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Parathyroid Hormone
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Parathyroid hormoneCalcium.
Calcium is important for muscle contraction and secretory cells and therefore
maintaining adequate levels of calcium plasma is important.
Calcium in the human body.
Between 1 and 2% of the entire human body mass is calcium.
Most of the calcium is stored in the bone, in the skeletal system as calcium
phosphate, specifically calcium hydroxyapatite.
Calcium= total calcium content, total calcium refers to calcium that is stored in
phosphate forms
Ca2+= ionized calcium, this type of calcium tends to circulate and cause muscle
contraction as well as many other functions.
Circulating calcium is less than 100 nM
Intersitial fluid (fluid that bathes cells)
Calcium concentrations outside the cell are generally higher than inside the cell.
There is a gradient of calcium concentration between the blood, interstitial fluid to
ionized calcium and calcium found inside the cell.
Calcium in the blood.
Calcium can be free or bound.
Normal range of calcium in an individual is 2.25 mM- 2.60 mM.
46% of calcium is bound to a protein, usually albumin.
54% is free calcium
Some of the free calcium is complexed, which means that it is bound to citrate and
phosphate (7%)
Most of it is totally free ionized calcium (47%)
Usually, total calcium and bound calcium are measured.
The value is adjusted if albumin levels are increased or decreased.
This is because the more albumin there is, the more calcium will be bound.
If we need to know how much ionized free calcium is in the blood, we can measure
ionized ca2+ with a ion-selective electrode.
Things can happen day to day which alters the levels of calcium in the blood.
Acidosis (decrease in fluid PH) increases ionized plasma calcium ion levels, this is
because hydrogen ions will bind to albumin which will displace Ca2+ ions.
The opposite happens in alkalosis- if the PH of the blood increases, they’re will be
fewer free hydrogen ions and this means that we will end up with more albumin
binding ionized calcium.
The relative levels of PH in the blood, will have an influence on how much calcium
will be bound and how much calcium will be free.
Blood phosphate also regulates calcium, so an increase in blood phosphate levels will
decrease ionized calcium levels in the blood.
Calcium balance.
In the extracellular fluid you would expect 900 mg of calcium.
, In the intracellular fluid, you have a larger amount in total (9000 mg), because there
are lots of cells. Even though the concentration is lower, the amount ( if you add up
all of the cells in the body) is higher.
Intracellular fluid and extracellular fluid are our two pools of calcium.
They are interchangeable, calcium can move from cells into the extracellular fluid.
And from extracellular fluid into cells.
This is an important process because this movement triggers something like
secretion of a hormone.
There are a number of different routes into and out of extracellular fluid where
calcium can either enter or leave.
All of these different routes contribute to calcium balance.
EG. We get calcium from our diet about a 1 g of calcium will enter our
gastrointestinal tract, but we will loose 90% of that calcium in our faeces, so we will
absorb around 10% of that calcium.
Collectively we might collect 400 mg of calcium over a daily period.
But we will secrete 300 mg from the extracellular fluid, so there is a two way process
of calcium in from the gastrointestinal tract and then back out into the
gastrointestinal tract from the fluid.
So we have a movement of calcium in both directions and this is regulatable.
So if we need more calcium, under hormonal control we can increase absorption
from the gut.
If calcium levels are too high in the blood, we can increase secretion.
This is a modifiable way of regulating calcium input and output into the gut.
The kidney is a key tissue in terms of regulating the blood concentration of
compounds.
Around 10 grams of calcium will be filtered and only 100 mg of that will be released
in urine.
This is because the vast majority of filtered calcium is reabsorbed from the tubules
back into the blood.
So the kidneys are very effective at maintaining calcium levels.
This is also modifiable, whether we reabsorb more or less calcium from the kidneys
is under hormonal control.
We have a giant store of calcium phosphatehydroxyapetite in the bones.
There is a two way process involved: either accretion which is the storage of calcium,
exchange and reabsorption of calcium, so around 600 mg can either be stored or
reabsorbed from calcium to build and maintain this extracellular fluid pool.
We have 3 different routes of either gaining calcium into the extracellular fluid or
removing calcium from extracellular fluid.
Particularly plasma which allows us to maintain calcium balance.
The hormones that regulate calcium balance:
Vitamin D, parathyroid hormone and calcitonin
Vitamin D regulates calcium uptake from the gut.
Vitamin D regulates calcium reabsorption from the kidney.
Parathyroid hormone regulates calcium reabsorption from the kidney.
calcitonin is involved in the process of the rate at which calcium is reabsorbed or
deposited from the bones.
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