The skeletal System

Skeletal System

Skeletal system is the biological system providing support in living organisms.

The skeleton functions not only as the support for the body but also in haematopoiesis, the manufacture of blood cells that takes place in bone marrow. This is why people who have cancer of the bone marrow almost always die. It is also necessary for protection of vital organs and is needed by the muscles for movement.

A. Functions of Bones
1. Support. Provide a hard framework.
2. Protection of many vital organs.
3. Movement. Act as levers with skeletal muscles moving them. Joints control possible movements.
4. Mineral storage. Especially calcium and phosphate, critical minerals for cellular function. Continuous deposition and withdrawal. Exquisite control of Ca++ (calcium ions) levels necessary for function of nerves, muscles, blood coagulation and other functions. Most of Ca++ in body in bones. Osteoclasts & osteoblasts controlled by hormones which regulate blood levels of Ca++.
5. Blood cell formation. Certain bones have active marrow.
B. Structure
1. Compact-Dense outer layer, looks smooth and solid. Contains cylinder of concentric layers with central canals.
a. Haversian system = circles of bone (lamella) with central canal (Haversian canal)
b. Central canal contain blood vessels & nerves. Connected at right angles to network.
c. Perforating small canals - blood vessels & nerves go through lamellar bone to supply osteocytes. Connect to periosteum.
d. Osteocytes live in bone, maintain it. Live in holes called lacunae. Connect to each other and central canal via canaliculi, little canals. Pass nutrients, waste products
2. Spongy- honeycombed, open spaces. Same structure as compact but less regular.
Withstand maximum stress with least weight. In bone interiors & weird weight bearing bones like head of femur. Not organized in lamella. Trabeculae are arranged along lines of stress. Osteocytes interconnected by canaliculi. Nutrients reach osteocytes by diffusing through the canaliculi from capillaries. Osteoporosis - More bone resorption than deposition, very weak bone.

Skeletal System

A. Axial skeleton
Principal supportive structure of the body includes skull, vertebrae, sternum & ribs. Central column of the skeleton from which arms and legs & bones that help them hang.
B. Appendicular skeleton
Provides fairly freely movable frame for upper & lower limbs. Includes pectoral (shoulder) & pelvic (hip) girdles, arms, forearms, wrists, hands, thighs, legs & feet.

Joints

Bones -> framework; muscles -> power; joints provide mechanism that allows body to move
A joint is where 2 adjacent bones or cartilages or combination thereof meet.
Most joints movable, some not.



Bone cells are called osteocytes, and the matrix of the bone is made of calcium salts and collagen. The calcium salts give bones the strength for its supportive and protective functions. The function of osteocytes is to regulate the amount of calcium that is deposited in or removed from the bone matrix.
Bone is an organ, it has its own blood supply and is made up of two types of tissue; compact and spongy bone.
 The names imply that the two types of differ in density, or how tightly the tissue is packed together. There are three types of cells that contribute to bone homeostasis. Osteoblasts are bone-forming cell, osteoclasts resorb or break down bone, and osteocytes are mature bone cells. An equilibrium between osteoblasts and osteoclasts maintains bone tissue.
Compact bone consists of closely packed osteons or haversian systems. The osteon consists of a central canal called the osteonic (haversian) canal, which is surrounded by concentric rings (lamellae) of matrix. Between the rings of matrix, the bone cells (osteocytes) are located in spaces called lacunae. Small channels (canaliculi) radiate from the lacunae to the osteonic (haversian) canal to provide passageways through the hard matrix. In compact bone, the haversian systems are packed tightly together to form what appears to be a solid mass. The osteonic canals contain blood vessels that are parallel to the long axis of the bone. These blood vessels interconnect, by way of perforating canals, with vessels on the surface of the bone.
Spongy (cancellous) bone 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. The canaliculi connect to the adjacent cavities, instead of a central haversian canal, to receive their blood supply. It may appear that the trabeculae are arranged in a haphazard manner, but they are organized to provide maximum strength similar to braces that are used to support a building. The trabeculae of spongy bone follow the lines of stress and can realign if the direction of stress changes.