Gill Structure Enabling the Countercurrent System

The fishes gills are used to extract oxygen from the water and in return excrete carbon dioxide and toxic metabolic wastes, like ammonia and acid. The gills are located behind the head and consist of arches and rows of filaments, which transport densely packed flat lamellae in rows (1). Oxygen is extracted from the water by moving it in the opposite direction to the blood via the lamellae.



Figure 1: Gill structure and the
countercurrent flow (3).

  »Fish employ a method known as the countercurrent system to extract oxygen from the water.

»This system moves water flowing across the gills, in an opposite direction to the blood flow creating the maximum efficiency of gas exchange.

»When he blood and water flows in the same direction, the co-current system, it will initially diffuses large amounts of oxygen but the efficiency reduces when the fluids start to reach equilibrium (2).

»The concentration of oxygen gained from this system would not meet the physiological needs of the fish; therefore the countercurrent system is used.
»This method removes almost all of the oxygen (80-90%) from the water that passes over the gills and then transfers it to the blood, compared to the co-current that is approximately 50% (4).

»When the blood has circulated to the gill the oxygen content is very low and still decreasing. Although dissolved oxygen levels are dropping as the water meets the gills, the blood has lower levels; therefore a sustained diffusion gradient is the result (3).

»The lamellae are very important in this process, as this is the only place where countercurrent occurs, they can be found protruding from the gill filaments dorsal and ventral surfaces (5).

»They comprise of two epithelia separated by pillar cells, which the blood can run through (5).

»The lamellae contain the flow of blood that diffuses the oxygen from the water and these have been measured at between 1-5 microns in size (2). The total number increases with the body size of the fish; active fish of around 1kg may have up to five million (5).

»The filaments are held by a gill bar, which is designed to force the water through the filaments into the lamellae, instead of around the outside (figure 1). An interlocking of filaments from different gill bars also stops water from passing around the lamellae (2).

»The surface size of the gills is very important as the larger they are with respect to the size of the fish, the more oxygen will be diffused.

»Obviously this theory would suggest that the largest gill possible with the thinnest barrier between water and blood would be fundamental. The average surface area for teleost gills is approximately 4.9cm2/g-1 body weight (6).

»The problem with this is the threat of water diffusion into, or out of, the blood. Limiting the osmotic gain or loss as well as optimising the uptake of oxygen will determine the ratio of gill surface to body mass a fish should employ (3).
References:
1) Levesque, M., Fralick, L., and McDowell, J. (1999). Respiration in water: An overview of gills. http://www.unb.ca/courses/biol4775/SPAGES/SPAGE13.HTM [on-line]

2) Smith, L.S. (1982). Introduction to Fish Physiology. TFH Publications Ltd, New York.

3) Wedemeyer, G.A., Meyer, F.P., and Smith, L. (1976). Diseases of fish: Environmental stress and fish diseases. TFH. Publications, Inc. Ltd, New York.

4) Fish, a quick course on Ichthyology. (1999). Fishes - How fish breathe. http://www.marinebiology.org/fish.htm [on-line].

5) Hoar, W.S. and Randall, D.J. (1984). Fish physiology Vol.X. Part A. Academic press, New York.

6) Hoar, W.S. and Randall, D.J. (1970). Fish physiology Vol.I.V. Academic press, New York.

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