Originally Posted by
foama
The coolant flows from the cylinder head where coolant gets hottest, then through the heater core, the throttle flap, and goes to the bottom of the block where it slowly recirculates within the block towards the cylinder head. The thermostat and radiator is being bypassed by this flow, but it is absolutely necessary for the coolant to actually flow within the block especially with a cold engine. Without that flow during warm-up, the cylinder head would overheat while the thermostat which is located in the lower block(!) remains cold.
Btw: The coolant and engine oil have large-surface passages right next to each other inside the lower block which are in effect a very effective oil/water heat exchanger! Thats why the oil temperature at high speed on the highway will not exceed coolant temperature, and also why the oil warms up comparatively fast during warm-up.