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Fish use specialised surfaces called gills to carry out gas exchange.
Gills are highly folded, giving them a large surface area and maximising the efficiency of gas exchange. The gill filaments have many protrusions called gill lamellae.
One of the ways in which gas exchange is carried out efficiently is by the countercurrent flow principle. Sounds complicated but it just means that water and blood are flowing in different directions. The water that passes over the gill lamellae flows in the opposite direction to the blood within the gill lamellae.
This system maximises the amount of oxygen diffusing into the blood by having the most oxygenated blood meet the most oxygenated water, and the least oxygenated blood meet the least oxygenated water.
This means that the concentration gradient is maintained the whole way through, allowing the maximum amount of oxygen to diffuse into the blood from the water.
This is important because there isn't much oxygen in the water, and fish need to absorb enough oxygen to survive.
In addition to this, the lamellae have a rich blood supply so that a steep concentration gradient can be maintained between the blood in the lamellae and the water through. Hence, oxygen diffusing into the blood is rapidly removed by the circulating blood supply and more oxygen is able to difuse into the blood.
Another way in which a steep concentration gradient is maintained is by ensuring water flows in one direction only. The fish opens its mouth to let water in, then closes its mouth and forces the water through the gills and out through the operculum (gill cover).
This allows for more efficient gas exchange than if the water had to go in and out the same way. This is important for fish becaus of the low oxygen concentration in water.
Efficient gas exchange in fish is due to:
-large surface area of gills due to gill lamellae
-rich blood supply of lamellae
-water being able to flow in one direction onlysee more
Reproductive separation occurs, this could be through geographical separation. (1)
Therefore no interbreeding in the population. (2)
This results in separate gene pools. (3)
Variation due to random mutation (4)
The separated populations of the species will be exposed to different environmental/abiotic/biotic conditions/ selection pressures (5)
The different environments will therefore favour different mutations, depending on what is required for survival. For example, one environment may be colder, therefore individuals with a thicker fur mutation will survive and be selected for. The other population may live in a warmer environment, where a thinner fur is more suited for survival.
Therefore there will be selection for different/advantageous alleles, depending on the environmental pressures. (6)
Therefore these selected organisms, with favourable alleles, are able to survive and reproduce. (7)
Over a long period of time, this will lead to a change in allele frequency. (8)see more
The alveolar epithelium is very thin, just one cell layer thick. This means the diffustion distance is small, therefore diffusion of gases occurs rapidly.
Remember that diffusion is the net movement of particules from an area of high concentration to one where its concentration is lower.
Diffusion will be faster if there is a greater difference in concentration between the two areas. This is known as the concentration gradient. A steep concentration gradient will increase diffusion rate.
Each alveolus is highly vasculated (many capillaries surround the alveolus). This mantains a steep concentration gradient. Once blood adjacent to the alveolus has been oxygenated, it is rapidly removed by action of the heart, and replaced with deoxygenated blood.
Similarly, the lungs maintain oxygen rich air in the alveoli through the process of ventilation.see more