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- [Voiceover] Now, one of the main functions that bone 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 blood stream 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 relatively to osteoblast activity, there is an increase in the liberation of calcium and phosphate from bone into the blood stream. And remember that osteoclasts cause bones to crash. So they take the calcium and phosphate from bone and put it into the blood stream. The opposite is true if this ratio reverses. As the activity of osteoblasts increase, you get calcium and phosphate going from the blood stream 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 calcium homeostasis, this balance between calcium and phosphate in the blood stream and calcium and phosphate in the bone? Well, the main players are parathyroid hormone, calcitonin, and a third hormone called calcitriol, which is really just the active form of vitamin D. These hormones basically help to regulate the amount of calcium that is either absorbed from the gut or reabsorbed from the kidneys. And they also affect the level of activity, or this ratio, of osteoclast to osteoblast activity. Parathyroid hormone, or P-T-H, 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 and 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 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 bloodstream. 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 the kidneys. So here, going 'cross the table, we'll 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, we'll put osteoclast activity, and then intestinal or renal absorption of calcium. So 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 blood stream 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 in the 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, 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 and phosphate from the bone and putting it in the blood. Calcitonin, on the other hand, remember, tones down calcium and 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, osteoclast 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 blood stream. And remember that parathyroid hormone stimulates this process. So the effect of parathyroid hormone on osteoclast activity is to increase it. And likewise, calcitriol is going to increase osteoclast activity. And then calcitonin, because it decreases calcium and phosphate being put into the blood stream, 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 in 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 and phosphate between the blood stream and bone? Well, the concentration of calcium ions in the blood actually has very important physiological effects. So, for example, too much calcium actually leads to hypoexcitable cell membranes. 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.