Tunas achieve endothermy by conserving the heat generated through normal metabolism. In all tunas, the heart operates at ambient temperature, as it receives cooled blood, and coronary circulation is directly from the gills. The "rete mirabile" ("wonderful net"), the intertwining of veins and arteries in the body's periphery, allows nearly all of the metabolic heat from venous blood to be "re-claimed" and transferred to the arterial blood via a counter-current exchange system, thus mitigating the effects of surface cooling. This allows the tuna to elevate the temperatures of the highly-aerobic tissues of the skeletal muscles, eyes and brain, which supports faster swimming speeds and reduced energy expenditure, and which enables them to survive in cooler waters over a wider range of ocean environments than those of other fish.