calcium homeostasis Flashcards
role of calcium in the body
- signalling (Ca2+ important signalling molecule)
- blood clotting
- apoptosis
- skeletal strength
- membrane excitability (Ca2+ decreases Na+ permeability)
Ca2+ and membrane excitability - hypocalaemia
increases neuronal Na+ permeability leading to hyperexcitation of neurones
in extreme cases causes tetany, if spreads to larynx and resp muscles - asphyxiation
Ca2+ and membrane excitability - hypercalaemia
decrease in neuronal Na+ permeability which will reduce excitability and depress neuromuscular activity and in extreme cases, trigger cardiac arrythmias
calcium distribution in body
bones 99%
Intracellular 0.9%
extracellular 0.1%
calcium in extracellular fluid
nearly half bound to protein so only 0.05% of calcium in body is free in solution and physiologically active
physiologically active calcium
0.05% in ECF
why is phosphate homeostasis also important in determining calcium balance
calcium stored in calcified extracellular matrix of bone is mostly in the form of hydroxyapatite
(Ca10(PO4)6(OH)2)
how much calcium in plasma
2.2-2.5mM
extracellular calcium stores
calcium has high affinity for proteins an in plasma around 40% is bound to plasma proteins
10% are neither bound to plasma protein or free in solution, they bind to plasma anions
how much calcium is physiologically active
about 1.2mM (~50% plasma calcium)
calcium distribution in the blood
40% protein bound (albumin 80%, globulin 20%)
50% free
10% complexed (anions)
how does pH affect binding capacity of plasma protiens
binding capacity is increased under alkalotic conditions
protein binding when alkalosis
plasma proteins bind more Ca2+ causing plasma concentration to fall
may precipitate hypocalcaemia
protien binding when acidosis
binding capacity reduces and free plasma Ca2+ rises
why does alkalosis cause more protein binding
hydrogen and calcium ions are postively charges and so compete for binding sites on the negative proteins
less H+ means less competition for binding sites
total body calcium =
calcium in - calcium out
Ca2+ from diet - Ca2+ excretion kidney and faeces
Ca2+ in bone
99% total body Ca2_ in bone, this can be added to or released as required
bone functions to provide mechanical support for body but its role in maintaining ca balance takes precendence over this
osteoblasts
bone building cells
highly active cells which lay down a collagen extracellular matrix which they then calcify
osteocytes
differentiates osteoblasts in established bone
regulate activity of osteoblasts and osteoclasts
osteoclasts
cells that are responsible for mobilising bone
they secrete H+ ions to dissolve calcium salts and provide proteolytic enzymes to digest the extracellular matrix
which two hormones act to increase calcium in plasma
parathyroid hormone
calcitriol
calcitriol
active form of viD3
steroid hormone produced from vitamin D by the liver and kidneys
parathyroid glands
4 on posterior surface of thyroid gland
essential for life
when it PTH released
in response to drop in plasma calcium
how does PTH act to increase Ca2_
- stimulate osteoclasts to release Ca2+ and phosphate in bone
- inhibit osteoblast to reduce Ca2+ deposition
- inc reabsorption of Ca2+ from kidney tubules
- inc renal excretion of phosphate - less phosphate = less calcium deposited back in bone
- stimulates kidney to synthesise calcitriol from vitamin D which promotes calcium absorption at gut and kidney
calcitriol production
steroid hormone produced in 2 steps (.1/ liver 2 kidneys) from dietary vitD or from precursors activated by sunlight on skin
actions of calcitriol
- inc absorption of Ca2+ from gut
- facilitates renal absorption of Ca2+
- mobilise calcium stores in bone by stimulating osteoclast activity
vitamin D deficiency
rickets in children
osteomalacia in adults
calcium and phosphate deficiency
how does VitD3 deficiency cause Ca2+ and phosphate deficiency
intenstinal malabsorption of Ca2+ so low plasma calcium which increases PTH which promotes phosphate deficiency (aggravates Ca2+ loss from bone)
which hormone acts to decrease plasma calcium
calcitonin
calcitonin
peptide hormone produced by thyroid gland
secretion stimulated by rise in plasma calcium
calcitonin actions
bind to osteoclats and inhibit bone resorption
also increase renal excretion
however, little evidence it is important in humans
other endocrine hormones altering calcium balance
cortisol
insulin
oestrogen
prolactin
how does cortisol affect calcium balance
inhibits osteoblasts, increase renal excretion and reduces intestinal absorption
so decreases plasma calcium
which increases PTH
which causes increased bone resorption
how does insulin affect calcium balance
increases bone formation and antagonises the action of cortisol
diabetics ma have significant bone loss
how does oestrogen affect calcium balance
constant stimulus for bone formation
how does prolactin affect calcium balance
promotes calcium absorption from gut by stimulating syntehsis of calcitriol
