All hydronic heating systems need to be pressurized to some degree to prevent the water from flashing to steam in the boiler. It depends on how high up it needs to pump, how much pressure it needs.
You often need to balance the lines so you can get the desired flow in each branch. You can do that with separate valves, then, once a loop is opened, the flows should be able to be balanced and you'll get even heat to all. Now, if the head is too high, you may need a bypass loop/valve to keep things within range of acceptable if say only one zone was calling for heat.