Atlantic bluefin tuna use a mechanism referred to as countercurrent exchange to keep its core muscles warm and prevent the loss of heat when swimming. Counter current exchange is the mechanism in which oxygen enters the blood in fish. Blood flows in the opposite direction to the water that flows over the fish's gills. Fish gills have gill filaments and these filaments have protrusions called lamellae which the water flows over. The lamellae increase the surface area of the gills meaning that there is a larger area for the oxygen to diffuse over. With the counter current exchange system the water is always richer in oxygen than in the blood therefore maintaining the concentration gradient. The oxygen then diffuses into the blood capillaries of the fish.