PTH, Calcitonin, Ca and Phosphate, VitD Flashcards
How much calcium is absorbed/ excreted by the intestines?
About 35% is absorbed from the intestines, and 90% is excreted in feces (with the addition of the ones secreted by GI juice)
How much calcium is absorbed/ excreted by the kidneys?
About 10% of ingested calcium is excreted via the kidneys. And 99% is reabsorbed by the renal tubules –> controlled by PTH
How is phosphate regulated in the kidneys?
If there is low phosphate in the blood, all phosphate will be reabsorbed. If there is more phosphate in the blood, the rate of excretion will be directly proportional to the increase.
PTH can greatly increase excretion
What are the 2 types of bone tissues?
- Compact (cortical)
- Trabecular (spongy) –> inner, where blood cell production occurs
Describe bone calcifcation
- secretion of collagen and proteoglycans by osteoblast cells
- uncalcified collagen = osteoid
- calcium salt precipitates after a few days, forming hydroxyapatite crystals
What’s the role of pyrophosphate?
It inhibits hydroxyapatite crystallization and calcification of bone
How is the level of pyrophosphate controlled?
regulated by molecules such as tissue-nonspecific alkaline phosphatase (breaks down pyrophosphate)
What is bone remodling?
The continuous formation of new bone by osteoblasts and breakdown by osteoclasts
What influences bone remodling?
- mechanical stress
- PTH and active Vit D (1,25(OH)2 D3) –> (+) osteoclast
- calcitonin –> (-) osteoclasts
What are the 3 steps of active Vit D regualtion?
- U/V can convert 7-dehydrocholesterol to D3 @ the skin
- D3 converted to 25-hydroxycholecalciferol @ the liver
- 25-hydroxycholecalciferol converted to 1,25(OH)2 D3 @ the kidneys, tightly regulated by PTH
What are the actions of 1,25(OH)2 D3?
- promotes intestinal absorption of calcium & phosphate
- weak effect to decrease renal excretion of calcium & phosphate
- important in bone resorption and deposition
- PTH bone resorption is diminished without it
How does PTH control extracellular calcium level?
- increased intestinal reabsorption of calcium
- decrease renal excretion of calcium
- exchange between bone and ECF
Where is PTH formed?
chief cells of the parathyroid gland
How does PTH mobilize calcium from bone?
2 phases:
1. fast phase, from existing osteocytes
2. slow phase, proliferation of osteoclasts & increased osteoclast activity
How does PTH influence renal calcium and phosphate?
- PTH decreases phosphate reabsorption at the proximal tubule
- PTH increases calcium reabsorption at the ascending loop of Henle and distal tubules
How does PTH influence intestinal calcium and phosphate?
PTH increased BOTH calcium and phosphate reabsorption at the intestines due to more 1,25(OH)2 D3 formation @ the kidneys
Where is calcitonin produced?
In the parafollicular cells in the interstitial tissue of the thyroid gland
What’s the function of calcitonin?
Opposite of PTH, though effects are much weaker
How is calcium regulated between ECF and bone?
If there is low ECF calcium:
- readily exchangeable Ca2+ diffuse into ECF
- PTH formation increases –> (+) osteoclasts
How is calcium regulated between ECF and GI?
If there is low ECF calcium:
- PTH increases, increases 1,25(OH)2 D3 –> increases calcium binding protein etc –> increased GI absorption
How is calcium regulated between ECF and kidneys?
If there is low ECF calcium:
- increased PTH –> increased reabsorption of calcium, decreases excretion
- decreases reabsorption of phosphate, increases excretion
What happens is there if a lack of PTH?
- inactive osteoclasts
- low 1,25(OH)2 D3
- decrease in GI calcium absorption
- renal calcium excretion > absorption
- calcium ECF falls below normal
- phosphate level = normal, or elevated
What happens with primary hyperparathyroidism?
- excessive PTH production
- excessive osteoclast activity –> bone breakdown most serious
- renal retention of calcium and excretion of phosphate
- increased 1,25(OH)2 D3
- ECF calcium = above normal, phosphate = low
What happens with secondary hyperparathyroidism?
ex. Rickets
- increases in PTH to compensate for lack for calcium in the diet or damaged kidneys (ie. 1,25(OH)2 D3 can’t be formed)
- leads to osteomalacia
What causes osteoporosis?
- reduced osteoblast activity, so osteoclast > osteoblast
3 main reasons:
1. lack of mechanical stress/ physical activity
2. lack of estrogen/ post menopausal
3. advanced age, other hormones and GH that contribute to bone growth diminishes greatly
Is calcium supplement effective for post menopausal osteoporosis?
nope
