Why is the skeletal system an organ system?
They are an organ system because the bones act like organs.
The best place to start here is to ask "what is an organ?"
Specifically, an organ is a collection of tissues that carry out certain functions in the body. But are bones organs? Yes they are! They are composed of both minerals and cells and they have SEVERAL very important functions in the body.
At first glance, it seems as though bones are just a collection of calcium and phosphorous, albeit a very well constructed collection! But bones are actually living tissue that is constantly changing shape! The major cell type in bone tissue is the osteocyte which, through the help of other cell types known as osteoblasts and osteoclasts, is constantly working to remodel bone and make it stronger. This is why our bones are able to repair themselves if they are fractured.
More than that, the bones act as a reservoir for calcium. The blood must have a relatively specific concentration of calcium in it at all times. When calcium levels are low, bone is broken down so that more calcium is released into the blood; but when blood levels of calcium are high, osteoblasts work to deposit calcium into new bone that it is forming.
Bone remodeling is not the only function of bone tissue. Bones are also the sight of the production of red and white blood cells (a process known as hematopoiesis).
Bones much more than meets the eye. They are critical, not just for protection and movement, but for the well-being of body chemistry as a whole.