Skeletal System

How are bones classified by their shape?

- *long* bones (fibula, tibia, metacarpals); - *short* bones (talus, scaphoid); - *flat* bones (scapula, sternum); - *irregular* bones (vertibrae, pelvis).

What is bone remodeling?

A lifelong process where mature bone tissue is removed from the skeleton (a process called *bone resorption*) and new bone tissue is formed (a process called *ossification* or *new bone formation*). These processes also control the reshaping or replacement of injuries like fractures and micro-damage.

How are bones divided by location?

1. *axial* The axial skeleton includes 80 bones divided into the skull, vertebral column, and thoractic cage regions. They carry other body parts, provide skeletal support and organ protection. 2. *appendicular* The bones that help us move around (shoulder baldes, pelvis and limbs).

The process of bone remodeling

1. The osteocytes release chemical signals that direct osteoclasts to the site of the damage. 2. When they het there, they secrete both a collagen-digesting enzyme and an acidic hydrogen-ion mixture that dissolves the calcium phosphate, releasing its components back into the blood. 3. When the old bone tissue is cleaned out, the osteoclasts then undergo *apoptosis*, where they basically self-destruct before they can do any more damage.

The structure of bones

All bones have a similar internal structure. They all have a dense external layer of *compact* (or cortical) bone around a porous *spongy* bone. This spongu bone tissue is made up of *trabeculae* that help the bone resist stress. And it is also where you typically find your *bone marrow*, which comes into two colors: *red* and *yellow*.

How many bones do humans have?

An average human body contains *206* bones, raging in shape and size from the tiny stapes of the inner ear to the huge femur of the thigh.

How is bone nourished?

Bone needs nourishment like any other tissue so there are central canals that hold nerves and blood vessels.

What are bones made of?

Bones are technically organs because they contain more than one type of tissue, while they are mostly made of *osseous* (bone) tissue, they also contain some *cartilage*, *muscle*, *nervous* and *epithelial* tissue. Bones are made of active connective tissue that is constantly breaking down, regenerating, and repairing itself throughout your lifetime. You basically get a whole new skeleton every 7 to 10 years.

Why is microgravity dangerous?

It causes serious damage to the bones. Up there, a person suffers 1-2% bone loss every month. By comparison, an average elderly person experiences 1-2% bone loss every year.

Why is exercising good for the bones?

It stimulates bone remodeling and bone strength.

What is the basic structural unit of a bone?

The basic structural units of bone - *osteons*. They are cylindrical, weight-bering structures that run parallel to the bone's axis. They are composed of tubes inside of tubes. Each one of thess tubes - *lamellae* - is filled with collagen fibers that run in the same directions.

What is tucked away between the layers of lamella?

Tucked away between the layers of lamella are "lacunae". These little gaps are where the real work of your skeletal system gets done, because they house your *osteocytes* - mature bone cells that monitor and maintain your bone matrix. They pass along commands to your skeleton's 2 main workhorses: the *osteoblasts* (bone makers) and *osteoclasts* (bone breakers). They allow your bones to regenerate.

What are bones' functions?

Your bones are basically how you *store the calcium*, *phosphate*, and other minerals you need to keep neurons firing and muscles contracting. They are also crucial to *hematopoiesis* - blood cell production. All of your new blood is generated in your *bone marrow*, which also helps store energy as fat. Bones even help maintain homeostasis by regulating blood calcium levels and producing the hormone *osteocalcin*, which regulates bone formation and protects against glucose intoleranes and diabetes.

Red bone marrow

produces blood cells.

Yellow bone marrow

stores energy as fat.