Calcium Metabolism and Calcium Oxalate Stones
Calcium metabolism in the body is a complex process that involves the regulation of calcium levels in the blood, bones, and other tissues. Calcium is essential for many physiological functions, including bone formation, muscle contraction, nerve transmission, and blood clotting. Here's an overview of how calcium metabolism works:
Dietary Intake: Calcium is primarily obtained from the diet. Foods rich in calcium include dairy products (milk, cheese, yogurt), leafy green vegetables (broccoli, kale), fortified foods (orange juice, cereals), and fish with edible bones (sardines, salmon).
Bone Resorption: Calcium can be mobilized from the bones when dietary intake is insufficient. This process is known as bone resorption.
Intestinal Absorption: Calcium is absorbed in the small intestine, mainly in the duodenum and jejunum. The efficiency of calcium absorption is influenced by several factors:
Vitamin D: Enhances calcium absorption by increasing the production of calcium-binding proteins in the intestinal lining.
Parathyroid Hormone (PTH): Indirectly increases calcium absorption by stimulating the activation of vitamin D in the kidneys.
Dietary Factors: Phytates, oxalates, and high levels of phosphorus can inhibit calcium absorption, while acidic conditions and certain amino acids can enhance it.
Transport in the Blood: Calcium is transported in the blood in three forms:
Free (Ionized) Calcium: About 50% of calcium in the blood is in the ionized form, which is biologically active.
Protein-bound Calcium: Around 40% of blood calcium is bound to proteins, mainly albumin.
Complexed Calcium: The remaining 10% is complexed with anions such as bicarbonate, phosphate, and citrate.
Regulation of Calcium Levels
Calcium homeostasis is tightly regulated by three main hormones: parathyroid hormone (PTH), vitamin D (calcitriol), and calcitonin.
Parathyroid Hormone (PTH):
Secretion: Produced by the parathyroid glands in response to low blood calcium levels.
Actions: Increases blood calcium by stimulating bone resorption, increasing renal reabsorption of calcium, and enhancing intestinal absorption of calcium via activation of vitamin D.
Activation: Synthesized in the skin from cholesterol upon exposure to sunlight, then converted to its active form in the liver and kidneys.
Actions: Increases intestinal absorption of calcium and phosphate, promotes bone resorption, and reduces renal excretion of calcium.
Secretion: Produced by the thyroid gland in response to high blood calcium levels.
Actions: Lowers blood calcium by inhibiting bone resorption and increasing calcium excretion in the urine.
Bone: The majority of the body's calcium (about 99%) is stored in the bones and teeth, providing structural support and serving as a reservoir that can be tapped when needed.
Soft Tissues: Calcium is also found in soft tissues, where it plays roles in muscle contraction, neurotransmission, and other cellular functions.
Kidneys: The kidneys filter calcium from the blood, and a portion is reabsorbed back into the bloodstream. The remaining calcium is excreted in the urine.
Feces: Unabsorbed dietary calcium is excreted in the feces.
Sweat: Small amounts of calcium are lost through sweat.
Disorders of Calcium Metabolism
Hypercalcemia: Elevated blood calcium levels, which can be caused by hyperparathyroidism, malignancies, excessive vitamin D intake, and certain medications.
Hypocalcemia: Low blood calcium levels, which can result from hypoparathyroidism, vitamin D deficiency, chronic kidney disease, and other conditions.
Osteoporosis: A condition characterized by weakened bones due to an imbalance between bone resorption and bone formation, often associated with aging and hormonal changes.
Rickets/Osteomalacia: Conditions caused by vitamin D deficiency, leading to poor bone mineralization.
Calcium metabolism is a dynamic process involving dietary intake, absorption, transport, storage, and excretion, regulated by hormones to maintain stable calcium levels essential for various physiological functions. Proper calcium balance is crucial for bone health and overall bodily function.
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