骨骼肌内分泌控制

now one of the main functions at buon performs is as a storage source of calcium so now you know why your mom told you that you needed plenty of calcium probably in the form of a tall glass of cold milk so that you could form strong bones so calcium homeostasis or the flow of calcium between the bloodstream and bone is actually under endocrine or hormonal control and these hormones these endocrine hormones actually alter the ratio of osteoclast activity to osteoblast activity so as osteoclast activity increases relative to osteoblast activity there is an increase in the liberation of calcium and phosphate from bone into the bloodstream and remember that osteoclast caused bones to crash so they take the calcium and phosphate from bone and put it into the bloodstream the opposite is true if this ratio reverses as the activity of osteoblasts increase you get calcium and phosphate going from the bloodstream back into bone so that's the gist for calcium homeostasis now what are the main players the hormones that are responsible for maintaining this help calcium homeostasis this balance between calcium and phosphate in the bloodstream and calcium and phosphate in the bone while the main players are parathyroid hormone calcitonin and a third hormone called calcitriol which is really just the active form vitamin d these hormones basically help to regulate the amount of calcium that is either absorbed from the gut or reabsorb from the kidneys and they also affect the level of activity or this ratio of osteo class to osteoblast activity parathyroid hormone or PTH and calcitriol have the same overall effect of increasing calcium and phosphate in the blood with calcitonin having the opposite effect it decreases the amount of calcium phosphate in the blood and the way I remember this is that calcitonin actually tones down the calcium in blood and so as you can see there are a couple of general themes here the first being that each time calcium increases in the blood you have a concurrent increase of phosphate they kind of go together they're buddies and the same thing for if calcium decreases in the blood phosphate will decrease the other thing to kind of keep in mind is that as calcium or phosphate increases in the blood there is a concurrent decrease of these two ions in bone it has to be coming from the bone if it's going to be increasing in the blood and the opposite is true for whenever calcium and phosphate is decreasing from the blood stream it's decreasing because it's being placed back into bone deposited in the form of bone so one thing that I think would be particularly helpful is if we draw out a table and go through how each of these hormones affects the cellular activity of osteoblasts and osteoclasts and also calcium absorption from either the intestines or kidneys so here going across the table will put parathyroid hormone calcitonin and calcitriol and then we'll put each of the things that is affected by the various levels of these different endocrine players osteoblast activity will put osteoclast activity and then intestinal or renal absorption of calcium so if first let's look at osteoblast activity recall that osteoblasts build up bone as their activity increases you have calcium and phosphate taken from the bloodstream and deposited into bone now keeping that in mind and also realizing that parathyroid hormone helps to increase calcium and phosphate in the blood in other words it breaks down bone to put calcium phosphate into blood we would expect osteoblast activity to decrease as parathyroid hormone increases so that's the effect parathyroid hormone has on osteoblast activity it decreases it and calcitriol remember it has the same overall effect that parathyroid hormone has it increases calcium and phosphate in the blood by taking it from bone so as calcitriol increases you would expect osteoblast activity the activity of building bone to decrease because you're taking calcium phosphate from the bone and putting it in the blood calcitonin on the other hand remember tones down calcium phosphate in the blood it takes these ions and puts it back into bone and so as calcitonin increases we would expect osteoblast activity to increase because more bone is being formed from calcium and phosphate being taken from the blood and you'll see this theme of parathyroid hormone and calcitriol having the same effect on either osteoblast activity osteoclasts activity or the absorption of calcium because they have the same effect of increasing calcium and phosphate in the blood so next let's go to osteoclast activity this is basically going to be the opposite of whatever osteoblast activity is because remember osteoclasts crash down bone they take the calcium and phosphate and put it back into the bloodstream and remember that parathyroid hormone stimulates this process so the effect of parathyroid hormone on osteo a class activity is to increase it and likewise calcitriol is going to increase osteo class activity and then calcitonin because it decreases calcium phosphate being put into the bloodstream is going to decrease this osteoclast activity so now we're on to intestinal and renal or kidney calcium absorption knowing that parathyroid hormone leads to an increase in calcium and phosphate in the blood we would expect parathyroid hormone to lead to an increase in the ability of the intestines and kidneys to absorb calcium and we know that calcitriol has this same effect calcitonin on the other hand we remember tones down calcium in the blood so we would expect calcitonin to decrease the rate of calcium absorption via the intestines and in the kidneys so why does the concentration of free calcium ions and the blood matter so much why does our body have this elaborate system of calcium homeostasis with these various hormones and altering the concentrations of calcium phosphate between the bloodstream and bone well the concentration of calcium ions in the blood actually has very important physiological effect so for example too much calcium actually leads to hypo excitable cell membrane and this results in lethargy fatigue and memory loss whereas too little calcium in the blood actually leads to muscle cramps and convulsions so as you can see the endocrine system plays a pretty critical role in calcium homeostasis through its effect on bone remodeling and its effect on calcium absorption