Parathyroid gland, calcium and phosphate regulation Flashcards
Where is calcium mostly stored?
Sequestered in bone
What form is calcium sequestered to bones?
Hydroxyapatite crystals
What is the purpose of the skeleton?
- Structural support
- Major reserve of calcium
- Helps to buffer serum levels
- Releasing calcium phosphate into interstitium
- Up taking calcium phosphate
What is the normal serum calcium level (range)?
2.2-2.6 mM
What are the three interconvertible fractional forms of calcium in plasma?
- Ionized
- Protein bound
- Complexed (with low-molecular-weight inorganic ions such as citrate)
List some uses of calcium in the body?
- Build and maintain bones and teeth
- Regulates heart rhythm
- Assists in normal blood clotting (factor 4)
- Help maintain proper nerve and muscle function
- Needed for activity of some enzymes
- Required for nerve transmission
What are the 3 hormones involved in the regulation of calcium serum levels?
- Parathyroid hormone
- Calcitriol (1,25(OH)2D)
- Calcitonin
How much calcium does an average 70kg man contain?
~1000g
How much calcium is excreted a day for an average 70kg man?
~1000g (825 from faeces and 175 from urine)
What effect does PTH have on serum calcium levels?
Increases
What is the half life of PTH?
4 minutes
Where is PTH synthesised?
PTH is CONSTANTLY synthesised in chief cells of parathyroid gland (also constantly degraded)
What are the 3 target organs of PTH and their subsequent effect?
- Bone = increased resorption
- Intestine = activates vitamin D thus increase transcellular uptake from GI (Vit D to Calcitriol)
- Kidneys = increased Calcium reabsorption (less lost from urine)
Describe the mechanism of PTH on bones
- induces osteoblastic cells to activate osteoclasts and protect them from apoptosis (through cytokines)
- PTH decreases osteoblast activity exposing bony surface to osteoclasts
- Reabsorption of mineralized bone and released of Pi and Ca 2+ into ECF
List sources of vitamin D
- Body (exposure to sunlight)
- Cheese
- Butter
- Fortified milk and cereals
- Fish
Describe how vitamin D is activated
- Vitamin D3 = precursor
- Hydroxylated in the liver
- Hydroxylated in the kidneys
What effect does calcitriol have on calcium serum levels?
Increase serum calcium by increasing absorption in the gut
What effect does calcitonin have on calcium serum levels?
Decrease
Where is calcitonin synthesised?
C cells in Thyroid gland
What is severe/chronic hypercalaemia?
Abnormally high serum calcium levels exceeding 3mmol/L
What are symptoms of hypercalaemia?
- Renal calculi
- Kidney damage
- Constipation
- Tiredness
- Depression
How can polyuria exacerbate hypercalaemia and what are the consequences?
Dehydration
- Lethargy
- Weakness
- Confusion
- Coma
- Renal failure
What is the usual treatment for hypercalaemia?
Rehydration
What is the main aetiology for hypercalaemia?
Malignant OSTEOLYTIC bone metastasis
-common cancers that metastasise to bone causing lytic lesions: breast, lung, renal, thyroid
Where are common sites for bone metastasis?
- Vertebrae
- Pelvis
- Ribs
- Proximal femur and humerus
- Skull
What is hypocalcemia?
Condition where serum calcium is abnormally lower than the normal range
What does hypocalcemia lead to and what are the symptoms?
Hyper-excitability of neuromuscular junction
(low serum calcium causes increase Na+ entry to neurons=depolarisation)
-Pins + needles
-Tetany (muscle spasms)
-Paralysis
-Convulsions
-Carpopedal spasm
What is hyperparathyroidism?
Secretion of too much parathyroid hormone
What are the two types of hyperparathyroidism?
- Primary = 1/4 glands develop an adenoma and secrete excessive PTH
- Secondary = all 4 glands become hyperplastic (vit D deficiency)
