Control of Bone remodeling - hormonal controls

What is calcium's importance to the body

Maintaining extra cellular fluid calcium levels within homeostatic levels is absolutely critical for maintaining the resting membrane potential of all cells

What happens when calcium levels are not at normal levels

Nerves cannot fire as needed and muscles are unable to contract

How much calcium is in the human body

1200-1400 g; 99% is in bone minerals, 0.9% inside cells, and 0.1% in extra cellular fluids

What do hormones use the vast amount of calcium for

A storage bank from which they can make withdrawals (resorption) or deposits as needed to maintain homeostatis balance

What is the normal blood calcium range

9-11 mg per dl (100ml) of blood

How is calcium absorbed

Through the intestines under the control of the active form of vitamin D, calcitriol

What is calcitriol

active form of vitamin D

What do the hormonal controls primarily involve

The parathyroid hormone (PTH), produced by the parathyroid glands

What happens when Ca2+ levels in the blood are decreased

PTH is released

What does the release of PTH stimulate

Osteoclasts to resorb bone which releases Ca2+ into the blood

What else occurs from the release of PTH

Because it is indirect, other cells respond to PTH secretion by producing another protein, RANK-L

What does RANK-L do

It stimulates the formation and activity of osteoclasts

What occurs as blood Ca2+ concentration rises

PTH hormone declines, which causes reverse effects and Ca2+ levels to drop

What happens if blood-calcium levels get too low

The osteoclasts with resorb so much bone that bone becomes demineralized and begins to develop holes

What is calcitonin

A hormone produced by parafollicular cells of the thyroid gland and helps to regulate Ca2+ blood levels as well

Does calcitonin, at normal levels, play a large role in humans

No, its effects at normal levels are negligible

When does calcitonin play a role in humans

When pharmaceutically administered at high levels it can lower blood calcium levels temporalriy

What other hormones stimulate bone remodeling

Glucocorticoids (hormones from the adrenal gland) and vitamin D directly stimulate osteoclast activity by increasing the synthesis of RANK-L and decreasing the synthesis of its antagonist

What is RANK-L's antagonist

Osteoprotegerin

How do sex hormones impact RANK-L

They can increase the synthesis of RANK-L antagonist, osteoprotegerin