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There are a number of important differences between spongy and compact bone, but the biggest ones have to do with what each is made of, how dense and heavy those contents are, and where each is located. Both are bone tissues, and many bones contain both; some do tend to have a higher proportion of one type over the other, though. This is particularly true of children. Young people often begin with mainly spongy bone, much of which will turn compact with maturity. People who suffer from certain bone and blood conditions or who experience chronic blood loss or starvation may also have slightly different bone compositions than the average human adult, though a lot of this depends on individual circumstances.
Most human bones look pretty similar from the outside, but their internal structures can occur as either spongy or compact and what they contain on the inside often depends on this classification. It’s important to note that the terms “spongy” and “compact” refer to bone tissue rather than bones themselves, and a single bone count contain both classifications. Many bones have spongy tissues at their knobby ends, for instance, but more compact and dense tissues in the middle. A lot has to do with where the bone is and its primary function, since the different tissue types tend to contain different things.
Spongy bone tissue gets its name because it often looks a lot like a sponge, at least when viewed as a cross-section. It is very porous and typically contains a lot of red bone marrow, which is known scientifically as medulla ossium rubra. Red marrow tends to be very flexible, and it’s responsible for producing and carrying red blood cells. Blood typically circulates through both the veins and arteries and bones, at least those parts with this sort of tissue.
Compact bone tissue, on the other hand, is mostly made up of yellow marrow, or medulla ossium flava. This is made up mostly of fat cells, which as s a general rule are more tightly packed than red blood cells. There isn’t usually much room for fluids or other substances to pass through these tissues.
The difference in the tissues’ densities — which is to say, the tightness and closeness of the marrow and its components — is another major distinguishing factor. In an average adult, roughly 80% of the skeletal system's weight comes from compact bone. Spongy tissues tend to be a lot less dense, and accordingly these weigh less in most cases.
Primary location is also a consideration. The outer surfaces of most bones are generally made up of compact bone, and shafts of long bones, such as femurs, are, too. This is the type of bone that makes the entire skeleton appear smooth and white, at least at first glance.
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. These sorts of tissues can also be found in the ribs, vertebrae, and skull.
Newborns and young children often have different spongy and compact bone compositions than do adults. At birth, most of the marrow in the 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 marrow begins to convert to yellow, and the nature of the bone tissue changes accordingly.
Most of the spongy bone in healthy adults contains red marrow, while the hollow areas surrounded by compact bones contain yellow marrow. Certain diseases and conditions may cause the body to convert yellow marrow back to red marrow. This is most common in situations of starvation or severe blood loss, but can also be a symptom of certain bone and blood cancers.
I didn't know that yellow marrow is used to store fat and blood. Why and how is it converted back to red marrow in cases of starvation or blood loss? Not enough blood or fat in the body?