Shivering

The shaking you get when you are cold has a name, shivering, and it is not a failure of the body: it is an order. It comes from the hypothalamus, the part of the brain that acts as the thermostat. In its back portion there is a motor center that orders the skeletal muscle, the muscle you normally use to move, to contract, and that contraction produces heat.

The hypothalamus does not find out on its own. It has informants on two fronts: sensors in the skin, the peripheral thermoreceptors, which measure how the surface is doing, and others inside, in the viscera and the spinal cord, which measure the temperature at the body's core. It decides based on those two reports.

And shivering almost never comes alone. Along with it, the body closes the skin arterioles so blood does not wander over the surface losing heat, and it raises muscle tone. Shivering itself produces a huge amount of heat: metabolic output can rise to several times the resting level.

That also explains its two limits. Shivering costs energy, and if there are no reserves it cannot be sustained. And when the temperature at the body's core drops to between 30 and 32 degrees Celsius, shivering usually shuts down.