Calcium is an electrolyte, a mineral that carries an electric charge, which is tightly regulated in the bloodstream to maintain a narrow and specific concentration. Although 99% of the body’s calcium resides in the bones and teeth, the small fraction circulating in the blood sustains life functions. When levels deviate outside the normal range of approximately 8.5 to 10.5 milligrams per deciliter (mg/dL), it can lead to severe medical conditions. Imbalances, whether too high or too low, require urgent medical intervention.
Essential Functions of Calcium
Circulating calcium extends far beyond its structural role in the skeleton, influencing the function of nearly every cell type in the body. Calcium ions are fundamental to muscle contraction, acting as the primary signal that triggers the sliding of actin and myosin filaments in both skeletal and cardiac muscle. For the heart, this function is particularly important, as calcium helps regulate the rhythm and force of every heartbeat.
In the nervous system, calcium is essential for communication between nerve cells. When an electrical impulse reaches the end of a neuron, the influx of calcium ions triggers the release of neurotransmitters into the synapse, allowing the signal to propagate. Calcium also acts as a cofactor for several coagulation proteins, accelerating the complex cascade of events that leads to blood clotting.
Hypocalcemia (Critically Low Levels)
Hypocalcemia is defined as a total serum calcium level below 8.5 mg/dL. The primary effect is increased excitability of nerve and muscle cells, leading to neuromuscular irritability. A reduced level of calcium ions makes it easier for a stimulus to trigger an action potential, which can cause tetany, characterized by involuntary, painful muscle spasms. Two specific clinical signs reflect this irritability: Trousseau’s sign (spasm of hand and wrist muscles when blood flow is restricted) and Chvostek’s sign (twitching of facial muscles when the facial nerve is tapped).
Low calcium also profoundly affects the heart’s electrical system, causing a prolongation of the QT interval on an electrocardiogram, which increases the risk of dangerous, erratic heart rhythms like Torsade de Pointes. Common causes include conditions that impair regulatory hormones or cause calcium sequestration. Hypoparathyroidism, often resulting from thyroid surgery, leads to insufficient production of Parathyroid Hormone (PTH), a main calcium regulator. Renal failure is another frequent cause, as damaged kidneys cannot retain calcium efficiently and fail to activate Vitamin D, necessary for intestinal absorption.
Acute pancreatitis can trigger severe hypocalcemia when fatty acids released by the inflamed pancreas bind to and precipitate calcium in the abdomen, effectively removing it from circulation. Immediate treatment for severe symptoms often involves the intravenous administration of calcium, typically as calcium gluconate, to quickly restore the circulating concentration and stabilize the patient.
Hypercalcemia (Critically High Levels)
Hypercalcemia is characterized by a total serum calcium level above 10.5 mg/dL. High calcium levels decrease the excitability of nerve and muscle tissues. The classic presentation is often summarized by the mnemonic “stones, bones, groans, and psychic moans.”
- Stones refers to the increased risk of nephrolithiasis, or kidney stones, due to the excessive calcium load filtered by the kidneys.
- Bones describes the pain and fractures that occur as calcium is pulled from the skeletal structure, especially in cases of chronic excess.
- Groans relates to gastrointestinal issues, such as constipation, nausea, and abdominal pain.
- Psychic moans encompasses neuropsychiatric symptoms, ranging from lethargy and confusion to depression and psychosis.
The two most frequent causes are primary hyperparathyroidism and malignancy. Primary hyperparathyroidism involves the overproduction of PTH, which stimulates the release of calcium from the bones and increases its reabsorption in the kidneys. Malignancy-associated hypercalcemia often involves tumors that secrete Parathyroid Hormone-related Peptide (PTHrP), mimicking PTH actions and leading to uncontrolled bone breakdown. Emergency management focuses on hydration with intravenous saline to promote calcium excretion, followed by medications like calcitonin or bisphosphonates to inhibit bone breakdown.
How the Body Maintains Calcium Balance
The body maintains the precise blood calcium concentration through a sophisticated feedback loop involving three primary organs and three hormones. The bones act as a reservoir, the kidneys manage excretion and reabsorption, and the small intestine controls dietary absorption. Parathyroid Hormone (PTH), secreted by the parathyroid glands, is the main regulator, released when calcium levels drop.
PTH acts on the bones to stimulate the release of stored calcium and on the kidneys to increase calcium reabsorption. In the kidneys, PTH also stimulates the conversion of inactive Vitamin D into its active form, calcitriol. Calcitriol then acts on the intestine to significantly enhance the absorption of calcium from food. Calcitonin, a hormone produced by the thyroid gland, acts as the counterbalance by inhibiting the release of calcium from the bones, though its role is considered less significant than that of PTH and calcitriol.