Anatomy of a Long Bone

0:00Hey, when people think of bone, a lot of times we can think of just a solid white structure that's just uniform throughout, no holes in it, no cavities, none of that sort of stuff. But in reality, bone is living tissue. It's got different types of marrow. It's got arteries and veins. It's got all kinds of stuff that living tissue has. So in this video, we're going to take a look at the anatomy of a long bone, see what all it's made of, and see what each part does. Let's jump to the whiteboard and get started. So let's start off with an outline of a bone, and this is going to be a long bone. In particular, we're looking at an outline of the femur. But all the parts that we go through exist in any of the long bones in our body, such as the humerus, or radius, or ulna, and a lot of this exists in other bones like the skull and the pelvis and all of that. Just not the exact same layout as the long bone here. Now there's three main regions of the bone. The middle or the main part of the shaft of the bone is called the diaphysis, and on either side of the diaphysis we have something called an epiphysis. The epiphyses are on either end, and the diaphysis is the long shaft of the bone. Now we have a name for each epiphysis depending on which side of the bone it's on. So the part of the bone that's closest to the either the shoulders or the hips or the point of attachment we call the proximal epiphysis. And in the case of the femur here, this proximal epiphysis will form a joint with your pelvis to make your hip joint. And the part of the bone that's the farthest from that point of attachment is the distal epiphysis. So in the case of the femur here, this would be connecting with your knee. Your kneecap or patellar bone would be right in there, and then your tibia and fibula would be distal to that. So we have the diaphysis, which is a long shaft of the bone, and then we have the two ends, which are the proximal epiphysis, closest to the point of attachment, and the distal epiphysis, farthest from the point of attachment. Now, on either end of the bone, we have something called the articular cartilage, and the articular cartilage is going to be a layer of cartilage that covers the end of the bone where it forms a joint with another bone. Our bones are very rough, and so if we had bones rubbing against each other, that would cause a lot of friction. That would cause osteoarthritis. That's what happens whenever this cartilage breaks down and the bones are rubbing against each other. So the whole purpose of the articular cartilage here is to form a smooth, low friction area, so that whenever we move our joints; it doesn't rub our bones together and cause pain and damage to the bone. And so we see the articular cartilage here, where it forms a joint with our hip, and we see the articular cartilage on the distal end, where it forms a joint with our kneecap or patella and our tibia. Up next, we have red bone marrow. Now, if you look at this, it looks like spots all over here, but it's really more like a sponge. So think of the inside of our epiphyses as sort of the sponge-like area. Then all the holes of the sponge are filled with red bone marrow. Now, when people think of bones, they usually think of structure and protection, like our skull protecting our brain and stuff like that. But our bones are also in charge of making all of our blood cells. All of your red blood cells, which carry oxygen and carbon dioxide throughout the body, and all of your white blood cells, which help fight off pathogens and bacteria and viruses and things like that, are all made in the red bone marrow in your bones. In a long bone, we find this red bone marrow in either epiphysis, but we also find bone marrow in our skull and our sternum and our pelvis and lots of other bones as well. So our red bone marrow makes all of our blood cells. Now, if you notice here, where I've got the red bone marrow drawn, there are these areas where there isn't any red bone marrow. For example, right here between those sections, or right here we have one, and then finally we have one down in here, and those are called epiphyseal plates. Epiphyseal plates are the location where your bone will grow as it grows longer. So when you were a kid and your bones were growing longer as you get taller and stuff, these are the areas where your bone cells were producing more calcium deposits in order for your bones to grow longer. They're not just growing longer throughout; they're growing longer on these ends along these epiphyseal plates. Now, once you reach the point where you're not growing any taller, these epiphyseal plates will kind of start to fade and form what we call an epiphyseal line. Doctors can actually look at X-rays, and as that line or epiphyseal plate starts to fade, that's when they know that okay, this person's not going to be growing anymore because that epiphyseal plate is no longer depositing new bone material to help the bones grow longer. And so, in the case of the femur, we've got a few of those: one right there, one right there, and then one down on this end. But depending on the bone, those epiphyseal plates will be arranged slightly differently. In addition to red bone marrow, we also have yellow bone marrow. Yellow bone marrow is going to run down the length of the bone or the diaphysis, and it's going to fill what we call the medullary cavity. A cavity is just a hollow space inside of the body, and so this is a hollow space inside of the bone, but it's filled with this yellow bone material. Now, a common misconception is that red bone marrow makes red blood cells, and yellow bone marrow makes white blood cells, which makes a lot of sense because we've got two colors of blood cells, two colors of bone marrow, but it's wrong. The yellow bone marrow doesn't make blood cells at all. Your white and your red blood cells are made in the red bone marrow. The yellow bone marrow has a different function, and that's going to be the storage of fat and nutrients to help the bones. So, red bone marrow makes blood cells. Yellow bone marrow, fat storage. Now, there's a couple exceptions to that. When you're first born, you actually only have red bone marrow, and by the time you reach the age of seven or eight, about 50% of your red bone marrow has converted into yellow bone. Marrow to work as fat storage and not to produce more blood cells. But if you ever get really badly injured and you're suffering lots of blood loss, and suddenly your body needs to make blood cells as fast as possible because you're losing blood, the yellow bone marrow can actually revert back to red bone marrow to increase the rate of production of blood cells to try to keep you alive. So red bone marrow makes blood cells, yellow bone marrow fat storage. But in times of emergency, it can convert to red in order to make blood cells. Now, there's two types of bone that we find in our long bone here. One is spongy bone, and I already talked about the red bone marrow sort of filling the area within the sponge. Well, we call this bone that's surrounding that red bone marrow spongy bone. The other type of bone is called compact bone, and the compact bone is going to fill anywhere in this diagram where it's just white space. So this is all compact bone in here, around here, along the epiphyseal plate is compact bone, and it's really important that we have both types of bones. So let's talk about the function of each. Compact bone is all about making our bones really strong. For as light as our bones are, they don't break that easily. Now you may have broken a bone. I've broken some bones before, but in general, our bones are really strong and they're really light. Compact bone is about making it strong. It's going to be dense bone, closely packed bone material, so that it doesn't break very easily. Spongy bone, on the other hand, has two main functions. One is well, we need a place for all of the red bone marrow to live, so we have to have that spongy bone for all that bone marrow to exist in. But second, spongy bone is also going to help make our bones lighter. Imagine if your bone was only compact bone, and it was just this solid, uniform, compact bone structure, or our bones would be incredibly heavy, and we wouldn't be able to run around. We'd have been eaten by predators back in the day, and we wouldn't exist now. So it's important that our bones not only are strong, think compact bone, but also really light, think spongy bone. Now we've talked about blood a lot, and so if our bones are making blood, well, they better have some blood vessels throughout the bone in order for the bone cells that our bone makes to get where they need to go to the rest of the body. So we'll have arteries and veins kind of throughout our bone here, both to deliver nutrients to the bone, but also to take those new blood cells and let them travel throughout the body. Two more structures, real quick. We've got an outer layer of the bone material. So this is going to surround the whole bone, and it's this thin layer of tissue called the periosteum. The word periosteum just means around the bone. Peri is around. Osteum is bone, so around the bone. The purpose of the periosteum is to have a layer of osteoblasts. Osteoblasts are bone-making cells. In other words, cells that deposit bone material on the bone, so that our bones can grow thicker, or they can remodel. In the case of like a broken bone, or if our blood calcium gets low and we need to be able to break down some bone material to increase the calcium in the blood, all of those cells are found in the periosteum. So as your bones grow stronger, they will deposit bone material along the outside on that periosteum layer. We've got another similar layer on kind of the inside called the endosteum. Endo means inside, and ostium means bone. So this means inside the bone, and it functions very much the same as the periosteum. If your compact bone area needs to grow thicker, it can deposit new bone material along the endosteum as well. So let's do a quick recap here. We've got a long bone, and it's got three parts to it. We have the proximal epiphysis, the diaphysis, and the distal epiphysis. The proximal epiphysis is closer to the point of attachment, so the shoulders or the hips, closer to that area. We've got articular cartilage on both ends of the bone in order to reduce friction, to prevent osteoarthritis and damage in our joints. We've got red bone marrow, which is all about making blood cells, both red and white. We've got yellow bone marrow, which is about storing fat, but it can revert to red bone marrow if we're suffering from intense blood loss in order to try to create more red blood cells as quick as possible. We've got the epiphyseal plates, which is where bone growth is going to occur, so the bones can grow longer, and we've got the periosteum and the endosteum, so that bones can grow thicker and stronger as needed. And we've got arteries running throughout the bone in order to transport oxygen and nutrients to all the different parts of the bone. All right, here's our diagram, but blank now. Take a moment, pause the video, see if you can identify all of the parts of our long bone. Here we have the diaphysis, or the shaft of the bone. We have the proximal epiphysis, the distal epiphysis. We have the articular cartilage, which lowers friction in our joints. We have the red bone marrow to make blood cells. We have the yellow bone marrow, which is for fat storage. We have epiphyseal plates, which is where bone can grow longer. We have compact bone, which gives our bone strength. We have spongy bone, which makes our bone light and also houses our red bone marrow. We have arteries throughout our bones to transport nutrients and blood cells. We have the periosteum and the endosteum, so our bones can grow thicker and stronger as needed. Hey, Alondra, do you know you have bones all throughout your body? You got bones in your hands, and bones in your head, and bones in your toes and your legs. Did you know that? Yeah, you knew that. You wave bye-bye to all the students.