calcium and bone Flashcards
PTH
secreted in response to decreased calcium
immediately increases Ca reabsorbed in the renal tubule
also causes Ca release from the bone
activates vit D and increases Ca absorption in the gut
overall role is to increase serum calcium and reduce phosphate
stimulates osteoclast activity
increase calcium, and decrease phosphate reabsorption from the kidney
increase renal production of 1,25-dihydroxy-vitaminD3 (calcitriol)
vitamin D
cholecalciferol
7-dehydrocholesterol converted in the skin into vit D by UVB in the sunlight
osteoclasts
form a seal and secrete acid onto the underlying bone leaving a pit behind, on which osteoblasts act to build new bone
hypercalcaemia
common
thirst, polyuria, renal stones, weakness, myalgia, bone pain, diarrhoea, anorexia
stones, bones, groans and moans
primary hyperparathyroidism
F:M 3:1 peak 50-70yrs 85% solitary parathyroid adenoma 15% 4-gland parathyroid hyperplasia <1% parathyroid carcinoma
can lead to:
kidney stones and renal impairment
osteoporosis at wrist and hip
osteitis fibrosa cystica
the high PTH removes Ca from the bone and moves it to the blood. this is then excreted by the kidneys, causing kidney stones as it passes through
severe hypercalcaemia
when someone first presents, blood tests can indicate cause
if PTH is high - primary hyperparathyroidism
if PTH low - cancer
albumin
calcium in the blood is bound to albumin. therefore patients who have low albumin will have an apparent hypocalcaemia on blood tests. their calcium is not low, rather their albumin is reduced
vit D and calcitriol
vit D hydroxylated in the liver into 25-hydroxy-vit D and again in the kidney into calcitriol, the biologically active form
calcitriol production is stimulated by decreased calcium, decreased phosphate and PTH
it increases calcium and phosphate absorption from the gut
inhibits PTH release (negative feedback)
it increases bone turnover
increases calcium and phosphate reabsorption in the kidney
role of phosphate
85% of the bodies stores found in bone as hydroxyapatite
15% stored as phosphoproteins, phospholipids and nucleic acids
involved in energy production and bone and tooth formation
levels are opposite to calcium. ie as calcium falls, phosphate will rise and vice versa
controlled less tightly than calcium, with wider variations due to diet
increased by vitD, decreased by calcitonin
serum phosphate controlled by reabsorption in the kidney,. which is decreased by PTH
bone
despite the role of bone as a structure, its metabolic function as a reservoir for calcium, phosphate, magnesium is its primary function
ie calcium will be removed from bone if needed regardless of the loss of structural integrity
magnesium
low magnesium prevents PTH release and may play a role in hypocalcaemia