Kalciumsystem

Kalciumsystem refers to the physiological network that regulates extracellular calcium levels in vertebrates. Calcium is essential for bone mineralization, muscle contraction, neurotransmission, blood clotting, and many enzymatic processes. The system maintains calcium balance through bone storage and release, intestinal absorption, and renal excretion or reabsorption, coordinated by endocrine signals.

Key components and interactions

The skeleton acts as a major reservoir of calcium in the form of hydroxyapatite, with bone remodeling controlled by osteoblasts and osteoclasts, balancing storage and release. The gut absorbs calcium from the diet, a process strongly influenced by vitamin D. The kidneys regulate calcium excretion and reabsorption, shaping the amount returned to the bloodstream. Hormones coordinate these processes: parathyroid hormone (PTH) increases serum calcium by stimulating bone resorption, enhancing renal calcium reabsorption, and promoting activation of vitamin D; vitamin D (calcitriol) increases intestinal calcium absorption and supports bone mineralization; calcitonin can lower serum calcium by inhibiting bone resorption, though its role in humans is comparatively minor.

Regulation

Serum calcium levels are sensed by calcium-sensing receptors on the parathyroid glands and kidneys, triggering adjustments in PTH release. This negative feedback maintains calcium within a narrow range: typically about 2.2–2.6 mmol/L for total calcium, with ionized calcium around 1.1–1.3 mmol/L.

Clinical relevance

Disorders of the kalciumsystem include hypocalcemia (which can cause neuromuscular symptoms), hypercalcemia (which can cause fatigue, kidney stones, or calcifications). Causes include hypoparathyroidism, vitamin D deficiency, chronic kidney disease, and hyperparathyroidism. Management targets the underlying cause and aims to restore calcium balance through diet, supplementation, or pharmacological therapy.

See also: calcium homeostasis, bone remodeling, PTH, vitamin D.