15 February 2011

Fish out of water

Why does a fish die if it is removed from the water too long? I presume the fish suffocates even though it is surrounded by air that has more oxygen in it than the water in which the fish lives.

I can think of 3 reasons why a fish outside of water suffocates:

(1) When a fish is in water, either the fish or the water moves constantly and so there is always fresh water passing thru the gills. But a fish out of water cannot move and the passive air flow across the fish's gills, unless it is windy, may not be fast enough to maintain a steady and high enough rate of gas exchange across the gills. If the air layer against the gills is stationary, then it will quickly be depleted of oxygen, while becoming saturated with carbon dioxide.

(2) Gills exposed to air, especially when it is windy, will eventually dry and the gas exchange across them will stop; gasses can't seem to diffuse across dry biological membranes. Because fish originated in the sea, they never needed and therefore never evolved a slime layer covering their gills, which, if it were present, would keep the gills wet until, of course, the fish ran out of water.

(3) The gill filaments and their lamellae float in water and therefore remain separated from each other. But if they are exposed to air, they collapse and stick to each other. This decreases the surface area available for gas exchange significantly.

For all these reasons, the majority of terrestrial animals don't have gills and the membrane surfaces they use for gas exchange are continuously kept moist either in a high humidity chamber inside their bodies or by copious slime production on the external surfaces of their bodies.

1 comment:

Paul said...

My Physiology & Anatomy professor taught us that it was #3, although I'm sure the other two play some role as well.