By Elizabeth A. Murray, Mount St. Joseph University
The skeleton is a fascinating collection of living organs. The combination of bones, along with the ligaments and cartilages that hold them together, make up the skeletal system. Because the skeleton forms the basic structural framework of the body, just seeing the outward shape and the pattern of movement of any animal with bones tells you a lot about that organism.
Bones Tell Us about the Past
Because bones and teeth are the most durable parts of the body, we also know a lot about the evolution of humans and other bony animals from the history left in their fossilized skeletons—features like how they stood, how they moved, how big their eyes were and where those eyes were placed, and how large their brain was relative to body size.
Not only does the skeleton support the varied soft tissues that make up the rest of the body, but because of the joints between them, bones also serve as levers in the leverage systems that cause movement. Muscles supply the force, and the joints serve as the fulcrums in these leverage systems.
Consider the rib cage, or the skull, and you’ll recognize the protective role that the bones provide for more vulnerable and soft internal organs. You might also know that one type of our bone marrow, known as red marrow—found in flat bones and in the ends of long bones—is a source for the production of new blood cells that move out of the bones and enter circulation.
The major long bones of the body also house yellow marrow, which is the fatty marrow within the hollow shafts of bones like the femur or the humerus. This yellow marrow serves as an energy reserve, since fats have over twice the caloric value of proteins or carbohydrates.
The Internal Warehouse
Among the most important roles of the skeleton—and the one that many scientists believe was the original function of bone within the animal body—is to serve as an internal warehouse of minerals. You probably know that calcium is the primary mineral in bone, but bone also houses phosphate salts, with phosphorus being another mineral important to good health.
Other essential salts and minerals in bones include carbonates, sodium, magnesium, and potassium. Still other minerals are held in the skeleton in trace amounts. These include iron, copper, zinc, manganese, fluoride, strontium, and boron—all of which are important in various metabolic and enzymatic pathways.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
The Value of Calcium
You might be wondering why experts in animal evolution hold that the original function of the bone was for mineral storage. Let’s just consider calcium as one example: Calcium and phosphate salts crystallize and, together with other minerals, make bones hard and durable. But calcium is also a required ingredient in the reactions that cause all of our muscle activity, including our heartbeat.
Calcium is also necessary for the propagation of electrical signals in the nervous system, and also aids in the body’s chemical messaging with the release of hormones from glands. As if that weren’t enough, calcium is also necessary for normal blood clotting, so minor injuries don’t result in the loss of too much blood. So, what better way for animals to have a supply of this important mineral than storing it within the body, with constant and ready access, even between meals?
The point is that these minerals don’t just sit there; remember, bone is a living tissue. When we have plenty of calcium in the blood—say, after we eat a spinach salad and drink a glass of milk—we are able to store some of that incoming calcium in our bones, at least when we’re young. Cells of the bone, called osteoblasts, under the influence of hormones, remove some of that calcium from the blood and store it away in the bone for a later time.
Then, if we go the rest of the day without dairy products, without sardines or canned salmon with its small edible bones, or if we eat no dark, leafy greens—then blood calcium levels fall. In this case, we have hormones that tell other bone cells, called osteoclasts, to get some of the calcium out of bone storage and put it back into the blood.
These cells don’t rob the bone to the point where your skeleton goes rubbery. Instead, they take just what they need to keep the blood calcium levels at their optimal range for all of the other body processes that need it.
This cycling of minerals is called bone remodeling, and it’s an important part of bone maintenance. About 10% of bone mineral is turned over each year—so your skeleton is constantly renewed by the process of remodeling. In fact, the minerals currently in your bones represent only about the last 10 years of your life.
Common Questions about the Skeletal System
The skeleton of living beings is what remains of them after they die, so they can be used to theorize about how those animals moved, what they looked like, where their eyes were, and how big their brain was.
The bones that make up our skeleton were originally used as an internal warehouse where minerals could be stored so when the body needs them for any reason, they can be released into the bloodstream.
Bone remodeling is the constant process of renewing the skeleton through cycling minerals in and out of the bones. The minerals that are stored in the bones are later used and others replace them and this cycle leads to 10% of bone minerals turning over each year.