Control Of Calcium Levels Flashcards
What are the main roles of calcium in the body?
- Hormone secretion
- Muscle contraction
- Nerve conduction
- Exocytosis
- Activation of enzymes
- Intracellular second messenger
How is calcium transported in the blood?
- 45% bound to serum proteins e.g. Albumin
- 45% free ionized form (ACTIVE)
- 10% complexed into low molecular weight anions e.g. Citrate, oxalate
Where is the majority of calcium stored?
Hydroxyapatite crystals in bone
Where is calcitonin released from?
C cells of the thyroid gland
Where is PTH synthesised and secreted?
Chief (principal) cells of the parathyroid gland
Where is vitamin D obtained from?
- Sunlight
- Food and supplements
What are the main actions of PTH?
- Stimulates reabsorption of Ca2+ from bone
- Stimulates Ca2+ reabsorption from the kidneys
- Inhibits uptake of phosphate from kidneys
- Stimulates hydroxylation of 25-hydroxyvitamin D3 to form Calcitriol
What are the main actions of Calcitriol (1,25-dihydroxycholecalciferol)?
- Increased absorption of Ca2+ from GI tract
- Increased bone reabsorption
- Increased reabsorption of Ca2+ from kidneys
Where are the parathyroid glands located?
- Behind the thyroid gland
- 2 superior and 2 inferior (4 in total)
Name 3 cell types you can identify in a histological image of the parathyroid gland
- Principal (chief) cells
- Adipocytes
- Oxyphil cells
What is the half life of PTH?
4 mins
How is the secretion of PTH controlled?
- NEGATIVE FEEDBACK LOOP
- Low serum calcium levels detected by parathyroid glands
- Increased secretion of PTH which increases calcium reabsorption from bone and kidneys, as well as activating vitamin D
- Serum calcium levels increase
How do the concentrations of phosphate and magnesium ions affect PTH secretion?
- Elevated phosphate INCREASES PTH secretion
- Elevated magnesium DECREASES PTH secretion
How does PTH increase reabsorption of calcium from bone?
Decreases osteoblast activity, exposing bony cavity to osteoclasts, which can break down hydroxyapatite crystals and release calcium
Why does PTH decrease uptake of phosphate from kidneys?
- Increases reabsorption of calcium
- Decreases absorption of phosphate to prevent formation of KIDNEY STONES
Which two hormones are involved in increasing serum calcium levels?
- Parathyroid hormone PTH
- Calcitriol (active vitamin D)
Why must dietary vitamin D undergo hydroxylation?
- Dietary vitamin D (D2 and D3) is BIOLOGICALLY INERT
- SHORT HALF LIFE so must be converted to 25-hydroxyvitamin D3 (half life of ~2 weeks)
How is vitamin D transported in the blood?
- LIPID SOLUBLE
- Bound to serum proteins CBP
What are the actions of calcitriol on the gut?
- Increases sensitivity of calcium receptors
- Increases absorption of calcium from the gut
How does Calcitriol affect bone turnover?
INCREASES rate of bone turnover by osteoclasts
What is the role of calcitonin?
- Little effect
- LOWERS SERUM CALCIUM
- May have a role in preserving the maternal skeleton during pregnancy
What are the signs and symptoms of hypocalcaemia?
- Increased excitability of nervous system (NM junction)
- TETANY
- Paralysis
- Convulsions
- Pins and needles
What are the signs and symptoms of chronic hypercalcaemia?
- Kidney stones
- KIDNEY DAMAGE
- Constipation
- Dehydration
- Tiredness
- Depression
What are the classic triad of symptoms of a patient displaying hypercalcaemia?
- STONES (kidney stones)
- MOANS (depression)
- GROANS (abdominal pain)
How is hypercalcaemia treated?
- FLUIDS as patient loses excess fluids in urine
- Removal of possible benign tumour on parathyroid gland causing PRIMARY HYPERPARATHYROIDISM
What is PTHrp?
- Parathyroid hormone related peptide
- Secreted from TUMOURS and acts in a similar way to PTH
- Can cause hypercalcaemia
How does rickets occur and how is it treated?
- VITAMIN D DEFICIENCY
- Calcium is not absorbed from the gut
- Serum calcium maintained AT THE EXPENSE OF BONE
- Treatment involves INCREASING DIETARY VITAMIN D
What is the relationship between control of calcium and phosphate levels?
- Calcium levels are tightly regulated but phosphate levels are not
- Vitamin D increases absorption of BOTH calcium and phosphate from the gut
- PTH increases absorption of calcium from kidneys BUT NOT PHOSPHATE
What are the main organs involved in controlling serum calcium levels?
- Bone
- Kidneys
- GI tract (duodenum and jejunum)
