Feb 25th Flashcards
99% of calcium is found in
Bones
- stably stored as hydroxyapatite
- calcium salts + phosphate provide structural integrity
1 % of calcium is found in soft tissues
- intracellular (0.9%)
- extracellular (0.1%)
extracellular calcium
- very tightly regulated
- 50% ionized (free)
- 40% protein-bound
- 10% complexed with phosphate & citrate
- extracellular fluid/plasma
- bone
intracellular calcium
- also highly regulated
- more abundant than extracellular
- largely associated with membranes in mitochondria, endoplasmic reticulum, & plasma membrane
intake vs output (calcium)
intake = diet
- 1/3 absorbed in small intestine
- absorption is hormone regulated
- recommended 1000mg/day
output = kidney
- body can’t make it s it has to be replaced
3 hormones regulate movement of calcium between bone, kidneys, and intestine
- parathyroid hormone (PTH)
- Calcitrol (vitamin D3)
- Calcitonin
1&2 most important in adults
What is the function of PTH?
Regulates blood calcium by increasing bone resorption, kidney reabsorption, and intestinal absorption.
Why can the thyroid be removed but not the parathyroid glands?
Parathyroid glands are essential for life because PTH is critical for calcium homeostasis.
How does PTH increase calcium levels?
Stimulates bone resorption, kidney reabsorption, and vitamin D activation (which increases gut absorption of calcium).
Where are the parathyroid glands first seen in evolution?
In terrestrial amphibians.
2 cell types in parathyroid glands
- Chief cells -produce PTH
- Oxyphils - function unknown
When plasma Ca2+ begins to fall, PTH acts to raise blood Ca2+ back to normal
via 3 mechanisms
- Stimulates osteoclasts to resorb bone (primary mechanism)
- Stimulates kidneys to resorb Ca2+
- Stimulates kidneys to produce enzyme needed to activate vitamin D,
which promotes better absorption of Ca2+ from food/drink across
intestinal epithelium
hypocalcaemia
- too low
- increased PTH secretion - stimulates resorption to get more calcium back into the blood
hypercalcaemia
- too high
- decreased PTH secretion
What is bone constantly doing?
Being formed and resorbed
What does bone contain?
Calcified extracellular matrix (ECM) that forms when calcium phosphate crystals precipitate & attach to a lattice support
What is the most common form of calcium phosphate in bone?
Hydroxyapatite
In what form is most Ca²⁺ found in bone?
Crystallized (as hydroxyapatite)
Is all calcium in bone crystallized?
No, a small fraction is ionized and readily exchangeable.
Bone deposition
- osteoblasts
- secrete a matrix of collagen protein, which becomes hardened by deposits of hydroxyapatite
bone resorption
- osteoclasts dissolve hydroxyapatite & return the bone calcium (& phosphate) to the blood
What does PTH stimulate the kidneys to do?
Resorb Ca²⁺ and produce 1-α-hydroxylase enzyme
What is the role of 1-α-hydroxylase?
Activates vitamin D (by converting it to its active form, calcitriol).
What does active vitamin D (calcitriol) do?
Increases Ca²⁺ absorption in the intestines.