1. Calcium homeostasis

2. Organs in calcium homeostasis
Small intestain
Uptake from dietary
Normal adult diet contains 25mmol, which absorped only 5 mmol /day   
Blood/plasma 
Coagulation 
Transporting with albumin 
Main carrier of protein- bound calcium  
The plasma total calcium concentration is in the range of  mmol/L
The normal ionized calcium is  mmol/L  (50-60%)
Protein-bound in albumin (35-50%)
Complexes in organic acids and phosphates (5-10%)

3. Organs in calcium homeostasis
Hormone system​
Intracellular secondary messenger​
Thyroid gland​
Calcitonin​
Parathyroid gland​
Parathyroid hormone (PTH)
Skeletal system
Storage
99% of body's total calcium
Calcium makes bones strength
Liver
Cell signaling
Extra cellular fluid    
Storage    
Transporting to the cells   
Cell signaling  
Muscle       
Contraction
Kidneys
Calcitriol
Remove to urine
In ions 5 mmol/day

4. Calcium homeostasis
Calcium concentration in blood is regulated by calcitonin and PTH activated calcitriol
Calcitriol is released from kidneys. It stimulate cells releasing calcium from bone, increase uptake to gastrointestinal tract and increase reabsorption in renal tubule
Calcitonin is released from thyroid gland and it stimulate calcium deposition in bones and reduce uptake in kidneys when calcium remove to urine

5. Vitamin-D as the regulatory factor of calcium homeostasis
Vitamin-D increases calcium intake from intestine, bones and kidneys
Calcitriol is a vitamin d metabolite that upregulates calcium binding protein expression causing uptake of calcium
Kidneys observe calcium level in blood and produce calcitriol
With the help of renin, calcitriol reabsorbs calcium in the kidneys
Calcium binding proteins can be intracellular and extracellular and they have many functions such as free calcium regulation, calcium transport and signalling (for example calmodulin).

6. Intracellular calcium homeostasisER
Cytosol
High concentration of Ca2+ in rest
IP3 activates calcium release to cytosol through ion-channels
In muscle the sarcoplasmic reticulum releases Ca2+ after synaptic signal
Active pumping of calcium into the ER
Low concentration of Ca2+
Calcium works as secondary messenger
Calcium can activate some proteins that regulate exocytosis
Intake from ER and extracellular space

8. Calcium pumps and calcium transport
Calcium is actively pumped out of cytosol with Plasma membrane Ca2+ ATPase (PMCA) and Sodium-calcium exchanger (NCX)
PMCA has high affinity but low capacity where as NCX has low affinity but high capacity
This makes PMCA slow and steady when NCX works fast in high Ca2+ concentrations (during signalling where Ca2+ is a secondary messenger)

9. Calcium regulated homeostasis and transport
Calcium has a major role in biomembrane fusion thus regulating many other homeostatic events in the organism
Most of membrane fusions require calcium, such as exocytosis, so the calcium can be released from the vehicle itself or from the ER
Calcium isn't required always and can also inhibit some membrane fusions like pre-golgi-fusion

10. Disorders in calcium metabolism
High
Deficiency
Hypercalcemia
Milk-alkali syndrome (Burnett's syndrome)
Calcinosis (Calciphylaxix,
     Calclinosis cutis)
Calcificaltion (Metastatic calcification,
    Dystrophic calcification)
Familial hypocalciuric hypercalcemia
Excitotoxicity
Hypocalcemia
Osteomalacia
Pseudohypoparathyroidism (Albright's hereditary, osteodystrophy)
Pseudopseudohypoparathyroidism

11. Hypocalcemia Hypercalcemia
[ 𝐶𝑎 2+ ] 𝐸𝐶𝐹 too low
Voltage-gated ion-channels opening spontaneously
Nerve and muscle cells come hyperactive
Involuntary muscle spasms (hypocalcemic tetany)
[ 𝐶𝑎 2+ ] 𝐸𝐶𝐹 too high
Voltage-gated ion channels don´t open so easily
Nervous system is depressed
Calcium react with phosphate ions
Forming calciumphosphate stones to the blood vessels and to the kidneys