The skeletal system Chapter Six Part A

Functions of Bone and the Skeletal System Bone tissue and the skeletal system perform several basic functions that include: Support Protection Assisting in movement Mineral homeostasis Production of blood cells Triglyceride storage/Energy Storage 2

Functions of Bone and The Skeletal System Support- The skeleton provides a framework for the body by supporting soft tissues and providing points of attachment for most skeletal muscles. Protection-The skeleton protects many internal organs from injury. For example: cranial bones protect the brain, vertebrae protect the spinal cord and the rib cage protects the heart and lungs.

Assisting in movement- Because skeletal muscles attach o bones when muscles contract, they pull on bones. Together, bones and muscles produce movement. Mineral homeostasis- Bone tissue stores several minerals, such as calcium and phosphorus. On demand, bone releases minerals into the blood to maintain critical mineral homeostasis and distribute other minerals through out the body.

Production of red blood cells-Certain bones contain a connective tissue called red bone marrow. Red bone marrow produces red blood cells, white blood cells and platelets through a process called hemopoiesis. Red bone marrow is made of developing blood cells, adipocytes, fibroblast and macrophages. It is found in developing bone of a fetus, and adult bones such as the pelvis, vertebrae, skulls and the ends of arm and thigh bones.

Triglyceride storage- Triglycerides are stored in adipose cells of yellow bone marrow. Yellow bone marrow is an chemical energy reserve that contains adipose cells and few blood cells. A fetus has mostly yellow bone marrow, with increasing age yellow marrow becomes red bone marrow.

Types of Bones There are four main types of bones that are classified based on their shape which are the following: Long Short Flat Irregular

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Long bones- have a greater length than width and consist of a shaft and a variable number of ends. May be curved for strength Examples: Femur, tibia and fibula, humerus, ulna and radius and phalanges.

Short bones- are somewhat cubed shaped and nearly equal in length and width. Examples: Wrist and ankle bones

Flat bones: Are thin and offer a lot of protection and surfaces for muscle attachments. Examples: Cranial bones, sternum, ribs, scapula

Irregular bones- have complex shapes and cannot be grouped into any of the other categories. Examples: vertebrae and some facial bones 07_38b.jpg

Structure of Bone The long bone- is made of the following parts: Diaphysis- the shaft of the bone Epiphyses-distal and proximal ends of the bone Metaphyses- regions of mature bone where the diaphysis joins the epiphyses. Has an epiphyseal plate and hyaline cartilage that allows the bone to grow. When growth is complete, cartilage is replaced with bone forming the epiphyseal line.

Articular Cartilage- thin layer of cartilage covering part of the epiphysis where the bone forms the joint with another bone. This reduces friction and absorbs shock at freely movable joints. Periosteum- made of dense irregular connective tissue that surrounds the bone surface wherever it is not covered by articular cartilage. Contains bone forming cells that allow bones to increase in width, not length which protect against fractures as well as provide nourishment. It also serves as an attachment point for ligaments and tendons.

Medullary cavity- is the space within the diaphysis that contains yellow bone marrow in adults. Endosteum- is a thin membrane that lines the medullary cavity and contains bone forming cells.

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Microscopic Structure of Bone Bone (osseous) tissue is made of cells that contain 25% water, 25% collagen fibers and 50% of crystallized mineral salts. When mineral salts are deposited into collagen fibers they crystalize causing the tissue to harden, this process is known as calcification. Calcification is initiated by osteoblast (bone building cells)

Bone hardness depends on minerals, however flexibility depends on collagen fibers. There are three major types of cells present in bone tissue: Osteoblast Osteocytes Osteoclast

Osteoblast- are bone building cells that create and secrete collagen fibers and other organic components need to build he matrix of bone tissue. Osyteocytes- are mature bone cells that are involved in the bones metabolism, such as exchange of nutrients and waste with the blood. Osteoclast- large cells that are derived from white blood cells found in the endosteum. Osteoclast release lysosomes (enzyme) and acid to digest protein and minerals of bone matrix. Osteoclast therefore break down bone, this is known as resorption.

Compact and Spongey Bone Bone is not completely solid and has many small spaces. Some of these spaces are channels for blood vessels or storage for red bone marrow. Depending on the size of the spaces, the bone may be classified as spongey or compact. Approx. 80% of the skeleton is compact bone while 20% is spongey bone.

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Compact bone contains few spaces. It forms the external layer of all bone. Provides protection, support and resist the stresses produced by weight and movement. Compact bone is arranged in units called osteons, also called haversian systems. In the perforating canals, blood vessels, lymphatic vessels and nerves pass though. Central (haversian) canals run longitudinally through the bone and are surrounded by concentric lamellae which are rings of hard bone matrix. Between the lamellae are small spaces called lacuna. This is where osteocytes are found.

Radiating in all directions from the lacunae are canaliculi. Canaliculi are filled with extracellular fluid. Canaliculi connect lacunae with one another via central canals. These little canals allow for many routes for nutrients and oxygen to reach osteocytes and for waste to diffuse away.

Spongy Bone Unlike compact bone, spongy bone does not contain osteons. Contains trabeculae- which is an irregular lattice work of thin columns of bone. Spongey bone makes up most of the tissue in short, flat and irregularly shaped bones. Forms most of the epiphyses of long bones

How is spongy bone different from compact bone? Spongy bone weighs less, therefore moves more readily. Trabeculae of spongy bone protect red bone marrow is found only in certain areas of hip bones, ribs, breast bones and the ends of long bones in adults.

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Bone Scans What are they and how do they work? A small amount of radioactive tracer compound is injected intravenously. The trace is picked up by bone and a scanning device detects the tracer and translated into a photograph.

Normal bone with have a consistent gray appearance through out, however bone that has darker areas, also called “hot spots” may indicate bone caner, abnormal healing of fractures, or abnormal bone growth. Light areas also called “cold spots”may indicate problems such as degenerative bone disease, fractures, bone infections, Paget’s disease, or rheumatoid arthritis. Benefits of bone scans- they can detect abnormalities 3-6 months sooner than x-rays and there is less exposure of radiation.