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Skeletal System

Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

The skeletal system serves many purposes in an organism's body, including supporting and protecting body movements and producing red blood cells that the circulatory system carries. Jellyfish and worms have skeletons which are hydrostatic and allow the organism to push water in or out of their bodies. Arthropods have exoskeletons. Humans are vertebrates and have an endoskeleton made mainly of collagen.

So to help you bone up on the skeletal system let's begin by looking at its functions. The skeletal system very obviously helps in supporting and protecting the other organs of your body. That would have killed me except for the bones of my skull. It also helps in movement obviously when my muscles contract by pulling on my bones it gives them an anchor plus some something to pull so my bones can act as levers to help either magnify the speed or the force that my muscles are able to exert.

Your bones store minerals calcium for example is involved in dozens and dozens of metabolic functions without throughout your body so in order for you to make sure that you have right amount of calcium in your blood at all times you store excess calcium in your bones and when you're starting to run out of calcium you can take some of that calcium out of your bones and replenish your blood supplies. And obviously your bone marrow is located in your bones and that's where your body makes red blood cells white blood cells et cetera.

Now the cells of the skeletal system the most important of those are the osteocytes which literally means bone cells. Within the osteocytes there's osteoblasts which build bones just remember osteoblasts build bones while osteoclasts you may have heard the word iconoclast that literally means you destroy icons well iconoclast break down or destroy bones.

Now there's three main tissues involved in the skeletal system. There's compact bone, spongy bone and cartilage. Let's take a look at this diagram over here of a cross section of a bone like a drumstick from KFC and here in the outer region of the bone like it will make up the long shaft of a femur bone for example, you'll find there's hard dense bone called compact bone. Now within it you'll find osteocytes sitting in this tiny little caves and they're being fed by these arteries and veins that run through the middle of these canals so you'll have these concentric rings of bone being made by the osteocytes with blood vessels running up the central canal of these concentric rings, collectively these series of concentric rings is called an osteon or some textbooks will call it a Haversian system I guess discovered by doctor Haversian and you can see if you look here that there's many of these concentric rings making up the dense compact bone. In the middle of the bone, around the medullary cavity that hollow space where you'll find the bone marrow, you'll see there's lighter spongy bone which instead of being hard and dense well it's still hard has lots of gaps and spaces and this helps hold the cells of the bone marrow in place and it helps greatly reduce the overall weight of the bone. Now the bone spikes that you see here are called trabeculae and within them are the osteoblasts and osteoclasts that are working on the bone constantly building and remodeling it.

Now if we continue, you can see that there's this thing called axial versus appendicular skeleton. This right here is my the long axis of my body if we take a look at this diagram, you can see here the long axis that is called the axial skeleton where in red we've highlighted the appendicular skeleton i.e. it's got true appendages on it and so a lot of times when you're studying the skeletal system you'll have to start memorizing the bones of the axial system like the ribs or the cranium or the bones of the appendicular the femur here or I've always found this one pretty funny the humerus bone aha!

If you come back over here there's two major kinds of bone formation or ossifications; there's endochondral ossification and intramembranous ossification. Now endo means inside chondra is a loop that means cartilage versus intra which means within, membranous means within a membrane.

If we take a look at this diagram over here, we can see how endochondral ossification works. When you were first born, most of your bones were actually made out of cartilage they formed a general model of the shape of the bone. Ultimately, bone cells started penetrating into that cartilage model and as the cartilage model continued to grow to elongate get a little bit wider, the bone cells slowly started building the hard framework of calcium phosphate and other minerals calcium carbonate et cetera, they started building that to create this hard dense thing that we know as a bone. Eventually all the cartilage gets replaced that at least the part that form the framework of the bone till finally the only cartilage that you typically find is at the ends to help form the cushioning joints surfaces of the bone and it elongates. Now this is one of the reasons why young children can actually take injuries that an older person couldn't because their bones are still little bit flexible so when they talk about a bouncing baby boy don't bounce babies but that's kind of what we're talking about.

Now intramembranous ossification that's the creation of bone in places like your cranium or other flat bones and there your brain originally was encased in this connective tissue membrane and then the bone cells grew into that to slowly expand and replace it and that's why you know that again talking about babies everybody who talks to you about babies and you're about to touch him said don't push there, there's that soft spot and that's where the bones haven't finished filling in the connected tissue membranes that surround the brain.

Now if we take a look I was talking about how the bones are fusing into each other. The bones of the cranium and I believe there's about 14 of them actually are separate bones but they've joined together, those joins are classified as immovable joints and they form a particular kind of a movable joint called a suture. There's many different kinds of classifications of joints based on function, based on form et cetera and I'm not going to go through every single one because I doubt you'll ever need to memorize all of them. But some examples of common ones that you'll encounter are the pivot joints such as the skull on top of the spinal cord it allows you to pivot like this. The hinge joints are ones like the elbow or knee where much like a door they can open up in one direction but not on the other way. Ball and socket joints are very important joints for our movement and those one can include our shoulder joints as well as things like our knee, our hip bone and it allows a really wide range of motion and that's the skeletal system.