So let's do a quick review and recap the two lectures, calcium levels being too high and calcium levels being too low. So the stimulus is there's some disruption that causes your calcium levels to change. Remember, calcium levels should be between 9 and 11 milligrams per deciliters. So let's go over when calcium levels are too high. Remember a little trick.
If it's too high, we want to drop it. Down we want to bring it back down to normal anytime we go from something that changes back to normal We're reversing the change Awesome, so what is going to detect that calcium levels are too high? Your options are going to be the glands in your throat. So is it? Thyroid or parathyroid gland?
You said thyroid. You are correct. Specifically, those cells in the thyroid gland are cells. Easier name, though, is C-cells. Not the copyrighted C, but that's what showed up.
Now, those C-cells detect that calcium levels were too high. So the C cells are going to send input. The input in this case is going to be a chemical signal, and that chemical signal is actually going to alert the control center. So it's targeting the control center. In this case, the control center is going to be C cells again, but it's a specific gene within these cells.
This gene... It's going to be turned on, and when the gene gets turned on, it's going to produce output. Now, the output in this case is a very specific hormone. So remember, hormones, chemicals, those are the endocrine system. This output is going to be released, and this output is going to help us bring our calcium levels down.
We're going to tone it down. So what is our chemical output? If you said calcitonin, you are correct.
Calcitonin tones down the calcium levels. It brings it down, it cuts it out. So remember, play on words are important.
It helps you remember situations. So calcitonin tones it down. What are we toning down? We're toning down calcium levels in the blood.
Calcitonin tones down calcium levels. Now the calcitonin is going to go target effectors. We're going to have three major effectors. So our calcium levels are too high, so let's think about the cells that are the effectors. Which bone cell do we want not to break down new bone anymore so we're not putting more calcium into blood?
If you said osteoclast, perfect. So we're going to tell osteoclast to increase or decrease. You got it.
We're going to decrease their activity. We want to stop putting more calcium from bone into the bloodstream. Now, osteoclasts are not going to be able to work. their own to fix our calcium levels being too high. So, calcitonin is also going to help by telling another bone cell to What cell is going to take calcium from your blood stream and put it into bone?
Osteo, you got it, blast. They're going to take calcium from the blood and place it into the bone. So this way, what we're doing is we are taking some of that high amount of calcium from the bloodstream and we're storing it elsewhere. Your bone is like a buffer system.
Now, what is the next effector? Because the cells can't do this on their own. You got it.
The next effector is the kidney. The kidney plays probably the largest role. Is our calcium levels are too high in our bloodstream, so is the kidney going to hold on to the calcium levels, or is it going to filter it and excrete it? You've got to excrete calcium in the urine.
By doing this, we're ridding calcium in three ways. So we're helping control the problem in three ways. Now what's going to be the result if we're putting more calcium back into the bone, if we're losing calcium via urine?
Yep, our calcium levels should go to normal. It's going to normal, going back down. You got it.
They should drop. So they should decrease. Lots of different ways that you can describe it. Since we're going back down to normal, it is a negative feedback system.
Now, calcium levels being too high are only one of the disruptions. What if it's the opposite? What if our disruption in our stimulus was that calcium levels were too low?
If they're too low, what are we going to want to do? If you said increase calcium levels, you're right. So we're going to do the opposite. If our goal, if our level are to be increased, then...
That means we're going back to normal. If we're going back to normal, what type of feedback is it? We were too low, and now we're going back up.
We're reversing that change. You got it. You said negative. You're correct.
So how do we get from too low back to normal? Well, we have to go through the feedback cycle and talk about the components. So calcium levels being too high in the blood were detected by thyroid gland cells.
So what's going to detect calcium being too low in the blood? If you said parathyroid cells, you are correct. These cells are going to detect that our blood calcium levels are too low. Once they detect it, they're going to send input.
Now, input in this case is a chemical. It kind of sounds like you're going out on the road. If you said cyclic amp, you're right. You can call it camp. And when calcium levels are low, the parathyroid cells will start to increase the production of cyclic AMP.
An increased production of cyclic AMP is going to target the control center. And what happens is there's a special gene inside of the cell called the parathyroid hormone gene. And this is in the cell. This gene, in the presence of high amounts of AMP, will be turned on.
Now, if cyclic AMP is present in high amount, then the PTH gene gets turned on, and the PTH gene is going to produce a chemical for output. What do you think that output chemical is? If you said parathyroid hormone, you are correct. PTH. Now what does PTH do?
PTH's goal is to help us pick up calcium levels in the blood. So remember, it's a play on words. PTH picks up calcium levels in the blood.
Why calcitonin tones calcium levels down. Any trick that you can use to help you is important. Now when PTH is released, it's going to go target the effectors.
The effectors are the same, but their actions are different. So PTH is going to go target osteoclast, osteoblast, and the kidney. Now when PTH targets osteoclast, think about what our problem is.
The problem is that our calcium levels are too low in the blood. So what are osteoclasts job? If you said, hey, they break down bone, that's great.
And we need to get more calcium into our blood. And that calcium is going to come mostly from bone. So we're going to tell osteoclasts to increase activity.
This way, when they increase activity, they are taking calcium. stored in the bone and placing it in the blood. But osteoclasts cannot do this on their own.
Osteoclasts are also going to have to work with other effectors. So the other effector is going to be osteoblasts. Now, our problem is that our blood calcium levels are too low, so we're not going to want to build new bone because that takes calcium from the blood. So we're going to use osteoblasts to do that. And that way they're going to stop.
Taking what little calcium we have in the bone. And not build any bone. But again, as I said, when our calcium levels are too low, are too high, these cells can't do it on their own. So we have to rely on the kidney.
Now, if our calcium levels are too low, we don't want to get rid of calcium. In our urine, we want to hold on to it, so the kidney needs calcium. It is not going to excrete any. But the kidney has two jobs. In the presence of PTH, the kidney not only retains calcium, but it releases another hormone.
That hormone is the active form of vitamin D known as calcitriol. Now what does this hormone do? This hormone is going to target, you got it, the GI tract.
And calcitriol tells the GI tract to increase absorption of calcium from food. So now we are putting calcium. from the bone into the blood by using osteoclasts. We're holding calcium in our blood by not excreting it through urination. And we're releasing calcitriol, which targets the GI tract, and we're taking calcium from our food in higher amounts.
All of this is going to help us increase our calcium levels. So our calcium level is going to go back up. Since it's reversing the change, we're going from low back to normal.
It is a negative feedback. So just to recap, This is an overview of how the two calcium homeostasis feedback cycles work. What happens when it's too high and what happens when it's too low.
When it's too high, calcitonin tones it down. When it's too low, parathyroid hormone picks it up. PTH also tells the kidney, hey, it's time to release calcitriol.
So PTH and calcitriol work together to bring your levels back up, picks it up. CT, calcitonin, works to bring it down, tone it down.