This is Kate's jacket. = This jacket belongs to Kate.
That's our neighbors' dog. = That dog belongs to our neighbors.
Belong with means that things/people are similar and should be together. If one person in a romantic relationship says "You belong with me," it is like saying that it is destiny for the two people to be together.
You can also use belong with or belong in for putting an object into a category with other, similar objects:
That book about humans traveling to Mars doesn't belong in the history section. It belongs with the other science fiction books.
An object belongs IN a category, and belongs WITH other similar things.
Betablockers, a type of medication most commonly prescribed for heart disease, can cause potassium levels to increase in the blood. High potassium foods should be consumed in moderation when taking betablockers. Consuming too much potassium can be harmful for those whose kidneys are not fully functional. If your kidneys are unable to remove excess potassium from the blood, it could be fatal.