The Skeletal System Overview

Study Notes

The skeletal system is a network of bones that work together to support the body, allow movement, protect organs, make blood cells, and store minerals.
The skeleton's protective function may be provided by structures situated on the body surface.
The skeleton supports the body against the pull of gravity.
It gives the body its shape and allows movement.
It provides a storage site for excess calcium.
Calcium may be removed from bone to maintain a normal blood calcium level, which is essential for blood clotting and proper functioning of muscles and nerves.

The skeleton is divided into two main parts: the axial skeleton and the appendicular skeleton.

The appendicular skeleton is composed of the bones of the upper and lower extremities.
The appendicular skeleton has a total of 126 bones.

Long bones support the weight of the body and facilitate movement.
Long bones are mostly located in the appendicular skeleton and include bones in the lower limbs (tibia, fibula, femur, metatarsals, and phalanges) and bones in the upper limbs (humerus, radius, ulna, metacarpals, and phalanges).

Short bones are cube-like in shape, approximately equal in length, width, and thickness.
The only short bones in the human skeleton are in the carpals of the wrists and the tarsals of the ankles.
Short bones provide stability and support as well as some limited motion.

Sesamoid bones are bones embedded in tendons.
These small, round bones are commonly found in the tendons of the hands, knees, and feet.
Sesamoid bones protect tendons from stress and wear.
The patella (kneecap) is an example of a sesamoid bone.

Irregular bones vary in shape and structure and do not fit into any other category (flat, short, long, or sesamoid).
They often have a complex shape that helps protect internal organs.

There are two main types of bone tissue: compact bone tissue and spongy bone tissue.

Compact bone tissue forms the extremely hard outside layer of bones.
It gives bones their smooth, dense, solid appearance.
Compact bone tissue accounts for about 80% of the total bone mass of the adult skeleton.

Spongy bone tissue is lighter and less dense than compact bone.
Spongy bone consists of plates (trabeculae) and bars of bone adjacent to small, irregular cavities that contain red bone marrow.

Besides compact and spongy bone tissue, bones contain several other tissues, including blood vessels and nerves.
Bones also contain bone marrow and periosteum.

Bone marrow is a soft connective tissue found inside a cavity called the marrow cavity.
There are two types of marrow in adults: yellow bone marrow (mostly consists of fat) and red bone marrow.
All marrow is red in newborns, but by adulthood, much of the red marrow has changed to yellow marrow.
In adults, red marrow is found mainly in the femur, ribs, vertebrae, and pelvic bones.
Only red bone marrow contains hematopoietic stem cells that give rise to red blood cells, white blood cells, and platelets in the process of hematopoiesis.