Often, blood tests check electrolytes, the minerals that help keep the body's fluid levels in balance, and are necessary to help the muscles, heart, and other organs work properly. To assess kidney function and blood sugar, blood tests measure other substances.
Tests for Electrolytes
Typically, tests for electrolytes measure levels of sodium, potassium, chloride, and bicarbonate in the body.Sodium plays a major role in regulating the amount of water in the body. Also, the passage of sodium in and out of cells is necessary for many body functions, like transmitting electrical signals in the brain and in the muscles. The sodium levels are measured to detect whether there's the right balance of sodium and liquid in the blood to carry out those functions.
If a child becomes dehydrated because of vomiting, diarrhea, or inadequate fluid intake, the sodium levels can be abnormally high or low, which can cause a child to feel confused, weak, and lethargic, and even to have seizures.
Potassium is essential to regulate how the heart beats. Potassium levels that are too high or too low can increase the risk of an abnormal heartbeat. Low potassium levels are also associated with muscle weakness.
Chloride, like sodium, helps maintain a balance of fluids in the body. If there's a large loss of chloride, the blood may become more acidic and prevent certain chemical reactions from occurring in the body that are necessary it to keep working properly.
Bicarbonate prevents the body's tissues from getting too much or too little acid. The kidney and lungs balance the levels of bicarbonate in the body. So if bicarbonate levels are too high or low, it might indicate a problem with those organs.
source : http://kidshealth.org/parent/general/sick/labtest5.html