What is the Role of the Skeleton? The main purpose of our skeleton is to provide a support framework for our organs & tissues. The skeleton also serves as a 'calcium bank', particularly relevant to nursing mothers. There are three different types of bone, categorised by their shape. Long bones, such as the femur (thigh), ulna and radius (lower arm) consist of a central tube (diaphysis) with blocks of cancellous bone at each end (epiphyses). The hole within the diaphysis is filled with bone marrow. A thin shell of cortical bone surrounds the two epiphyses. Flat bones, such as the mandible (jaw) consist of a sandwich of cancellous bone between two layers of cortical bone. Irregular bones, such as the spinal vertebrae and calcaneus (heel), consist predominantly of cancellous bone with a thin shell of cortical bone.

long bones	flat bones	irregular bones

How does Bone Function (Physiology)

Bone is a living tissue and constantly changes in response to a large number of varied stimuli. Some of these are mechanical, hormonal and genetic influences. Continuously throughout life, old bone is removed and new bone added, a process called remodelling. Two main cell types are responsible for this: osteoclasts remove old bone and osteoblasts add new bone. As our skeletons develop during childhood, more bone is added than is being taken away. During early adulthood, the amounts removed and added are the same. If however, more bone is removed than is being added, we have a condition called osteoporosis. Since cancellous bone has a higher surface area than cortical bone, the remodelling process is approximately 8 times faster and hence cancellous bone is lost at a faster rate than cortical bone.

normal cancellous bone		osteoporotic cancellous bone

If we become more active and exercise, thus stressing our bones more, they automatically build up strength to avoid breaking under the new level of stress. It takes time for this build-up to occur. If we increase the stresses too fast, not giving the bones enough time to build up, they may crack and a ‘stress fracture’ occurs. This is one of the reasons why it is so important to build up slowly when exercising. Alternatively if we suffer long periods of disuse (e.g. being bed-bound or in a wheelchair), we lose bone since our body thinks that the bone is not needed anymore. This also explains why Astronauts lose bone.

Next Section - Osteoporosis