 Bone organs, holy dynamic. They don't stay the same. They don't even look the same. In fact, I'm going to tell you this little fast fact right now, 20% of your skeleton, of your bone organs, is fully replaced every single year. So in five years, your body completely replaces your skeleton. I mean, don't you think of your skeleton as like, this is my permanent, like, now I have it. It's not going to grow anymore because my epiphyseal plates are closed, but it's my skeleton, and it's been my skeleton the whole time. But the tissue that's making up your skeleton, the bone tissue was literally replaced five years ago. My skeleton five years ago is not the same skeleton that's in my body right now. This is crazy. How cool is that? How does it do this? Why? You have the hint. From our last lecture on bone tissue, you know who's responsible for this dynamic nature of our bones. First of all, I'm going to tell you two processes that happen. First of all, there's bone deposition and then there's bone resorption. Okay, which is which? Bone deposition. Basically, you're building bone. You're building more bone. How, like, what exactly does that look like? Your bones get thicker, they get denser. More dense is denser a word. So what is resorption? You're reabsorbing your bone. Reabsorbing your bone tissue. Now think back, think back into the glory days of the last lecture and tell me, who does this? Who deposits, who builds more bone tissue? The blasts were the builders. That's your story, good work. Who reduced bone tissue? Remember the clasts? The clasts were the crushers. So if you have more blast activity in your bones, then you are building more bone tissue. If you have more clast activity in your bones, then your bones are going to shrink, they're going to become thinner, they're going to become more brittle, more easily breakable. Now let's think about that. Osteoblasts, building more bone tissue. Tell me true. Okay, you ready? Here's the question. Osteoblasts are making it happen. They're in there, they're building more bone tissue. And here's my question for you. A little bit of Pictionary. Uh-huh, for real, what is that? It's not a snake. That is a blood vessel. And my question for you is, what happens when osteoblast activity increases? What's going to happen to blood calcium concentration? Okay, stretch your brain. The osteoblasts are active. They're building bone. They're building bone out of calcium. Where do they get it? They get it out of the blood. Where did the blood get the calcium? It got it out of your food. And a little bit it might have gotten it out of your bones. But do you agree that if osteoblasts are active, you're actually going to decrease calcium concentration in your blood because you got to get the calcium from somewhere to build more bone tissue. What happens if osteoclasts are active? This is scenario number two. Here's scenario two. This one was scenario number one. Scenario number one. Now we're at scenario number two. If the osteoclasts are more active, osteoclasts are in there crushing your bone. Where does all the bone crush go? All the calcium that they crushed out of your bones. Where'd it go? It goes into the blood. Do you agree? Which causes blood calcium concentrations to increase. So bone resorption results in increased calcium concentration in your blood. Bone deposition results in decreased calcium concentration in your blood. Now when you get to physio, you're going to learn all the places that calcium ions in your blood why it's important. Dude, calcium is involved in nerve transmission. It's involved in muscle contraction. It's a very important ion that you better have enough of it so that you can do your thing. If you get too little, you actually start getting all tingly and funky feeling like your muscles start getting all twitchy, which is obviously not a good sign. So maintaining calcium homeostasis in your blood is really important and the osteoblasts and the osteoclasts work together to do this. Now, if the blasts are active, they're building more bone. They can actually build bone bumps. What? They can. If you have increased bone deposition, then an outcome of that can be bone bumps. And here's what I want to tell you. When a bone experiences force, when you're running and applying force to your leg bones, your osteoblasts get a message from the nervous system that say, dude, we're experiencing a lot of force here. So take the bone stronger in this area and your bones will build stronger, thicker structures in order to deal with the fact that you increased the force that they were experiencing. If, per chance, you never did any weight-bearing activity, your body will be like, why bother having all this important calcium, this important mineral in why have it in our bone? Let's use it. Let's do something else with it. And your bone actually will become thinner if you don't use it. This is why women are advised to do weight-bearing activity. Swimming is fantastic for your joints. However, if you do nothing but swim for your exercise, you are not applying enough force to your bones to keep them good and solid and thick. So you have to decide, do you want to deal with joint issues because you're doing weight-bearing activity? Or do you want to deal with osteoporosis issues because you're not doing weight-bearing activity? This is a very interesting question that everybody has to contemplate, particularly women, because of our hormones and what happens how hormones actually affect this whole thing. So if you look at your external brain today, you have a little chart, there it is right there. You have a little chart that asks you to analyze a couple of situations to figure out who's going to be more active in this scenario. Are the blasts going to be more active or are the clasts going to be more active? Definitely worth your time to think through applications of how this whole process works. When a muscle is attached to a bone, a bone bump forms, a bump for the muscle to attach to forms because the muscle is pulling on the bone. Do you agree with that? Which is applying a force to that bone. And when that force is applied to that bone, the bone says, oh, we got to make this stronger because there's a massive bicep that is pulling on this part of the bone so that part of the bone needs to be tougher and stronger and have a bigger attachment point. The rest of this lecture is going to be spent talking about the attachment points, talking about the bumps on the bones that basically we're going to use to attach muscles to. And why are there bumps there in the first place? Because the osteoblasts got the message that there is a force experienced at this location because there's a muscle that attaches there so we need to make that location bigger and thicker so we can handle the force and not break apart. Want to go learn bone bumps? Let's do it.