Asked by Mark Wrangham
Like humans, ﬁ sh need oxygen to survive, but unlike us, they are capable of getting the oxygen they need from water. To do this they use an arrangement of ﬁlaments on either side of their neck called gills. Fish gulp in oxygenated water and force it through their gills and then out through the gill openings while also continually pumping deoxygenated blood from the body into the gill ﬁ laments. As the oxygenated water passes by the deoxygenated blood, oxygen diffuses into the gills. Efﬁciency is increased by the blood and water ﬂowing in opposite directions; this is known as a counter-current exchange mechanism. Using this, a ﬁsh can extract up to 70 per cent of the oxygen dissolved in the water. Waste products in the blood can also be removed from the blood by the same process, passing from the gills into the water.
James Maclaine, Zoology department, The Natural History Museum, London