Nonshivering thermogenesis is activated by sympathetic activity or the release of norepinephrine . In the brown fat cells, ATP utilization rises through ion pumping. ATP synthesis would be stimulated, fatty acid oxidation to acetyl coA takes place and Kreb's cycle and electron transport occur. But no ATP is synthesized because the respiratory chain is uncoupled by proton ionophores, which allows the protons to return back into the mitochondrial matrix without stimulating the ATP synthase. Thus brown fat heats rapidly and the heat is dispersed by the blood flow through the tissue to the body core.
Countercurrent exchange is also important for preventing heat loss in the extremities .
The arterial blood entering the limb is at core temperature, while the venous blood from the peripheral tissue is significantly cooler. The arterial flow into the limb lies next to the venous flow from the limb, such that the warm arterial blood gives up heat to the returning venous blood. By the time the arterial blood reaches the periphery, it has been precooled to just a few degrees above the ambient temperature and little heat is lost to the environment because the returning venous blood has been warmed to near the core temperature. As a result, the extremities are maintained at temperatures that are far below the core temperature. A different set of metabolic responses is necessary for endothermy in hot environments.