L50- calcium homeostasis

describe the distribution of calcium in the human body

• 99% of calcium is in the Skelton

• and 1 is in the blood and other tissue but that’s the most important

what are the three main forms that calcium is found as

• protein bound calcium

• complexed calcium

• free ionised calcium ( ca²⁺ ₎

which form of calcium is the most physiologically active

the free ionised calcium

what are the functions of calcium in the body

explain calcium metabolism in humans

• through food we get calcium and it goes into out stomachs

• the majority of calcium is excreted inside the faeces

• the minority is absorbed and excreted inside the urine

what are the three hormones which regulate calcium

• calcitriol ( vitamin d )

• calcitonin

• parathyroid hormone - main regulator

explain what the parathyroid hormone is

• the hormone is produced by the parathyroid glands cheif cells

• its a peptide hormone

• its sythesised as preproPTH → proPTH→ active PTH

• it has a rapid regulation due to its short half life

what are the two types of PTH secretion regulation

• minute to minute

• long term

explain the minute to minute regulation of the PTH secretion

• the chief cells on the parathyroid glands sense the concentration of calcium in the blood

• when the calcium concentration is high it will bind to its receptors which are g- protein coupled

• this leads to the increase of intracellular calcium

• this inhibits the release of PTH inside the cell from being released

• the opposite happens when calcium is decreased which cause the PTH to be released

explain the long term regulation of the PTH secretion

• the PTH increases the vitamin d ( calcitriol )production inside the kidney

• the calcitriol leads to the increase of calcium in the plasma - leads to negative feedback

• the calcitriol decrease the preproPTH MRNA on a gene level to increase the synthesis of PTH

• this leads overall to the increased gut absorption of calcium and increased reabsorption of calcium

what are the two main sources of vitamin d and the two forms

• from diet

vitamin d₂ ( ergocalciferol )

• from sunlight ( skin production )

vitamin d₃ ( cholecalciferol )

explain how vitamin d is synthesised

• in our skin there’s 7-dehydrocholestrol and when exposed to the sun the uv light converts it into cholecalciferol ( vitamin d3 )

• then the vitamin d is transported into the liver

• in the liver 25- hydroxylase converts it into calcidiol which is the storage form of vitamin d

• the storage happens in the muscles and the adipocytes

• in the kidneys the activation happens where calcidiol → calcitriol which is active

explain calcitriol production

calcitriol levels are regulated b the balance between its synthesis and its degradation

1. rate of conversion of calcidiol to calcitriol ( activation step )

2. rate of conversion of calcitriol to calcitroic acid ( inactivation step )

once produced in the blood what does calcitriol do

• it increases intestinal absorption of calcium ions resulting in increase in concentrations in the blood

• it increases reabsorption of calcium ions into kidneys reducing the amount that’s lost in the urine

• it increases the action of osteoclasts so increases the calcium ions release in the plasma

• its actions lead to a rise in plasma calcium ions

what is calcitonin

• its a peptide hormone that’s secreted by the parafollicular cells of the thyroid glands

• its secretion is regulated the plasma calcium ion levels

• its released when calcium is high to help lower it

• its main physiological role is uncertain but potentially is protects Skelton from bone destruction

where and what is the main action sites of calcitonin

bone

• it inhibits osteoclasts activity decreasing bone resorption decreasing the calcium ions released from the bone into the plasma

in the kidneys

• it decreases the renal calcium reabsorption which increase the amount lost through urine

overall all it decreases calcium

how ( direct/indirect ) does the calcium regulating hormones regulate it in the bone

• all three hormones act on the bone cells

• calcitonin acts directly on the osteoclasts to inhibit their action

• but PTH and calcitriol affect the osteoclasts indirectly on the osteoblasts

what is hypercalcemia

what is hypocalcaemia