![]() It is through this mechanism that calcium is used to stabilize the cardiac cell membrane against depolarization in severe hyperkalemia. The contraction of one cardiac muscle cell is communicated to adjacent cells through intercalated disks, thus allowing for the synchronized contraction of cardiac muscle. The action potential triggers an inward flow of calcium that potentiates additional calcium release from the sarcoplasmic reticulum. Cardiac muscle is governed both by action potentials and extracellular calcium influx. Calmodulin binds calcium ions and activates myosin light chain kinase to phosphorylase the myosin head, which then binds actin and causes smooth muscle contraction. Additionally, ligand-gated and voltage-gated calcium channels on the smooth muscle membrane allow for extracellular calcium to enter the cell. In smooth muscle, second messenger systems trigger the release of calcium from the sarcoplasmic reticulum. This calcium then binds to tropomyosin and allows for the interaction of myosin and actin in the sarcomere, leading to muscle contraction. Skeletal muscle function is governed by an action potential that releases calcium stored in the sarcoplasmic reticulum. Ionized calcium plays an important function in muscle contraction. These processes lead to a decrease in serum calcium. Finally, calcitonin also inhibits calcium absorption in the intestines. Calcitonin also inhibits renal reabsorption of calcium, increasing urinary calcium excretion. Calcitonin acts on the bones to stimulate osteoblasts to deposit calcium in bones. All of these processes contribute to the rise in serum calcium.Ĭalcitonin is released by the thyroid parafollicular cells (C-cells) in response to an increase in serum calcium. PTH acts on the bones to stimulate osteoclasts involved in bone reabsorption and the release of free calcium. The kidney also responds to PTH by increasing secretion of Vitamin D3, which in turn stimulates calcium absorption through the gut. PTH acts on the kidneys to increase calcium reabsorption in the ascending loop of Henle, the distal convoluted tubule, and the collecting duct. ![]() The parathyroid glands release parathyroid hormone (PTH) in response to a decrease in serum calcium. ![]() The 3 primary hormones are parathyroid hormone (PTH) 1,25-dihydroxyvitamin D-3 (Vitamin D3), and calcitonin. Ĭalcium homeostasis is maintained by actions of hormones that regulate calcium transport in the gut, kidneys, and bone. If the serum calcium concentration exceeds the 8.8 to 10.4 mg/dL, then the body is considered in a state of calcium toxicity. Finally, free calcium, which makes up 51% of the serum calcium, is utilized by the body to maintain physiologic functions. Serum calcium is often chelated into the ionic complexes of calcium phosphate, calcium carbonate, and calcium oxalate. Chelated calcium, which accounts for 9% of the serum calcium, allows calcium to be absorbed by various tissues or carried between parts of the body. Albumin and globulin are the primary calcium-binding proteins in the serum whereas calmodulin is the primary calcium-binding protein in the cell. Protein-bound calcium, which accounts for 40% of the serum calcium, cannot be used by tissues. In serum, calcium exists in 3 forms: protein-bound, ionized (free), and complexed (chelated). ![]() Calcium in this form provides skeletal strength as well as a reservoir for calcium to be released into the serum. More than 99% of the calcium in the body is stored in bone as hydroxyapatite. Calcium is an essential element that serves an important role in skeletal mineralization. ![]()
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