Adult skeleton: Contains 206 bones
Bone marrow: Red marrow makes blood cells. Yellow marrow stores fat
Two types of bone: Dense compact bone and lightweight spongy bone
Your bones are strong enough to support your weight, but light enough to allow movement. They protect your delicate internal organs and act as a storehouse for minerals, such as calcium. Despite appearing dry and lifeless, your bones are a hive of activity.
Your bones contain blood vessels, nerve cells and living bone cells known as osteocytes. These are held together by a framework of hard, non-living material containing calcium and phosphorous. A thin membrane called the periosteum covers the surface of your bones.
Running along the centre of long bones, such as your femur (thigh bone), is a cavity filled with bone marrow. Red bone marrow is a soft tissue that produces blood cells and yellow bone marrow is a store for fat.
Bone can either be spongy or compact. Spongy bone is lightweight and made up of a mesh of needle-like pieces of bone with large spaces between them. Compact bone is dense and forms the outer layer of all your bones.
An adult skeleton is made up of 206 bones, which come in several different shapes and sizes. The shape of a bone reflects its role within your body:
- Long bones, like in your arms and legs, are mostly made of compact bone
- Short bones, like the bones in your wrists and ankles, are mainly made of spongy bone
- Flat bones, like your rib and skull bones, are made of a layer of spongy bone sandwiched between two thin layers of compact bone
- Irregular bones, such as your butterfly-shaped vertebrae, are unusual shaped bones that don't fit into the other three groups
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