Gross Anatomy of Bone: Compact and Spongy Bone - Video Tutorials & Practice Problems
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Compact and Spongy Bone
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When talking about the anatomy of bone, we need to realize that all bone is made of two main structural arrangements. Spongy bone and compact bone. We'll talk about spongy bone. First, spongy bone is called spongy bone because it looks like a sponge makes sense, right? It's filled with all sorts of holes. And we can see that in this image here, we have the uh end of a femur. And if you look at the middle of the bone here, you can see that it's filled with all sorts of these little holes with little pieces of bone connecting around those holes like a sponge. Now, really importantly, spongy bone is not soft like a sponge. That's a misconception. Some people get when they hear the name spongy bone is hard, just like bone. It just in a cross section is filled with all sorts of ho holes like a sponge. Now, we're gonna say, rather than uh likening it to a sponge, we're gonna say it's built more like scaffolding. And when we think of scaffolding, right, you put scaffolding up the outside of a building when you're doing construction on it and it's built with all these pipes and bars that are connected and in different ways altogether, that scaffolding is really strong. But when you look at it, it's mostly open space that's like spongy bone, the struts connecting in different ways are what give it its strength, spongy bone, we're gonna call those struts, trabecula. So trabecula are the bone struts of spongy bone building spongy bone this way with these struts that connect the space in between is gonna reduce weight. Bone is really heavy tissue. And if your bones were solid bone, your skeleton would weigh a lot more. So by making spaces inside you lighten the skeleton and you also give room to put stuff inside the bone. The stuff you put inside the bone is bone marrow. We'll talk about bone marrow more. In another video, spongy bone is going to be found in the middle of all bones. If you take a bone cut it in cross section, you should see spongy bone in the middle. The only exception to this is the shaft of a long bone, but even long bones at the ends just like the end of the femur that we see here will have spongy bone on the inside. Now, in contrast, we have compact bone, compact bone is called that because it looks solid. Now I say look solid because there's no visible spaces. If you look at a skeleton, you're looking at compact bone, we say no visible spaces because there actually are lots of little spaces and, and canals through the compact bone. But it's all at the microscopic level, at least it's mostly at the microscopic level. So, compact bone is really much more like solid bone and that's gonna be optimized for strength and hardness and because it is so strong and so hard, it's gonna be on the outside edge of all bones. So we put spongy bone in the middle and then you can see here on the end of this femur here all along the edge, there's a layer of compact bone. Now, if you want, if you have a bone, so you get an impact in your impact on your bone, you want that compact bone on the outside because it is so strong and hard, it's gonna present uh a a lot of protection for the bone. The other place you're gonna find compact bone is in the shaft of long bones. That's the only place where you don't have spongy bone in the middle and it's only compact bone. OK? So with that remember, compact bone looks solid, spongy bone filled with holes like a sponge but not soft, like a sponge. We're gonna go into more detail in both for both bones going forward and I'll see you there.
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Gross Anatomy of Bone: Compact and Spongy Bone Example 1
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This example tells me that spongy bone and compact bone are sometimes compared to scaffolding in a brick or concrete wall. And it wants me to explain that analogy. So let's take a look. All right. Over here on the left, we have scaffolding and we have a little picture of scaffolding and it has all these bars or pipes that are connected in different directions. It has a ladder going up too, but all these bars and pipes, they individually, they're quite strong. But when you connect them in all these different ways, you can create a structure that together is incredibly stable and strong, stronger than any one pipe or bar could be on its own. And yet most of the structure is empty space that reminds me a lot of spongy bone, spongy bone we said are these bony struts that are pointed in all different directions and connected to each other with space in between them together. They create this quite rigid and quite strong structure even though most of the space is filled with marrow. So my reasoning, both of them are gonna be struts surrounding space. Now, we can compare that to this brick wall over here and this brick wall shows a little picture of some bricks and brick wall is hard. It is heavy. It is solid. That reminds me a lot of compact bone, compact bone is all those things. It's really hard. It's solid. Now, technically there's microscopic spaces in there and we're gonna learn all about those microscopic spaces in future videos. But compared to spongy bone, it's much, much more solid, which also makes it heavier. But because it's heavy and hard, that's why it's on the outside of the bone, protecting that scaffolding the spongy bone on the inside. That's a little bit more delicate. So for my reasoning, I'm just gonna say those things, it is solid. It's hard. It's heavy. Oh, spell heavy correctly though. All right. So we're gonna rely on these two analogies a little more going forward. So keep them in mind for now. There's a couple more problems to all. Give them a try.
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Problem
Problem
If a person's spongy bone were all replaced by compact bone, how may that affect the function of their bones and skeleton?
A
The skeleton would be stronger but also heavier.
B
The interior of the bones would lose their softness meaning breaks would happen more easily.
C
The skeleton would have less room for marrow meaning the function of joints would be affected negatively.
D
Long bones would become heavier, but other bones would not be affected.
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