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In the skeletal system, spongy and compact bone are two types of osseous tissue, or bone tissue. Compact bone is generally dense and heavy, and it makes up the hard outer surfaces of the bones. Spongy bone is less dense than compact bone, and it is typically very porous, giving it a sponge-like appearance. This type of bone is typically surrounded by compact bone. Also, in an adult, spongy bone typically contains more red bone marrow, while compact bone may contain yellow bone marrow.
The human skeletal system contains more than 200 bones, which are responsible for supporting the rest of the body, protecting vital organs, and storing minerals. Spongy and compact bone can be distinguished by their densities. Compact bone, or cortical bone, is generally very dense, with almost no gaps or holes in it. Spongy bone, or cancellous bone, is much less dense, and it is typically very porous, with several large gaps and cavities. This type of bone is referred to as spongy bone because it strongly resembles a sponge.
There is also a noticeable weight difference between spongy and compact bone. Compact bone is much heavier than spongy bone. In an average adult, roughly 80% of the skeletal system's weight comes from compact bone. Since it contains so many gaps and air pockets, spongy bone is generally very lightweight.
Another difference between spongy and compact bone is where each one is located. The outer surfaces of most bones are generally made up of compact bone. Shafts of long bones, such as femurs, are also usually comprised of compact bone tissue. This is the type of bone that makes most bones appear smooth and white.
Spongy bone, on the other hand, is generally located inside some bones. The end of a long bone, known as the epiphysis, is primarily made up of spongy bone surrounded by thin layers of compact bone. It can also be found in the ribs, vertebrae, and skull.
When a person is first born, most of the marrow in his bones is red marrow, which is responsible for making red and white blood cells, as well as platelets. As a person gets older, however, this red bone marrow begins to convert to yellow bone marrow. Yellow marrow is used as a place to store fat and blood.
In an adult, most spongy bone contains red marrow, while the hollow areas surrounded by compact bones contain yellow marrow. Sometimes, the body may convert yellow marrow back to red marrow. This generally only occurs in certain situations, such as starvation or severe blood loss.