I know that the blood needs both Potassium and Sodium. However, we use only added Sodium with our food (when we salt it). The medical saline is also water + sodium chloride and contains no potassium. Why is the imbalance? Should I avoid added NaCl in my food since it is not paired with K and, as such, will be drained by kindeys anyway or Should I look for added Potassium salt to recover the balance?
Your kidneys will take care of that for you. They regulate the concentration of electrolytes in your blood. Usually, enough potassium is available in your food (e.g. coffee and bananas are rich in potassium) to prevent a potassium shortage.
Too much medical saline can cause hypernatraemia (too high concentration of sodium) and hypokalaemia (too low cencentration of potassium). However, one needs quite some quantities for that, and the balance is usually quite easily recovered by supplementing potassium.
TL:DR; don't worry about it.