The marrow is responsible for making around 2 million red blood cells every second. It also produces lymphocytes or the white blood cells involved in the immune response. 


There are five types of bones in the human body:

Long bones: These are mostly compacted bone with little marrow and include most of the bones in the limbs. These bones tend to support weight and help movement.

Short bones: Only a thin layer of compact bone, these include bones of the wrist and ankle.

Flat bones: Usually bones that are thin and curved. They consist of two outer layers of compact bone and an inner layer of spongy bone. Flat bones include most of the bones of the skull and the sternum or breastbone. They tend to have a protective role.

Sesamoid bones: These are embedded in tendons, such as the patella or kneecap. They protect tendons from wear and stress.

Irregular bones: As their name implies, these are bones that do not fit into the first four categories and are an unusual shape. They include the bones of the spine and pelvis. They are often protecting organs or tissues.

Bones of the skeleton are split into two groups:

Appendicular skeleton — bones of the limbs, shoulders, and pelvic girdle.

Axial skeleton — bones of the skull, vertebral column, thoracic cage.


Bone is always being remodeled. This is a two-part process:

1. Resorption when osteoclasts break down and remove bone.

2. Formation when new bone tissue is laid down.

An estimated 10 percent of an adult’s skeleton is replaced each year.

Remodeling allows the body to fix damaged sections, reshape the skeleton during growth, and regulate calcium levels.

If one part of the skeleton is put under increased stress over time, for instance, during sport or exercise, the sections of bone under most pressure will become thicker in response.

Remodeling is under the control of several hormones, including parathyroid hormone, calcitonin, vitamin D, estrogen in women, and testosterone in men.