Bone Function

Support and Protection

Bones provide the structure for our bodies. Otherwise we would be mainly limp bags of skin! (yuck!) Our bones give our bodies shape. Have you ever heard someone say that so-and-so's face has great bone structure? Or that someone else is big-boned? Our bones provide shape to who we are.

Our bones also protect our bodies. For example, if someone were to be punched in the chest (my Mom loves boxing, so I used that example), there would not be any damage to their lungs or heart. That's because our rib cage protects our thoracic cavity organs. Similarly, our pelvic girdle protects most of our reproductive organs. In addition, our vertebral column protects our spinal cord and our skull protects our brain; if someone "breaks their back," no one is worried about the bones, but instead people are worried about the loss of protection to that person's spinal cord. This last point reminds you that your bones are not foolproof-- and for that reason humans are often seen to wear seat belts or helmets when doing dangerous things (like going faster in a car than we could ever go on foot, or zooming along on a motorcycle or rollerblades).

Body Movement

The only reason that our muscles can work to move our bodies is that they are attached at their ends to bone (via tendons). This way, when a muscle shortens, it affects bone positions, and our bodies are seen to move.

Figure 7.13 and the text in Hole go farther into this topic than I care to. I would like you to understand that bones act as levers, and that joints are the fulcrums (pivot points) about which they move. I would also like you to understand that the force that acts to create the movement may be either a muscle or an outside perturbation (like someone else who shakes your hand). But the details of a first-, second-, and third-class lever are unnecessary.

Blood Cell Formation

Blood cells are made wherever their precursor cells are. These cells (you'll have to learn next semester that they are called hemocytoblasts) are found in red marrow in adults. Red marrow is simply a loose connective tissue that contains these blood cell precursors and the cells that they are making. It is red, like the red blood cells it makes, because red blood cells contain the protein hemoglobin which is a red pigment protein.

Early on in our lives, most of our bones contain red marrow. It is slowly replaced by yellow marrow as we get older, so that we only have enough red marrow to meet our needs in blood cell production. Yellow marrow is simply a fat storage area.

Inorganic Salt Storage

For those of you who had a hard time with the chemistry material you had to learn, this may be a bit more difficult to understand-- even though it is very simple. Inorganic salt storage is also a very important function of bone.

what is stored?

    We never really discussed what made up the extracellular matrix in bone. All I ever said was that it was secreted by osteoblasts and osteocytes, and that it was tough. You also learned that there are plenty of collagenous fibers in it (helping to make it strong). Another component of the extracellular matrix in osseous tissue is "inorganic mineral salts." If you think back to your basic chemistry material, salts are compounds formed by ionic bonds. Many salts dissolve easily in water. But, the salts in bone are highly compacted into large crystals, so that they do not readily dissolve.

What's the point of having salts in bone?

    The main salt in bone extracellular matrix is hydroxyapatite. This is simply the name for a salt that is made up of calcium ions and phosphate ions (Ca²⁺PO₄^2-). And if you think about it, you already knew that bone contained calcium... now you just know that the calcium is in the form of hydroxyapatite.

    Most of the weight of bone is due to its extracellular matrix, and 70% of that weight is due to inorganic salts. That's a lot of weight to be carrying around in just salt! In fact, the weight of an average-person's salts is one of the assignment questions.

    Why bother storing calcium? Wow! We need calcium for a lot of things in our bodies. See, the tricky thing about calcium is that our cells keep it in very low concentration within themselves. It is a highly-regulated material. Because of this, if the levels of calcium suddenly change, you should be able to see how it could be excellent as a signalling mechanism. Calcium is also used to help form blood clots and to assist in certain metabolic reactions. So it is very important, but it is not needed in huge amounts at all times... just at some times.

    Because calcium needs such tight regulation, but also must be available when we need it, we have to have a calcium reservoir in our bodies. Now you can see that this reservoir is our bones.

    We also store small amounts of other inorganic salts in our bone, besides just hydroxyapatite. For example, we have some other normal ions (like Na+ and K+) in our bones. We also have some other odd elements, like lead, in our bones; if our bodies unexpectantly ingest lead, whatever we can extract from our blood can go into our bones for safer storage.

homeostasis of stored material

    Now that calcium is stored in bone, how do we get it out? And how do we stop from taking out too much? Can we add any calcium back in?

    This is no big trick. The key players are the main types of cells in osseous tissue: osteoclasts, osteoblasts, and osteocytes. Osteoclasts (you'll see more on these in the embryonic development section) can crawl around, invade bone, and chew it up. When our bodies need more calcium, a hormone called parathyroid hormone activates the osteoclasts and they increase their labor to chew up bone, removing calcium from storage and returning it to the blood.

    When we have enough calcium (or too much) in the blood, a hormone called calcitonin inhibits osteoclast activity. That slows them down so that they no longer remove calcium from bone.

    We can also replenish our calcium store in our bones. Osteoblasts remain available and help to lay down more calcium even later in life. And our osteocytes are also available to replenish our calcium stores.