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 Compartmentalization of calcium  

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Regarding compartmentalization of calcium in the body:

First :
In Guyton it says:
" Extracellular fluid has 0.1 % of the total body calcium. "

Although in In Kaplan lecture notes of physiology it says:
" The interstitial fluid has 0.1% of total body calcium. "
" The plasma has <0.5% of total body calcium. "

This makes the ECF contain 0.6% instead of 0.1%

Second :
In Guyton it says:
" The cells contain 1 % of the total body calcium. "
" The extracellular fluid has 0.1 % of the total body calcium. "

This makes the ECF calcium concentration higher than the ICF

Although in In Kaplan lecture notes of physiology it says:
" Calcium concentration in the ( interstitial ) fluid is 10^3 to 10^4 higher than
than the ( intracellular ) fluid. "

O.o I am really confused o.O
Please, can anyone clarify this for me.


lets be clear and simple about few facts!

Guyton isn't recommended to study for the boards!

Guyton Physio contradicts itself when you go through it word by word during regular school!

Kaplan Or Goljan can answer your question.

Whenever there is a confusion just compare from reliable website and that should clear confusion.

P.S Have a smart approach rather then a deeper one!


The vast majority of body calcium is stored in bone so very little is outside of it but is very important from an endocrinological point of view. The critical thing about ECF calcium is its concentration. Concentration matters as calcium can act as a second messenger inside cells. Usually the calcium concentration in serum is buffered by albumin and so, sometimes you have to correct by the serum protein concentration to have an idea about the free serum calcium that is the one that most organs see. There are many formulas to calculate protein bound calcium but roughly is about 45%. The rest will be controlled or acted upon by calcium channels, calcium binding proteins (that contain EF hands like calmodulin, etc). Free calcium is an important second messenger because the intracellular concentration is about 10,000-fold lower than extracellular calcium. When Ca manages to get inside the cell, the cells knows right away and 'gets the message' that something is going and needs to do something, like activating other second messengers like Protein Kinase C or phosphoinositol degradation, or secretion via vesicle fusion to the membrane. As soon as calcium gets in, it's immediately buffered with calbindins (calcium binding proteins D9K y D28K and many others like caldesmon, etc), calcium is expelled from the cell via the Calcium Pump or Calcium ATPases, or via a Calcium Exporter in exchange of Sodium, which in turn is extruded via the Na-K ATPase. Calcium is also stored in the mitochondria and the endoplasmic reticulum where is awaits until its release via stimulation of the Rianodine receptor. Some people talk about the calciosome.

The most interesting aspects about calcium regulation occur in organs like the Parathyroid glands, absorption by the intestine and excretion by the kidney. There's a calcium sensor receptor (CaSR) in parathyroid glands and if damaged can cause disease either by gain of function or loss of function. There are in fact drugs that stimulate it (calciomimetics) or that inhibit it (calcilytics). There's a lot of interesting biology about calcium.

This is just what I remember. Good luck.


Thank you.

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