This is a typical engine-cooling system. Coolant is in contact with virtually every nook and cranny of the water jackets and radiator. Heat is transferred to the coolant that's moved by the water pump through the block, heads, and radiator. It circulates in the engine's water jackets, taking on heat. When coolant temperature reaches the thermostat's rated temperature (160, 180, or 192/195 degrees F), the thermostat opens, releasing hot coolant to the radiator via the upper radiator hose where it's cooled and returned to the engine through the bottom hose and water pump.