Now we're going to discuss what happens when calcium blood levels fall too low. Now, if they're falling too low, what do you think the body's response is going to be? If you said do something to bring it up, you are correct. Since we're reversing the action, we're bringing it back to normal, what type of feedback does this represent? Negative is correct too.
So remember that calcium levels should be between 1. 9 and 11 milligrams per deciliter. So anytime that we get near this physical limit, the physiological limit of 9, your body's going to start taking immediate action. And in all honesty, your body's always monitoring. Remember, the feedback cycle is what monitors, and it's cyclic, so it's constantly occurring. Now, Once it gets to that low number, to where the body believes it should take action, we basically have disrupted our controlled condition.
The controlled condition was your blood calcium level. Our calcium levels are too low. What is going to detect that the calcium levels are too low? You got it, the sensory receptor. Now, the sensory receptor in this case is going to be different than it was when calcium levels were too high.
When calcium levels were too high, the sensory receptor was the thyroid gland. Now, the sensory receptors are going to be the parathyroid gland. They're going to be specific cells that are in the parathyroid gland.
Now, the parathyroid gland is going to be these little four kind of green dots on the back of the thyroid gland. The only relationship is position between the thyroid and parathyroid glands. They do not...
function dependently on each other. Now, when the parathyroid gland cells detect that our blood calcium level is low, they're going to send out a chemical signal, and we know this signal is going to be the input. The chemical signal they send out is known as cyclic AMP.
Now, When the PT gland cells detect a low blood calcium level, they're going to start increasing their production of cyclic AMP, also known as CAMP. The increased production of this chemical is what the control center detects and how it evaluates the situation. So when cyclic AMP is increased, the control center detects this. The control center is also the parathyroid gland cells, but it's more specifically a gene in the parathyroid gland.
There's a gene in the cells that's known as the parathyroid hormone gene. And in the presence of cyclic AMP, this gene gets turned on. So it's basically like a light switch.
When you... Have a signal the light switch turns on. When you don't have a signal, the light switch is off.
So you can think about the clapping light switches. You clap your hands, it turns on. You clap your hands again, it turns off. When cyclic AMP is present, the parathyroid hormone gene gets turned on. Now, cyclic AMP is always present a small amount, so it takes an increased production to turn this gene on.
Now, once the gene gets turned on, it's going to start to produce I'll give you one guess. You got it. The parathyroid hormone, known as PTH.
Now, the parathyroid hormone is going to be the output. And we know that our blood levels are too low, so our goal is to bring them back up to what's considered normal. So a way to remember what PTH does, parathyroid hormone does, is parathyroid hormone picks it up, picks up blood calcium levels.
So it's all about a play on words. So think alliteration, parathyroid hormone, PTH, picks it up. Remember that calcitonin tones it down. So calcitonin was the opposite.
Now the output, which is a chemical, is going to target the effectors. Now remember that our blood calcium levels were too low, so we need to increase our blood calcium levels. Now what effectors can be helpful in doing this? Think about bone cells.
Hey, our blood level calciums are too low because the calcium blood level is too low. So what bone cell would help us increase it? If you said osteoclasts, you are correct. Osteoclasts are one of the effectors. When osteoclasts are targeted, they will increase bone resorption.
Basically, they keep breaking down bone. Bone contains a lot of calcium, and that calcium gets placed into the bloodstream. Now, PTH also... Osteo... If our blood calcium is low, we do not want to take what little calcium is in the blood and put it into burns.
So we're basically going to inhibit osteoblast activity. And it's kind of the same thing. So we're going to take a little bit of calcium and put it into burns.
Once it's inhibited, it doesn't totally stop, but its function is going to decrease, and we're going to increase osteoclast activity. The problem is bone cells can't do this enough on their own, so we have to rely on other structures in the body. And in this case, we're also going to rely on the kidney. Now, our blood calcium levels are too low, so we don't want to be getting rid of any calcium in our urine.
So the kidney, in this case, is going to retain calcium. It's going to make sure that we don't filter it out and get rid of it within our urine. Now, the kidney actually has another job.
In the presence of PTH, the kidney is also going to produce a hormone called citriol. And you may remember this in Chapter 5. It is the active form of vitamin D. So once PTH targets the kidney. The kidney produces calcitriol, and calcitriol will target the GI tract.
So what we have to remember is the control center gets turned on in the presence of CAMP. The control center is the parathyroid hormone gene. That gene gets turned on. And it's going to release more PTH. PTH will target the effectors.
It will slow down osteoblasts, increase osteoclast activity. And it will tell the kidneys to do two things. One, it's going to tell the kidneys to hold on to calcium. The second thing it's going to tell the kidneys to produce calcitriol.
So calcitriol, the active form of vitamin D, is going to be released in the presence of PTH. Calcitriol will target the GI tract and when it targets the GI tract it tells the GI tract to increase absorption of calcium from your diet. Now if we are putting more calcium back into the bloodstream by osteoclast breaking down bone.
If we're having the kidneys hold on to calcium in the blood instead of urinating it out. And we're having the kidneys produce calcitriol, which targets the GI tract to increase calcium absorption. We're putting more calcium back into the blood.
In this particular case, with all that calcium going back into the blood, what should happen? You got it. We should increase our blood calcium levels. As our blood calcium levels increase, we're going to be heading back to normal.
As we head back to normal, bring it back up to what's considered normal. We're reversing our change. So what type of feedback is it?
You got it? It's a negative feedback. So let's go over this again in a different diagram. We can see that your blood levels with calcium should be between 10 and, or 9, between 9 and 11. In this case, it's showing you 10. But what happens when this is disrupted?
The sensory receptors are going to detect that. So we fall below. what's normal or we fall near what's normal. And what are the sensory receptors in this case? If you said the parathyroid cells, you got it.
So here's our gland and the sensory receptors are cells in this gland. When those cells detect it, they're going to release a signal known as input. That signal is, you got it, camp. Camp is going to target the parathyroid hormone gene which is in the cells and when it targets that gene increased amounts of it causes that gene to turn on and that gene is going to release more pth now pth is going to go target the effectors and in this case we're going to have three major effectors we're going to have bone cells what are your two bone cells Which cell builds bone?
You got it, blast. And which cell breaks down bone? You got it, class.
So we have osteoblast and class. Well, remember, our calcium blood levels are low, so we're going to tell osteoclasts to increase activity. That way, we can break down more bone and add more calcium.
to the blood. We're going to tell osteoblasts to inhibit or slow down activity. We don't need to take what little calcium you do and store it.
We're also going to target the kidney and tell the kidney to retain calcium or hold on to it. And we're going to tell the kidney to release calcitriol, the active form of vitamin D. which targets the GI tract. It tells the GI tract, hey, let's hold on to this. Let's absorb more calcium and put it into the bloodstream.
So we stimulate calcium release from bone. We uptake calcium in the kidneys, basically. We're retaining it. And we release calcitriol, the active form of vitamin D.
All of these actions are going to do what? You got it. We're going to increase our blood calcium level until it rises to what's considered normal.
So you should definitely take some time and talk about this, write it in your own story. So this is showing us our stimulus. What is going to get us from this step of recognizing that we have low blood calcium level to the output when we release the pH? Okay, so let's talk about the steps in between here. So what is going to detect this?
What is our receptor? If you said parathyroid cells, you got it. Once the receptor detects it, it's going to release input. What's the input? You got it.
Cyclic ink. Cyclic amps released and in high amounts of cyclic amp, it's going to target your control center. What is the control center in this case?
The parathyroid hormone gene. In the presence of high amounts of cyclic amp, the PTH gene turns on and it causes output release, which is... PTH.
Now remember our blood calcium level is low so PTH has one major job. PTH will help us pick it up. Alliteration. Play on words.
So we're going to pick up our blood calcium levels. How does PTH do this? PTH is going to target different things.
It directly targets your bone cells. So we're going to take calcium from your bone and put it into the blood. What cells take calcium from bone and put it into blood?
You got it, osteoclasts. It's also going to tell bone cells to stop building bone, so it also tells which bone cell not to build. You said blasts, you're correct. So no blasts, decrease blasts, increase blasts.
PTH directly targets the kidney. It's going to tell the kidney to do two things. The first thing it's going to tell the kidney to do is, hey, hold on to that calcium.
Don't hang it out. We want to keep calcium in our blood. The second thing it's going to tell the kidney to do is, hey, let's release vitamin D.
What's the active name of vitamin D? You got it. Calcitriol.
Calcitriol targets your intestines. And it tells your intestines to increase absorption from food. As we're increasing all of this calcium intake, what should happen to your blood levels?
If you said they should increase, you got it. Remember blood levels? PTH picks it up.
We go up for PTH.