Fluid and Electrolytes (1)

Fluid and electrolyte disorders occur when water balance, electrolyte concentrations or electrolyte distribution become abnormal. Three different problems hide under that sentence, and keeping them apart is the first step in reasoning clearly. The body can hold too little or too much total water and salt, which is a volume problem. It can hold the right amount of water but the wrong concentration of one electrolyte, which is a concentration problem. Or it can hold the right amount of an electrolyte in the wrong place, for example potassium shifted into cells, which is a distribution problem. A serum result only reports concentration. It cannot tell you by itself which of the three you are looking at. Why does this matter at the bedside? Electrolytes are the ions that let nerves fire, muscles contract and the heart conduct. Sodium and water set the volume of every cell, and the brain is the cell population least able to tolerate swelling or shrinking inside a rigid skull. Potassium sets the resting electrical potential of heart and skeletal muscle. Calcium and magnesium control how...