USA Surgery Today

Calcium is the major constituent of bone and is thus the most abundant mineral in the body. Almost 99% of the body’s calcium is in the bone and therefore unavailable for biochemical interactions. The other 1% remains in flux with the mineralized fraction and is estimated by serum calcium levels. The nonosseous extracellular calcium is either bound to albumin (40%), complexed to small molecules such as sulfates and phosphates (10%), or free and ionized (50%). It is the free ionized form that is physiologically active and thus serum measurements of ionized calcium are most reflective of calcium balance. Hyperproteinemia and alkalosis result in an increased proportion of protein-bound albumin and a decrease in ionized calcium. Chelators such as the citrate used in exchange transfusions or with massive blood resuscitation after trauma bind free calcium as well. Conversely, hypoalbuminemia decreases serum measurements of total calcium due to a decrease in protein-bound calcium, whereas the ionized fraction often remains constant.

The gastrointestinal tract is responsible for the intake of calcium and is regulated directly by the activated form of vitamin D and indirectly by the parathyroid glands. When calcium stores are low, the parathyroids are stimulated to secrete parathyroid hormone (PTH). PTH increases hydroxylase activity in the kidney, which in turn converts the inactive form of vitamin D to the active form. PTH also stimulates osteoclasts in the bone to reabsorb calcium. Both vitamin D and PTH induce renal resorption of calcium in the proximal convoluted tubule (85%) coupled with sodium resorption, and in the distal tubule (15%), which is independent of sodium. Increased calcium absorption may be found in conditions associated with increased activated vitamin D levels, such as sarcoid, leukemia, and multiple myeloma. Calcium loss is mainly via the urine, and is increased by diuretic administration, growth hormone, thyroid hormone, and glucagon. As with most other electrolytes, high-output stomas or fistulae may result in a large calcium deficit.