 This is a thyroid gland. Now if you look at it from behind, you will find these tiny, tiny glands on the backside of the thyroid gland, which is called the parathyroid gland. In this video, we'll see how the parathyroid gland regulates calcium levels in our blood. So let's begin. So parathyroid gland, like any gland, secretes a particular hormone, and it's called, any guesses, it's called the parathyroid hormone. I love this name. This is how everything should be named in biology. So para-thyroid hormone. What's its job? Well, its main job is to increase. So it's specifically, its job is to increase the calcium ion levels in blood. How does it do that? Well, let's see. Well, it turns out that the parathyroid, the PTH receptors are specifically found in some of the cells of your bones and your cells in the kidneys. So this is where they have the most effect. So what happens in your bones? Well, when the specialized cells of the bones receive the bind to PTH, what it does is that it basically takes the calcium from the bones and puts it back into the blood. That's all it does. So it takes calcium from the bone and puts it into the blood. And there you go. It, that increases the calcium levels in the blood. Okay, what does it do in the kidneys? Well, in the kidneys, what it does is that it asks the cells of the kidneys to reabsorb calcium from the urine. And once it reabsorbs it, well, the kidney puts it back into the bloodstream. Into the bloodstream. Again, calcium levels in the blood has increased. But it also has one more effect. It also asks the kidneys to create active vitamin D. Active vitamin D. And yeah, turns out that there's something called active and inactive vitamin D. But you may ask that, what does this have anything to do with what we're saying? Well, active vitamin D turns out that when it goes into the intestine, it actually makes the intestine absorb more calcium from the food. And in doing so, eventually, there'll be more calcium in the bloodstream. So it doesn't directly act on the intestine, but it does so indirectly. And notice in all the three cases, it has increased the calcium levels in the blood. And that's how it ensures that the calcium level comes back to normal if it was reduced. But now we may ask, what if the calcium levels in the blood has increased? What do you do? Well, the parathyroid detects that and it stops producing PTH, which means all these activities stop. But guess what? Guess what? We care so much about the calcium levels in our blood that we have another hormone. This time, thyroid gland also is going to secrete another hormone that's going to affect the calcium. This hormone is called the thyrocalci... Tonin. And I wanna just double check if that's what it is. Yep, that's the name. I got it right. I can't believe I usually don't get the bio names right. All right, anyways, thyrocalcitonin, and you know what it does? It does the exact opposite of what the PTH does. Its job is to decrease the calcium ions in blood. Decrease the calcium in blood. So now imagine we have high levels of calcium in blood. What's gonna happen? Well, one is the PTH is going to reduce because we want to reduce this activity, but not just that. Now the thyrocalcitonin is released into the bloodstream. And you know what happens when these receptors bind to these cells? It does the exact opposite of what PTH does. So over here, it asks the cells to reabsorb calcium back into the bone. It puts the calcium from the bloodstream back into the bone, reducing the calcium in the blood. What happens over here? Well, it tells the kidneys to not absorb calcium from the urine. And so this doesn't happen. Decreases the calcium in the blood. And now it might be reasonable to think that the calcitonin also stops the kidneys from releasing active vitamin D and stops that. But turns out life is not that simple. It's not just, it's gonna do everything opposite. I couldn't find any resource which talks about its effect on vitamin D, so we'll just leave it as it is. And this way, both of these hormones work together to regulate the calcium levels. So in summary, if imagine your calcium in the blood has reduced, what will happen? Well, this hormone gets secreted a lot and this hormone will not get secreted. As a result, the calcium from the bones will go into the bloodstream. The calcium from the urine gets reabsorbed and goes back into the bloodstream. It also ensures that more calcium is absorbed from the food. And what if the calcium levels are high in the blood? Well, now the PTH will not get released. That levels will reduce. The thyrocalcitonin, its levels will increase. And what does it do? The exact opposite. It ensures the calcium from the blood goes back into the bone. It ensures that no absorption, no reabsorption of the calcium happens from the urine in the kidneys. Sometimes I just can't believe that we evolved from a unicellular being into such complex, complex systems.