just to eb a complete geek i've prepared an essay (ie i googled then copied and pasted )

The tests for blood electrolytes measure certain essential elements and chemical substances that are required for basic body functions. Typically the test measures the levels of sodium, potassium, chloride, and bicarbonate in the body. In addition, certain other related substances in the blood are often measured along with the electrolytes, such as blood urea nitrogen (BUN), creatinine, and glucose. Sometimes this entire group of tests is referred to as blood chemistries, electrolytes, or just lytes. Taken all together, these substances determine how the liquid parts of the blood behave as a chemical solution.



ps look here its quite a good read on bloodtests:

http://kidshealth.org/parent/system/.../labtest5.html