a site for study..who'd a thunk it?

Dearest Bloggie

you will be pleased to know that i found you infinitely and suprisingly useful in my biology studies a few posts ago. Therefore, bear with me, or better yet, ignore me, dear reader; for no doubt the following will be long and dreary, and of no particular use to the non-biologist.

GAS EXCHANGE IN AQUATIC ANIMALS

Gas exchange in all animals is essential for respiration; to supply cells with oxygen and remove waste carbon dioxide. Small or thin organisms with a large SA:V ratio are able to obtain oxygen through simple diffusion. Often, extensions such as external gills aid diffusion by increasing SA:V ratio, thus increasing the site of gas exchange. These gills are fine and thread-like networks, which fan out and are supported by the waters buoyancy. Because the concentration of oxygen is relativley lower than in air, an increased surface area is very useful to water-dwelling creatures. However, external gills have a crucial drawback in that they are highly susceptible to damage. Should they be destroyed, then the organisms access to oxygen will be limited, or obliterated entirely. Without this vital gas, cell respiration cannot occur; and therefore it will ultimately die, unless it has other means of obtaining oxygen.
Aquatic creatures have adapted to cope with this difficulty through internal gills, commonly known as the site of gas exchange in fish. Gills are highly vascularised- ie. they have a rich supply of blood capilleries. They consist of several pairs of gill arches on either side of teh opercular cavity. Multiple gill filaments extend from the arches, held by a solid base. Gill lamellae on each filament increase surface area, increasing the amount of O2 than can be diffused. Water is pushed over the gills between filaments. Blood in the vessels on the filaments flows in the opposite direction to the water, thus creating a counter-current mechanism. This means that the deoxygenated blood from the body is constantly being met with higher O2 concentrations from the water. Therefore a high and constant concentration gradient is maintained.
As i have previously stated, there O2 is not highly soluble in water, necessitating measures to increase O2 uptake. Gill-bearers have combated this problem by making the gill lamellae of adjacent gill arches overlap at their tips. This slows the flow of water, maximising the amount of O2 diffused to up to 96%.
As fish gills are protected, they require some sort of mechanism to draw water over their surface. They do this through alternating pressure in separate body cavities.
1. For inspiration, the floor of the buccal cavity is lowered, and water rushes in the the mouth
2. The mouth is closed, and the buccal floor cavity is raised, causing the water to flow into the region of low pressure in the opercular cavity. The opercular flaps remain closed, and the water passes over the gills, where dissolved oxygens diffuse into the lamellae.
3. The opercular valve compresses inwards, creating an opening, and increasing pressure. Water is expelled. The water does not flow back into the buccal cavity, as it retains a high water pressure than the outside water pressure.
THUS the water is pushed past the gills by a double-pumping mechanism.
As this mechanism is regulated through a difference in pressures, no energy is expended.

Other ways by which aquatic animals obtain oxygen that i failed to mention before...
1. sponges have flagella which beat water-currents
2. worms use rhythmic wavings of the body to increase water circulation (HAHHAHA! DISCO WORM)
3. echinoderms have outgrowth on their body to increase SA
4. crustaceans have special appendages which beat to pull water over gills.

eh. that's enough for now. sorry to be a pain. hungry. eat sushi now.

but i shall never deprive you of your...
QUOTES OF THE DAY: Oscar Wilde quotes

I never travel without my diary. One should always have something sensational to read on the train.

Work is the curse of the drinking classes.

This wallpaper will be the death of me; one of us will have to go.