Gills are made efficient in a number of ways.
(1) A large surface area for gaseous exchange means that more oxygen can enter the bloodstream over a given period of time.
A single gill of a bony fish consists of a curved gill arch bearing a V-shaped double row of gill filaments.
Each filament has many minute folds in its surface,
giving it a sort of fuzzy appearance and increasing the amount of surface area along a given length of filament.
Consequently, the surface area of the gills is commonly 10 to 60 times more than that of the whole body surface.
(2) A short diffusion, or travel, distance for the oxygen increases the rate of oxygen entry into the blood.
The blood traveling in the folds of the filaments is very close to the oxygen-containing water,
being separated from it by a very thin membrane usually 1 to 3 microns (4/100,000 to 1/10,000 in) thick,
and possibly less.
(3) By using countercurrent circulation in the gill, the blood in the filament folds travels forward,
in the opposite direction to the water flow,
so that a constant imbalance is maintained between the lower amount of oxygen in the blood and the higher amount in the water,
ensuring passage of oxygen to the blood.
If the blood were to flow in the same direction as the water,
oxygenated blood at the rear of the gills would be traveling with deoxygenated water and not only could not extract oxygen from the water but would even lose oxygen to it.
(4) Gills have little physiological dead space.
The folds of the filament are close enough together so that most of the water passing between them is involved in the gas-exchange process.
(5) Water flows continuously in only one direction over the gills,
as contrasted with the interrupted, two-way flow of air in and out of lungs of mammals.
我喜欢学习，但讨厌读书！
尤其是为了考试而读书的读书！
书上讲的又不清楚！
上网找的还像样些。。。。
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