Why fishes can't breathe in air...

As a child I always used to wonder how do the aquatic organisms survive underwater,do they hold there breath?How could they hold it for so long??As an answer of which I was taught in school that they have special respiratory organs named 'Gills' for breathing which helps them survive under water by taking in 'oxygen' and giving out 'carbon-dioxide' as we all know. Gills work on the same principle as lungs. In the lungs, there are small sacs called alveoli that are approximately 70% capillaries. These capillaries carry de-oxygenated blood from the body. As oxygen and carbon dioxide pass across the alveoli’s membrane,the capillaries take the newly oxygenated blood back to the body.Similarly,gills have small rows and columns of specialized cells grouped together called the epithelium. Deoxygenated blood in the fish is supplied directly from the heart to the epithelium via arteries,and even yet smaller arterioles. As sea water is forced across the epithelium membranes,dissolved oxygen in the sea water is taken up by tiny blood vessels and veins, while the carbon dioxide is exchanged.

Now the question arises if the fishes could breathe in oxygen dissolved in water,which is a difficult task why can't they breath the oxygen present out in the open air??There could be possibly two theories for this-

1. Evolutionary aspects state that fishes are more primitive to humans,they evolved earlier than us and hence developed body features which help them survive in water and not on land. Streamlined body,fins,gills and many more anatomical and morphological features have helped them survive under water.The fins due to which fishes turn out to be expert swimmers are of no use when brought on land.

2. The gills are designed in manner to breathe under water. It is divided into several arches and the whole surface area of gills is covered by minute blood vessels which absorb oxygen from the water rushing through the arches. Moreover a slimy substance is simultaneously secreted to lubricate the gills,which is constantly washed off by the out going water. If the fish is brought on land,due to lack of water the gill arches collapse and there's no source of oxygen to be absorbed by the blood vessels and this ultimately causes the fish to die.

Hope this article helps you resolve queries regarding fish and it's breathing pattern.