by Ben Davidson
Here I’m going to present two mechanisms by which fish obtain oxygen from the environment and get it to the rest of their body. The first is the function of the gill, and I will use the Pacific Salmon as the example species. The second is the adapted swim bladders found in the African lungfish.
Salmon have two sets, of four gills, on either side of their head. Water flows in through their mouth and is pushed out of their gills. The gills are composed mostly of chloride cells (involved in ion exchange) and densely packed capillaries for gas exchange. The water is pumped opposite to the blood flow, creating a counter-current exchange of oxygen from the water to the blood.
Deoxygenated blood is pumped from the heart to the gills for the gas exchange, and the oxygenated blood is pushed from the gills to the rest of the body, without returning to the heart. This simple design for distributing oxygen across the body allows for a two-chambered heart.
The lungfish has basic lungs and must swim to the water’s surface to breath. They have evolved two specialized swim bladders that are surrounded by capillaries, which allows for oxygen to diffuse directly into their blood stream. The capillary system allows them to use the same two-chamber heart as other fish, but with an added bonus; their lungs allow them to survive should their habitat dry up. When their habitat dries up they form a cocoon of mucus to cover the upper part of their body, which allows them to survive out of water for more than 20 weeks!
Bruton, M. N. 1998. The lung fish. In Encyclopedia of Fishes (ed. J. R. Paxton and W. N. Eschmeyer), pp 70-72. San Diego: Academic Press.
Romer, A. S., and T. S. Parsons. 1977. In The vertebrate body, pp. 316-327. Philidelphia, PA: Holt-Saunders International.
Smith, L. S. and G. R. Bell. 1975. The practical guide to the anatomy and physiology of pacific salmon. Fisheries and Marine Service Special Publication 1-13.
Wilkie, M.P. et al. 2007. The African lungfish (Protopterus dolloi): ionoregulation and osmoregulation in a fish out of water. Physiol Biochemical Zoology 80(1): 99-112.