RESPIRATION
In order to live, fish must extract oxygen from the water and transfer it to their
bloodstream. This is done by gills, lungs, specialized chambers, or skin, any of which
must be richly supplied with blood vessels in order to act as a respiratory organ.
Extracting oxygen from water is more difficult and requires a greater expenditure of
energy than does extracting oxygen from air. Water is a thousand times more dense (heavier
per unit volume) than air, and at 20 deg C (68 deg F) it has 50 times more viscosity
(resistance to flow) than air and contains only 3% as much oxygen as an equal volume of
air. Fishes, therefore, have necessarily evolved very efficient systems for extracting
oxygen from water; some fishes are able to extract as much as 80% of the oxygen contained
in the water passing over the gills, whereas humans can extract only about 25% of the
oxygen from the air taken into the lungs.
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.
Distribution - Anatomy - Circulation -Air Breathing
Body Temperature -
Water
Balance -
Swimming -
Gas Bladder
Lateral Line System -
Evolution
- Reproduction