 Calcium. I want you to pause. This just started. Pause and make a list of all the places where you know calcium is a critical player. Holy moly. Like, I mean, I just feel like, wow, we've seen calcium so many times in so many different places. So see if you can make a list while I make you my list after I make you my list. I'm going to start my list. I hope you stopped and made your list first. Calcium. It is involved in all muscle contractions, right? Remember how it was involved in cardiac muscle contractions and skeletal muscle contractions. And I can tell you that it's also involved in smooth muscle contractions, even though we didn't learn about that. Calcium. Doggies. We just learned about the role of calcium in blood clotting. Indeed. How about, I mean, this is probably the coolest, I don't know, maybe it's just the most memorable to me, synaptic transmission. All of our neurotransmitters were exocytosed because of calcium. In fact, that's something that we see frequently that calcium stimulates exocytosis. How does it do it? It causes some little receptor that it binds to and bubbles of stuff are exocytosed into the environment. Calcium is involved in, I mean, we just looked at it, in insulin production and insulin release. The calcium caused the exocytosis of insulin. If you have low calcium, hypo-calcemia, low calcium involved or resulted in a depressed nervous system. So things slowed down, you're not as reactive. If you have hyper-calcemia, you had increased nervous system reactivity. So you get twitchy and you get wonky. It doesn't take much to get you out of calcium homeostasis. Now, calcium is actually stored in your system, so this is important, and you have three places where you're going to find calcium. First of all, you're going to find it in cells, in the intracellular fluid. And usually when it's in a cell, it's like bound up somewhere. So it's saved. Remember in our muscles, it actually stored it in a sarcoplasmic reticulum and used it to generate a concentration gradient that would allow muscle contraction to take place. What this means is that you actually don't end up with much of it in the intracellular fluid, but ultimately about 1% of all your calcium can be accounted for in the intracellular fluid. This is crazy. 0.1% can be found in your extracellular fluid. I mean, that's a tiny amount of calcium, and yet that still is more calcium, enough calcium to create a concentration gradient so that calcium will rush into many cells enabling exocytosis. So that's kind of wild. And then the remaining part, where's the rest of calcium in your body? The 99 minus 0.1% of your calcium is found in your bones. And your bones are not like, oh, it's not like you used concrete to make your bones. You used the calcium to make the concrete that went into the building and that's the end of the story. Bones are considered a reservoir for calcium. So calcium can go into the bones and make them all buff and strong, but then you can take calcium out of the bones if you need it. And maintaining the proper concentrations of calcium is so important that your body's like, dude, bones, shmones, we need to make sure that we've got enough calcium in our system. We're going to steal it from the bones if we have to. So let's remind ourselves of a little bit of how bones deal with calcium. And then we'll talk about some hormones that affect calcium homeostasis.