Physiology of Bone and Calcium Homeostasis Flashcards
What is osteoid?
A collagen and protein mix produced by osteoblasts to which hydroxyapatite binds
What are the major preventable risk factors for osteoporosis?
Low dietary Ca
Lack of exercise
Smoking
Which cells release calcitonin?
C cells of the thyroid
How does PTH cause increased activation of Vitamin D3
Activating kidney enzymes required for activation of D3
When is calcitonin released?
Only in extreme hypercalaemia
What are the progenitor cells to osteoblasts?
Fibroblasts
The effect of PTH on Ca levels that is seen within minutes is due to what?
The increase in reabsorption in the kidneys
What are chondrocytes?
Collagen producing cells of the cartilage
What are the effect of PTH hyposecretion?
Hypocalcaemia
Hyperphosphataemia
Rare by often fatal
What are the effects of calcitonin?
Decrease Ca and phosphates by:
Decreasing bone resorption
Increase Ca excretion
Through which organ systems is the effect of PTH mediated?
Bone
Kidneys
Small intestines
Explain the process by which chondrocytes lengthen bones
Chondrocytes replicate in vertical lines
Old chondrocytes at the end of those columns die leaving space
New bone is laid down by osteoblasts in the space left by chondrocytes
Why do you get hypophosphataemia in a PTH hypersecretion state?
Because PTH blocks phosphate reabsorption (despite increase bone resorption that provides phosphate
Does calcitonin have an effect on the small intestine?
No
What percentage of total body Ca is extracellular?
0.1%
What is osteocalcin and osteonectin?
Proteins produced by osteoblasts that aid bone deposition
What are the main controllers of Ca reabsorption in the kidneys?
PTH = stimulates reabsorption
Calcitonin = downregulates reabsorption
What makes up the major non-cellular component of bone?
Calcium phosphate (most commonly hydroxyapatite) crystalising on a collagen matrix
What do osteoclasts produce to dissolve bone matrix?
H+ and enzymes
Where in the kidney is phosphate reabsorbed?
Proximal tubule
On which aspect of a bone is new bone depositied?
The outer aspect
Is PTH or vitamin D3 the mediator of increased Ca absorption in the gut?
Vitamin D3 - but PTH helps activate vitamin D3
What effect does PTH have on phosphate levels?
Decreases plasma phosphate
What are the major signals for Ca acquisition from bone?
PTH
Calcitriol
Cortisol
What effect does PTH have on Ca absorption in the small intestine?
Increase indirectly increases it via activating vitamin D3
What is PTH secreted in response to?
Low Calcium
Calcitriol is another name for what?
Vitamin D3
Describe the appearance of osteoclasts
Large, multinucleated, mobile cells
Why isn’t blood Ca low in vitamin D deficiency?
Because Ca levels are maintained by PTH stimulated resorption of bones - therefore you get bone demineralisation
What are the main functions of Ca?
Intracellular signalling
Structural support
Cofactor in blood coagulation
Required for normal nerve excitability and function
