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SURFACE AREA AND ITS IMPACT ON BIOLOGY

Fish Respiration














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HOW DO FISH RESPIRE?
















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Adult fish have a pair of gills. Each gill is covered by a boney lid (removed from the picture). A fish draws in water by closing the lid over its gills and opening its mouth. When the fish closes its mouth and opens the gill lid the water is forced out and over the respiratory surfaces of the gill filaments.    
















HOW IS OXYGEN AND  CARBON DIOXIDE  EXCHANGED IN THE GILLS?

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In order to live, fish must extract oxygen from the water and transfer it to their bloodstream. This is done by gills which are 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 denser than air, and at 20 deg C it has 50 times more viscosity 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. The oxygenated water flows through the tinny gill filaments and it exchanges the carried oxygen for carbon dioxide through a process called diffusion. 

WHAT IS THE STRUCTURE OF A GILL?

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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.

 
HOW ARE GILLS EFFICIENT?

1)  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) or less thick.
 
 
 
2)  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. 
 
 
 
3)  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.
 
 
 
4)  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.

IN CONCLUSION...

The surface area of the gill filaments is a factor that means death or life to a fish since the water contains much less oxygen than air therefore fish must have an organ with large surface area in order to absorb enough oxygen from the water to survive.