Serum sodium is the concentration of sodium ions found in the blood plasma. Sodium is the most abundant positively charged ion, or cation, in the fluid outside of the body’s cells, known as the extracellular fluid. As a powerful electrolyte, it plays a fundamental role in maintaining the electrical neutrality and fluid balance necessary for life. The concentration of sodium in the blood directly reflects the balance between the amount of sodium and the amount of water in the body.
Core Physiological Role
Sodium’s primary functional mechanism involves regulating osmotic pressure, which is the force that controls the movement of water across cell membranes. Because sodium is highly concentrated outside the cells, it acts as the main driver for attracting and retaining water in the extracellular space. This mechanism ensures that the volume of water surrounding the cells remains stable, preventing cells from either shrinking or swelling excessively.
Sodium ions are also indispensable for the generation of electrical signals in nerve and muscle cells. Specialized channels in the cell membranes open temporarily to allow a rapid influx of sodium, which creates the action potential necessary for transmitting nerve impulses. This rapid shift is fundamental for all muscle contraction, including the rhythmic beating of the heart. The constant, tightly regulated movement of sodium across cell membranes is managed by a mechanism known as the sodium-potassium pump.
Measuring Serum Sodium
Serum sodium measurement is usually performed as part of an electrolyte or basic metabolic panel blood test. Blood is drawn from a vein and analyzed using automated methods to determine the concentration of sodium ions. The standard normal reference range for serum sodium is between 135 and 145 milliequivalents per liter (mEq/L) or millimoles per liter (mmol/L).
Results outside of this range define specific clinical conditions. A sodium level below 135 mEq/L is defined as hyponatremia. Conversely, a level above 145 mEq/L is defined as hypernatremia. Determining the numerical threshold helps guide diagnosis and management of fluid and electrolyte disorders.
Understanding Low Sodium (Hyponatremia)
Hyponatremia, a serum sodium level below 135 mEq/L, is caused by a relative excess of water compared to the body’s sodium content, which dilutes the blood. One common cause is the Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH), where the body inappropriately retains water. Other factors include certain medications, such as diuretics and some antidepressants, which interfere with the hormonal and kidney processes that regulate water excretion.
In hyponatremia, the lower concentration of sodium outside the cells creates an osmotic imbalance, causing water to shift from the blood plasma into the cells. This influx of water leads to cellular swelling, which is particularly dangerous in the brain because the skull limits the room for expansion. The resulting cerebral edema, or brain swelling, causes many of the symptoms associated with the condition.
Mild symptoms include nausea, headache, and malaise. As cellular swelling progresses, severe symptoms develop, such as confusion, loss of energy, muscle weakness, and cramps. In profound cases, particularly when the sodium level falls below 120 mEq/L, the swelling can lead to seizures, coma, and death.
Understanding High Sodium (Hypernatremia)
Hypernatremia, defined as a serum sodium concentration above 145 mEq/L, results from a deficit of total body water relative to the amount of sodium. This imbalance is caused by inadequate water intake or excessive fluid loss that is not adequately replaced, such as from severe diarrhea, vomiting, or high fever. A relatively rare cause is diabetes insipidus, which impairs the kidneys’ ability to conserve water. Sustained hypernatremia occurs when the body’s natural thirst mechanism is impaired, or when an individual cannot access water, which is common in infants and older adults with altered mental status.
The high sodium concentration increases osmotic pressure, pulling water out of the cells and causing them to shrink. Symptoms of hypernatremia are primarily neurological due to this shrinkage of brain cells. Initial symptoms include intense thirst, lethargy, and confusion, progressing to severe confusion, muscle twitching, seizures, and coma if the sodium level rises rapidly or exceeds 160 mEq/L.