Sodium is a chemical element (atomic number 11) that in its pure form is a dull, highly reactive metal. However it is found in many natural compounds, including the compound that provides the body with most of its dietary sodium - sodium chloride or table salt. Blood is about 3% sodium chloride.
Sodium is vital for the transmission of impulses through the nerves. It's naturally removed from the body through urination and perspiration and must be replenished. Patients with low blood sodium suffer from fatigue and lightheadedness as the nerves lose their ability to transmit signals to the muscles and brain. Patients with high blood sodium suffer from high blood pressure as the heart attempts to supply the kidneys with more blood to assist them with removing the sodium from the body.
Sodium levels must be watched closely with patients with diabetes mellitus (too low), diabetes insipidus (also too low), kidney diseases (too high), and patients on intravenous saline, which contains sodium in an attempt to keep the body's sodium level more or less constant.