Chapter 5 The Skeletal System

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Chapter 5 The Skeletal System

Bones: An Overview

Be Able To:

•Identify the subdivisions of the skeleton.

•List the functions of the skeletal system.

•Identify the major anatomical areas of long bone.

•Explain the role of bone salts and organic matrix of bone.

•Describe the process of bone formation and remodeling.

•List and describe the various types of fractures.

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Bones: An Overview

• Our skeleton is both strong and light.

• The skeleton is subdivided into:

– the axial skeleton: comprised of the skull, vertebral column, and the thorax

– the appendicular skeleton:

comprised of the bones of the limbs and girdles.

• The skeletal system also includes joints, cartilages, and ligaments.

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Bones: An Overview

Functions of Bones

1) Support – bones provide a hard framework that supports the body and cradle its soft organs.

2) Protection – bones protect soft body organs.

3) Movement – skeletal muscles use the bones as levers to move the body and its parts.

4) Mineral Storage – serve as a

reservoir for fats and minerals such as calcium phosphate.

5) Blood cell formation –

hematopoiesis occurs in the marrow of certain bones.

Stored minerals can be released into the

bloodstream as ions for distribution to all parts of the body as needed.

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Classification of Bones

• Every bone has a dense outer layer compact bone and an internal honeycomb like layer spongy bone.

• The unique shape of each bone fulfills a particular need.

• Bones are classified

according to shape into four groups:

– Long bones – Short bones – Flat bones

– Irregular bones

1 2

3

4

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Gross Anatomy of Long Bone

• The diaphysis is the shaft of the bone surrounding a central medullary cavity.

• The periosteum is a double-layered membrane that covers and protects the diaphysis.

• The periosteum is richly supplied with nerve fibers, lymphatic vessels, and blood vessels that enter the diaphysis through the nutrient foramen.

• The epiphyses are the bone ends covered with articular cartilage and constructed of compact bone

surrounding spongy bone.

• The epiphyseal line spans the

epiphysis where the growth plate once resided in childhood.

The endosteum is a connective tissue membrane that covers internal bone surfaces such as the medullary cavity.

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Bone Markings

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Bone Markings

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Microscopic Anatomy of Long Bone

• The structural unit of compact bone is an osteon or Haversian system.

• The core of each osteon is a central or Haversian canal.

• Volkmann’s canals connect the periosteum to the central canals.

• Mature bone cells or osteocytes occupy small cavities or lacunae.

• Lacunae are arranged in concentric circles called lamellae.

• Lamellae surround the central canal.

• Canaliculi are small canals that

connect lacunae to each other and to the central canal.

The strength of bone comes from its inorganic components of such durability that they

resist decomposition even after death.

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Bone Formation and Growth

• At 6 weeks the skeleton of a human embryo is constructed entirely from fibrous membranes and hyaline

cartilage.

• Bone tissue begins to develop at about this time.

• Intramembranous ossification is the process of bone formation from fibrous membranes.

• Bone formation that occurs by

replacing hyaline cartilage is called endochondral ossification.

• Flat bones develop from fibrous membranes at about 8 weeks.

• Most bones of the skeleton develop by endochondral ossification late in the 2^nd month.

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• Endochondral ossification begins at a primary ossification center at the center of the hyaline cartilage shaft in 5 stages:

1) Bone forming cells called osteoblasts of the periosteum secrete bone

matrix forming a bone collar around the external surface of the diaphysis.

2) Cells in the shaft signal the cartilage matrix to calcify resulting in the

creation of small cavities within the shaft.

3) The cavities are invaded by arteries, veins, nerve fibers, osteoclasts, and osteoblasts, which form

bone-covered cartilage in the form of young spongy bone.

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4) As the primary ossification centers elongates, osteoclats break down

spongy bone to form a medullary cavity.

5) Shortly after birth secondary ossification centers appear in the epiphysis which retains spongy bone

• Articular cartilage persists for life and the epiphyseal plate provides for longitudinal growth during

childhood.

• Appositional growth or diameter growth is controlled by hormones such as growth hormone and sex hormones during puberty.

• The epiphyseal plate closes at 18 in females and 21 in males.

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Bone Remodeling

• Bone remodeling is essential if bones are to retain normal

proportions and strength as the body increases in size.

• Osteoblasts produce new bone matrix and osteoclasts break down bone matrix.

• In healthy adults the rates of bone deposit and bone

resorption are equal.

• Calcium levels in the blood and the pull of muscles on bones influences bone

remodeling.

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Bone Fractures

• Closed or simple fractures are clean breaks that do not penetrate the skin.

• Open or compound fractures are breaks that penetrate the skin.

• Reduction is the realignment of broken bone ends.

• In closed reduction the bone ends are manually manipulated into their normal positions.

• In open reduction the broken bone is secured with surgery.

• Healing time ranges from 6 weeks to many months depending on the severity and age of the patient.

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Bone Fractures

• Repair occurs in 4 stages:

1) Broken bones also have broken blood vessels that form

hematomas.

2) Breaks are splinted by a

fibrocartilage callus which acts as splint by filling in the space with cartilage matrix.

3) Osteoblasts and osteoclasts create a bony callus of spongy bone.

4) Mechanical stresses reinforce and strengthen the fracture site.