The Skeletal System (Chapter 6)

The Skeletal System Has 5 Primary Functions
(6-1)

Support

Storage

Blood Cell Production

Protection

Leverage

The Skeletal system provides structural support for the entire body. Individual bones or groups of bones provide a framework for the attachment of soft tissues and organs.

The calcium salts of bone represent a valuable mineral reserve that maintains normal concentrations of calcium and phosphate ions in body fluids. In addition, bones store energy reserves as lipids in areas filled with yellow marrow.

Red blood cells, white blood cells, and other blood elements are produced within the red marrow, which fills the internal cavities of many bones. The role of bone marrow in blood cell formation will be discussed wen we examine the cardiovascular and lymphoid systems.

Soft tissues and organs are often surrounded by skeletal elements. The ribs protect the heart and lungs, the skull encloses the brain, the vertebrae shield the spinal cord, and the pelvis cradles delicate digestive and reproductive organs.

Many bones function as levers that change the magnitude and direction of the forces generated bu skeletal muscles. The resulting movements range from the delicate motion of a fingertip to powerful changes in the position of the entire body.

Bones are Classified According to Shape and Structure (6-2)

Bone is a supporting connective tissue that contains specialized cells and a matrix consisting of extracellular protein fivers and a ground substance.

Macroscopic Features of Bone.

The human skeleton has four different shapes

Long Bone

Short Bones

Flat Bones

Irregular Bone

longer than they are wide

roughly equal

thin and relatively broad

have complex shapes that do not fit easily into any other category

Compact Bone

relatively solid

Spongy Bone

resembles a network of bony rods or struts separated by spaces.

Microscopic Features of Bone

Osteocytes

Lacunae

Lamellae

Canaliculi

In compact and spongy bone

found between narrow sheets of calcified matrix

calcified matrix

small channels, radiate through the matrix, interconnecting lacunae and linking them to nearby blood vessels

Compact and Spongy Bone

Osteon

Central Canal

Perforating canals

Trabeculae

the basic functional unit of compact bone

contains one or more blood vessels

provides passageways for linking the blood vessels of the central canals with those of the periosteum and the marrow cavity

rods or plates

Three Primary Cell Types

Osteocytes

Osteoclasts

Osteoblasts

mature bone cells. maintain normal bone structure by recycling he calcium salts in the bony matrix around themselves and by assisting in repairs.

giant cells with 50 or more nuclei. Acids and enzymes secreted by osteoclasts dissolve the bony matrix and release the stored minerals through osteolysis, or resorption. This process helps regulate calcium and phosphate concentrations in body fluids

the cells responsible for the production of new bone, a process called osteogenesis, Osteoblasts produce new bone matrix and promote the deposition of calcium salts in the organic matrix. At any moment, osteoclasts are removing matrix and osteoblasts are adding to it. When an osteoblast becomes completely surrounded by calcified matrix

Ossification and appositional growth are mechanisms of bone formation and enlargement (6-3)

Ossificaiton

Calcification

the depostio of calcium salts, occurs during ossification, but it can also occur i tissues other than bone

Replacing other tissues with bone

Inramembranous Ossification

when osteoblasts differentiate within embyonic or fetal fiborous connective tissue.

Ossification Center

place where ossification first occurs

Endochondral Ossification

begins as cartilage then turns into true bone

Step 1

starts when chondrocytes within the cartilage model enlarge and the surrounding matrix begins to calcify. The Chondrocytes die because the diffusion of nutrients slows through the calcifed matrix

Step 2

Bone formation occurs at the shaft surface. Blood vsesels invade the perichondrium, and calls of its inner layer differentiated osteoblasts that begin producing bone matrix.

Step 3

Blood vessels invade the inner region of the cartilage, and newly differentiate into osteoblasts form spongy bone within the center of the shaft at a primary ossification center. Bone development proceeds toward either end filling the shaft with spongy bone.

Step 4

As the bone enlarges, osteoblasts break down some of the spongy bone and create marrow cavity. The cartilage model does not completely fill with bone because the epiphyseal cartilages on the ends continue to enlarge, increasing the length of the developing bone. Although osteoblasts from the shaft continuously invade the epiphyseal cartilages, the bone grows longer because new cartilage continuously added in front of the advacing osteoblasts. This situation is like a pair of joggers, one in front of the other: As long as they run at the same speed, the one in back will never catch the one in front, no matter how fa they travel.

Step 5

The centers of the epiphyses begin to calcify. As blood vessels and osteoblasts enter these areas, secondary ossification filled with spongy bone. A thin cap of the original cartilage model remains exposed to the joint cavity as the articular cartilage

Bone growth and development depend on a balance between bone formation and resorption and on calcium availability (6-4)

Remodling

bone being removed and replaced

The Role of Remodeling In Support

Regular mineral turnover gives each bone the ability to adapt to new stresses. Heavier stressed bones become thicker and stronger and develop more pronounced surface ridges.

Repair of Fractures

bones will crack or even break. Bones usually heal even after they have been severely damaged, repair process may take from 4 months to well over a year

Step 1

A Large blood clot soon forms and closes off the injured blood vessels

Step 2

cells of the periosteum and endosteum undergo mitosis, and the daughter cells migrate into the fracture zone. There they form localized thickenings, an external callus, and an internal callus

Step 3

Osteoblasts replace the new central cartilage if the external callus with spongy bone. the external and internal calluses form a continuous brace of spongy bone at the fracture site.

Step 4

the remodeling spongy bone at the fracture site may continue over a period of 4 months to well over a year. When the remodeling is complete, the fragments of dead bone and the spongy bone of the calluses will be gone.

Osteopenia has a widespread effect on aging skeletal tissue (6-5)

Bones become thinner an relatively weaker as a normal part of the aging process. Inadequate ossification is called osteopenia, and all of us become slightly osteopenic as we age

The bones of the skeleton are distingujised by surface markings and grouped into two skeletal divisions (6-6)

Bone markings

Skeletal Divisions

axial skeleton forms the longitudinal axis of the body

elevations, projections, depressions, and openings

the appendicular skeleton includes the bones of the limbs and those of the pectoral and pelvic girdles

The bones of the skull, vertebral column , and thoracic cage make up the axial skeleton (6-7)

The Axial skeleton creates a framework that supports and protects organ systems in the brain and spinal cavities, and the ventral body cavities.

8 cranium bones. the cranium encloses cranial cavity, a fluid-filled chamber that cushions an supports the brain.

The frontal bone of the cranium forms the forehead and roof of the orbits, the bony recesses that contain the eyes. A supraorbital foramen is an opening that pierces the bony ridge above each orbit forming a passageway for blood vessels and nerves passing to or from the eyebrows and eyelids

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The pectoral girdle and upper limb bones, and the lower limb bones make up the appendiuclar skeleton

the humerous extends from the scapula to the elbow

the wrist, palm, and fingers arre supported by 27 bones.

The eight carpal bones of the wrist are: the scaphoid bone, the lunate bone, the triquetrum bone, the pisiform bone, the trapezium bone, the trapezoid bone, the capitate bone, and the hamate bone

the radius ad ulna are the bones of the forearm the radius lies along the laterial side of the forearm while the ulna provides medial support of the forearm

Joints are categorized according to their range of motion or anatomical organization (6-9)

joints are also called articulations

joints are classified as fibrous, cartilaginous, or synovial

An immovable joint is a synarthrosis a slightly movable joint is an amphiarthrosis

ligaments joining bone to bone may be found outside or inside the joint capsule

bursa is a small pouch containing synovial fluid

Anatomical and functional properties of synovial joints enable various skeletal movements (6-10)

Gliding, two opposing surfaces slide past each other

Flexion, movement in the anterior-posterior plane that reduces angle between the articulating elements

Extension, occurs in the same plane but it increaces the angle between articulating elements.

hyperextension, when extension is continued past the anatomical position

Abduction, movement away from the longitudinal axis of the body in the frontal plane.

adduction is moving back to the anatomical position

Interverteral articulations and appendicular articulations demonstrate functional differences in support and mobility (6-11)

medial and lateral menisci lies between the deoral and tibial surfaces