen.
This way, the blood is absorbing more and more oxygen as it moves along. Even as the blood reaches the
end of the lamella and is 80% or so saturated with oxygen, it is flowing past water which is at the
beginning of the lamella and is 90 or 100% saturated.
Therefore, even when the blood is highly saturated, having flowed past most of the length of the lamellae,
there is still a concentration gradient and it can continue to absorb oxygen from the water.
The gas exchange surface of a mammal is the alveolus.
There are numerous alveoli - air sacs, supplied with gases via a system of tubes (trachea, splitting into
two bronchi - one for each lung - and numerous bronchioles) connected to the outside by the mouth and
nose.
These alveoli provide a massive surface area through which gases can diffuse. These gases diffuse a very
short distance between the alveolus and the blood because the lining of the lung and the capillary are both
only one cell thick.
The blood supply is extensive, which means that oxygen is carried away to the cells as soon as it has
diffused into the blood. Ventilation movements also maintain the concentration gradients because air is
regularly moving in and out of the lungs.
This breathing in (inspiration) and breathing out (expiration) is controlled via nervous impulses from
the respiratory centre in the medulla of the brain.
Both the intercostal muscles (in between the ribs) and the diaphragm receive impulses from the
respiratory centre. Stretch receptors in the lungs send impulses to the respiratory centre in the brain giving
information about the state of the lungs.
Process of inspiration (breathing in)
1. external intercostal muscles contract
2. ribs and sternum move up and out
3. width of thorax increases front to back and side to side
4. diaphragm contracts
5. diaphragm moves down, flattening
6. depth of thorax increases top to bottom so the...
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volume of thorax increases.
pressure between the pleural surfaces decreases.
lungs expand to fill thoracic cavity.
air pressure in alveoli is less than atmospheric pressure.
air is forced in by the higher external atmospheric pressure.
As the lungs fill with air the stretch receptors send impulses to the expiratory part of the respiration centre
to end breathing in.
Process of expiration (breathing out)
1. External intercostal muscles relax
2. ribs and sternum move down and in
3. width of thorax decreases front to back and side to side
4. diaphragm relaxes
5. diaphragm moves up
6. depth of thorax decreases top to bottom. So the ...
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volume of thorax decreases.
pressure between the pleural surfaces increases.
lung tissue recoils from sides of thoracic cavity
air pressure in alveoli is more than atmospheric pressure.
air is forced out.
As the air leaves, the stretch receptors are no longer stimulated. The inhibition of breathing in (via the
expiratory part of the centre) stops so breathing in can start again.
Chemoreceptors
There are also chemoreceptors in the medulla and certain blood vessels that are sensitive to changes in
carbon dioxide levels in the blood.
If the level is too high (the pH would drop, enzyme action would be affected with serious results),
impulses are sent from these cells to the inspiratory part of the centre so that breathing rate increases.
This means that carbon dioxide is got out of the body as quickly as possible and more oxygen comes in.