 Okay, let's talk about bones. All right. There are two kinds of cells in your bones that we're going to deal with. And the reason why we're talking about this is because these cells are affected by calcium homeostasis hormones. And so if we need more calcium in the system, we're going to send a message to a certain kind of bone cell. If we need less, we're going to send a message to a certain kind of bone cell. So the two types of bone cells are the osteoblasts and the osteoclasts. And you know this already from anatomy. So the osteoblasts, remember, what do they do? They build bone. If they're going to build bone, what are they going to need? Calcium. So if they're super active, they're going to pull calcium from the blood and use that calcium to build more bone. Osteoclasts crush bone. This is how I remember the difference between my blasts and my clasts. It's crush things. They destroy the bone. They break it down. Now you might be like, dude, kill the clasts. Like why would we ever want that to happen? Well, there's actual remodeling of your bones that happens based on the environment that you're in, based on the stresses that you put your bones under. And there's no point in having super strong, like thick places on your bones if you're not actually using them. So your bones end up having this kind of dynamic shaping that takes place based on what you need. Well, you can imagine that if you crush bone, if you're breaking down bone, that's going to free up calcium and you're going to be able to deposit calcium into the blood. So think about this. You got low calcium. You're feeling kind of sad. Who should your hormones activate? Low calcium? Let's get more calcium into the scene, dude. Activate the clasts. Life is great. You've got tons of calcium. Way to go, rock star. You're drinking lots of milk and eating lots of broccoli and almonds and beans because those all have calcium in them, too. And now you've got tons of calcium. Who should you activate? My blasts should start building some bone so that we can store that calcium for the days when you don't get all your calcium, then you can break down your bone again. You got it? Now we're going to look at some hormones that act on blasts and clasts and other things and help send the message. Dude, we got to do something.