Calcitonin effects question

[leeney]

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I understand Calcitonin has two effects.

1. inhibits osteoclast -> inhibits bone resorbtion -> lowers blood Ca2+ level.

2. inhibits reabsorption of Ca2+ in the kidney tubule -> increased rate of its loss in urine.

If I only try to comprehend the first one, it makes perfect sense.
However, when I try to understand second one on the top of first one, it gets complicated.

So, if reabsorption of Ca2+ is inhibited, it will lower blood Ca2+ and trigger PTH which has opposite effect of number 1. -> this is the only conclusion i can come up with.

I just feel like those two effects contradict each other.
Can someone explain this to me and suggest a complete relationship between Calcitonin, blood Ca2+ level, osteoclast, and PTH?

Thank you! 🙂

 

[Rabolisk]

I'm not sure that logically makes sense. Let's say that the only effect of calcitonin was effect 1. That also lowers blood [Ca2+], so by your logic, that will trigger PTH release. But then PTH will increase blood [Ca2+], so it will trigger calcitonin. But then calcitonin lowers...

You're not going to get this sort of wild fluctuation. What will happen is that within normal homeostatic calcium levels, the activity of PTH will be more or less equal to that of calcitonin, resulting in no real change in [Ca2+]. Think of it as a steady state. If something disturbs this steady state (e.g. you ingest large amount of calcium containing food), then your body will adjust the hormone levels to regain homeostasis.

 

[GreenRabbit]

I think your confusion is from focusing too much on bones. The purpose of calcitonin is to lower blood Ca2+, not simply to strengthen your bones. Both of the effects you listed accomplish this. A third effect is that it decreases intestinal Ca2+ absorption, which again, serves to lower blood Ca2+.

PTH serves to raise blood Ca2+ levels, not just from bone, but also by acting on the kidneys and intestines with the opposite effect.

 

[golgiapparatus88]

Like Rabolisk said, it's all about homeostasis. The further you go from equilibrium, the more the opposing mechanism will work. If you urinated a liter of pure calcium, obviously PTH would being working like crazy. If you ate a 1000M calcium tablet, calicitonin would being working in overtime. Normally your body will undergo small fluctuations unless you have some specific disease.

To answer your question, if the level of X fluctuates, the restoring mechanism (hormones in this case) will be activated to return to normal levels.

 

[leeney]

I guess I was thinking it too hard!
Just considering them as factors to affect blood Ca2+ helps me.
It makes sense to see it as maintaining homeostasis when the question arises as to what triggers what.
Thank you guys so much!

 

[Rabolisk]

Like Rabolisk said, it's all about homeostasis. The further you go from equilibrium, the more the opposing mechanism will work. If you urinated a liter of pure calcium, obviously PTH would being working like crazy. If you ate a 1000M calcium tablet, calicitonin would being working in overtime. Normally your body will undergo small fluctuations unless you have some specific disease.

To answer your question, if the level of X fluctuates, the restoring mechanism (hormones in this case) will be activated to return to normal levels.

Not that this is important for the MCAT, but it is not really equilibrium. Rather, it is steady state.