I am studying this circuit in my course:

![enter image description here][1]

AB is opened, E1 = E5 = 10V, E4 = 40V, R1 = 2 Ohm, R2 = 4 Ohm, R3 = 6 Ohm, R6 = 6 Ohm

I want to write KVL for the Loop 1, what I do usually is defining the voltage drops polarity across each element (resistors in this circuit) but here I have many voltage sources.

Now the question that confused me is: How do I define the voltage drops polarity?
According to an origin voltage source (The highest one, or the one the problem is referring to)? Or I just define the polarity of each element according to its branch and its source separately?

Thank you very much in advance.
  [1]: https://i.sstatic.net/cYngm.jpg