The physiological importance of calcium cannot be understated. We've already mentioned how calcium and phosphorus are integral components of bone. They give bone strength, but calcium also functions in nerve impulse transmission, muscle contraction, blood clotting, or blood coagulation. It is used as a very important cellular signal inside of cells.
But keep in mind that more than 99% of the calcium in your body is stored away as hydroxyapatite in the bone. So it exists as bone minerals. When you look at intracellular calcium, that is the calcium that is inside of cells, it represents less than 1% of the total.
But the majority of that is actually in modified organelles. So for example, In skeletal muscle, these modified organelles are basically modified smooth endoplasmic reticulum that warehouses or stores that calcium. So blood levels of calcium, which represent less than a tenth of a percent, is carefully regulated by a number of hormones. Now We can get, of course, calcium from diet, but that requires intestinal absorption. And even that is going to be carefully controlled, and it highly depends on the vitamin D.
Now, of course, this is the reason why milk, which is rich in calcium, is fortified with vitamins D. Also with vitamin A, but more so the vitamin D, promotes the... dietary absorption of calcium.
So if we look at two scenarios, let's say you have low blood calcium versus elevated blood calcium. We have an antagonistic control system here between a number of hormones. So low blood calcium is actually detected by the parathyroid gland and in response to this low blood calcium, the parathyroid gland will secrete a hormone called PTH. PTH has three main effects. As it relates to bone, PTH is going to target the osteoclasts, and because of that it's going to promote bone resorption, or the breakdown of bone, so we can leach the needed calcium from the bone.
The integrity of bone structure, the strength of bone structure, is secondary to the needs of the body. It's not even in the picture. PTH, as shown in this particular figure here, so here's its role with the osteoclasts. PTH also has a role in the kidneys. So in the kidneys, it's going to promote calcium.
reabsorption. So whatever calcium we filtered out, we're going to try to reabsorb that in a controlled manner in the kidneys. PTH also signals the small intestine. So in the small intestine, it's going to promote absorption. And that's where vitamin D serves as a very important coenzyme to promote that absorption of calcium.
When you have elevated calcium levels, this actually is detected by the thyroid. In fact, there are a group of cells within the thyroid called the parafollicular cells. Different from... follicular cells.
These follicular cells were actually what secrete the thyroid hormone that we mentioned in a previous video that was needed for bone growth. So these parafollicular cells will actually secrete a hormone called calcitonin. So calcitonin actually will decrease the activity of PTH. So it will no longer promote calcium absorption.
And so indirectly, it promotes calcium excretion. And calcitonin will decrease osteoclast activity. It doesn't promote osteoblasts.
blast activity, but you're decreasing osteoclast activity. So the net result is more calcium is deposited into the bone. So we focus so far on calcium, but phosphate homeostasis is going to be actually carefully linked with calcium homeostasis.
So even very minor changes in calcium can have huge effects. You can have neuromuscular problems. So if you have too much calcium, a condition we call hypercalcemia, muscles will become hyper excitable. And you can think of the cardiac muscle, if it's hyper excitable, that could lead to very inefficient pumping. If you have, excuse me, if hyper excitable, if you have low calcium levels, And if you have too much calcium, the muscles will become non-responsive.
So if you have very elevated levels of calcium within the blood vessels, you can start to have calcium crystals form in the blood vessels and in the kidneys, and that will muck up their activity. So again, let me just reiterate my correction. If you have too much calcium, This can lead to non-responsiveness of muscles. And if you have very low, you might have some hyper excitability.
So that hyper excitability specifically in cardiac muscle can lead to tetany and ultimately heart failure. So all things to consider with calcium homeostasis.