COUNTER-CURRENT

Fish utilise a counter-current system in order to maximise the amount of oxygen that can extracted from water. Water has a much lower oxygen content than air and as such fish need a much more efficient system that can extract a much higher proportion of oxygen from the surrounding water. [image:http://i.imgur.com/cnC2onql.png?1] =Concurrent Exchange= [image:http://i.imgur.com/ksWRw5Bl.png] If blood flowed through the lamellae in the same direction as the water passing over the gills, only 50% (maximum) of the available oxygen could be utilised. Oxygen would diffuse down it's concentration gradient from an area of higher concentration (the water) to an area of lower concentration (the blood). However, as the water and blood continue to flow across the gas exchange surface an equilibrium would be reached with roughly equal concentrations of oxygen on either side of the gas exchange surface. At this point there would be no net movement of oxygen as there is no longer a concentration gradient. [image:http://i.imgur.com/mGai61w.png?2] =Counter-Current Exchange= [image:http://i.imgur.com/tPFpdB8.png] In reality the blood flowing through lamellae travels in the opposite direction to the water passing over the gills. This ensures that a concentration gradient is maintained across the entire gas exchange surface. Oxygen deficient blood entering the gills is exposed to oxygen poor water that is about to leave the gills. Because this water still has slightly more oxygen than the neighbouring blood oxygen diffuses into the blood. As blood flows through the gills it continues to 'pick up' oxygen. As the blood oxygen concentration increases the blood also continues to encounter water with an even higher concentration of oxygen and thus a concentration gradient is maintained. Oxygen rich blood that is about to leave the gills (to go to respiring body cells) encounters the most oxygen rich water (water that has just entered the gills). This is much more efficient and allows a much higher proportion of the dissolved oxygen in water to be absorbed. [image:http://i.imgur.com/XwF5fW9.png?1]