Calcium & Phosphate homeostasis
Phosphate homeostasis
-Average adult has 500 to 800 g phosphorus -85% to 90% of phosphate is in the bones -normal plasma concentration is 3.5 to 4.0 mg/dL -phosphate levels are not regulated as tightly as calcium levels; no immediate functional disorders -Calcitriol promotes its absorption by small intestine and promotes bone deposition -lowers blood phosphate level by promoting its urinary excretion
Other factors affecting bone
-at least 20 or more hormones, vitamins and growth factors affect osseous tissue -bone growth especially rapid in puberty and adolescence; surges of growth hormone, estrogen, testosterone occur and promote ossification -these hormones stimulate multiplication of osteogenic cells, matrix deposition by osteoblasts and chondrocyte multiplication -girls grow faster than boys and reach full height earlier; estrogen stronger effect than testosterone on bone growth
Hypocalcemia
-blood calcium deficiency
Anabolic steroids
-cause growth to stop epiphyseal plate "closes" prematurely -results in abnormally short adult stature
Hypercalcemia
-has a wide variety of causes, blood calcium excess -hormone
Parathyroid Hormone (PTH)
-secreted by the parathyroid glands which adhere to the posterior surface of thyroid gland -released with low calcium blood levels -raises calcium blood level by four mechanisms; binds to receptors on osteoblasts, promotes calcium reabsorption by the kidneys, less lost in urine -promotes the final step of calcitriol synthesis in the kidneys, enhancing calcium-raising effect of calcitriol -inhibits collagen synthesis by osteoblasts, inhibiting bone deposition
Calcitriol (hormone)
a form of vitamin D produced by the sequential action of the skin, liver and kidneys -produced by: 1. epidermal keratinocytes use UV radiation to convert a steroid (previtamin D3) 2. Liver adds a hydroxyl group converting it to calcidiol 3. Kidneys add another hydroxyl group, converting to calcitriol (most active form of vitamin D) 4. Calcitriol behaves as a hormone that raises blood calcium concentration 5.Necessary for bone deposition - need adequate calcium and phosphate 6. Abnormal softness of bones in children (rickets) and in adults (osteomalacia) without adequate vitamin D
Calcium homeostasis
depends on a balance between dietary intake and urinary and fecal losses and exchanges between osseous tissue -is regulated by three hormones; calcitriol, calcitonin, parathyroid hormone
Calcitonin (hormone)
secreted by C cells (clear cells) of the thyroid gland when calcium concentration rises too high -lowers blood calcium concentration in two ways 1. osteoclast inhibition: reduces osteoclast activity as much as 70 % 2. Osteoblast stimulation: increases number and activity of osteoblasts, deposits calcium into the skeleton
