1. Calcium intake and GI absorption are critical for the maintenance of total body calcium.
2. GI absorption of calcium (and phosphate) is increased by 1,25(OH)2Vitamin D.
a. Assuming calcium intake is 1000 mg/day, about 350 mg/day is absorbed in the GI tract.
b. About 250 mg/day is lost into the GI tract due to secretions (e.g. bile and pancreatic juices), such that the net absorption of calcium per day is about 100 mg/day.
c. Therefore, approximately 900 mg/day is lost in the feces.
3. Once calcium is absorbed into the extracellular fluid (including blood plasma), it can be exchanged with three general compartments. Calcium circulates in the plasma bound to plasma proteins (primarily albumin, complexed to anions, and in the free (ionized), biologically active form).
a. Intracellular fluidwhich contains about 13,000 mg calcium.
b. Bonewhich contains most of the total body calcium (1,000,000 mg = 1 kg)
i. Calcium (and phosphate) deposition in bone is about 500 mg/day.
ii. Calcium (and phosphate) removal from bone – called resorption– is controlled primarily by parathyroid hormone, although 1,25(OH)2Vitamin D also is involved.
iii. Calcium deposition and resorption are tightly coupled and, when in calcium balance, equal to each other in the steady state.
c. Like most ions, ionized calcium (and phosphate) in the plasma is filtered in the kidney.
i. About 9980 mg/day of calcium is filtered in the kidney each day. If this calcium was not reabsorbed, the body would run out of calcium very quickly.
ii. About 9880 mg/day of calcium is reabsorbed in the kidney each day. This is stimulated primarily by parathyroid hormone, although 1,25(OH)2Vitamin D also has a small effect.
iii. Net loss of calcium in the urine in the steady state is about 100 mg/day.