The Skeletal System and Bone Functions

The Skeletal System

• The skeletal system includes bones (skeleton), joints, cartilage, and ligaments
• The skeleton has two subdivisions: axial and appendicular.

Functions of Bones

• Provides support for the body
• Protects soft organs, for example:
  • The skull and vertebrae protect the brain and spinal cord.
  • The rib cage protects organs in the thoracic cavity.
• Allows movement due to skeletal muscles being attached
• Stores minerals such as calcium and phosphorus
• Contains fat in the internal marrow cavity
• Facilitates blood cell formation (hematopoiesis)

Bones of the Human Body

• The adult skeleton has 206 bones
• Two types of bone tissue, compact and spongy, make-up bones
• Compact bone is homogeneous
• Spongy bone has small needle-like pieces with many open spaces

Classification of Bones

• Bones can be classified as long, short, flat, and irregular

Long Bones

• Typically longer than wide
• Have a shaft with heads on both ends
• Contain mostly compact bone
• Include all the bones of the limbs, except wrist, ankle, and kneecap bones
• Examples include the femur and humerus

Short Bones

• Generally cube-shaped
• Contain mostly spongy bone
• Includes bones of the wrist (carpals) and ankle (tarsals)
• Sesamoid bones are a type of short bone that form within tendons, like the patella

Flat Bones

• Thin, flattened, and usually curved
• Two thin layers of compact bone surround a layer of spongy bone
• Examples include the skull, ribs, and sternum

Irregular Bones

• Have an irregular shape
• Do not fit into other bone classification categories
• Examples include the vertebrae and hip bones

Anatomy of a Long Bone

• Diaphysis:
  • The shaft of the bone
  • Composed of compact bone
• Epiphysis:
  • The ends of the bone
  • Composed mostly of spongy bone
• Periosteum:
  • Outside covering of the diaphysis
  • Fibrous connective tissue membrane
• Perforating (Sharpey’s) fibers secure the periosteum to underlying bone

Long Bone Arteries

• Supplies bone cells with nutrients

Anatomy of a Long Bone:

• Articular cartilage
  • Covers the external surface of the epiphyses
  • Made of hyaline cartilage
  • Decreases friction at joint surfaces
• Epiphyseal plate
  • A flat plate of hyaline cartilage seen in young, growing bone
• Epiphyseal line
  • Remnant of the epiphyseal plate
  • Seen in adult bones
• Marrow (medullary) cavity
  • Cavity inside of the shaft
  • Contains yellow marrow (mostly fat) in adults
  • Contains red marrow for blood cell formation in infants
• Red marrow in adults is situated in cavities of spongy bone and epiphyses of some long bones

Bone Markings

• Surface features of bones that are:
  • Sites of attachments for muscles, tendons, and ligaments
  • Passages for nerves and blood vessels
• Bone markings can be categorized as projections/processes or depressions/cavities

Bone Projections or Processes

• Grow out from the bone surface
• Terms often begin with "T"

Bone Depressions or Cavities

• Indentations in the bone
• Terms often begin with "F"

Microscopic Anatomy of Compact Bone

• Osteon (Haversian system)
  • A unit of bone containing central canal and matrix rings
• Central (Haversian) canal
  • Opening in the center of an osteon
  • Carries blood vessels and nerves
• Perforating (Volkmann's) canal
  • Canal perpendicular to the central canal
  • Carries blood vessels and nerves
• Lacunae
  • Cavities containing bone cells (osteocytes)
  • Arranged in concentric rings called lamellae
• Lamellae
  • Rings around the central canal
  • Sites of lacunae
• Canaliculi
  • Tiny canals that radiate from the central canal to lacunae
  • Forms a transport system connecting all bone cells to a nutrient supply

Formation of the Human Skeleton

• In embryos, the skeleton is primarily hyaline cartilage
• During development, much of this cartilage is replaced by bone
• Cartilage remains only in isolated areas such as the bridge of the nose, parts of ribs, and joints

Bone Growth (Ossification)

• Epiphyseal plates allow for lengthwise growth of long bones during childhood
  • New cartilage is continuously formed
  • Older cartilage becomes ossified
  • Cartilage is broken down
  • Enclosed cartilage is digested away, opening up a medullary cavity
  • Bone replaces cartilage through the action of osteoblasts
• Bones are remodeled and lengthened until growth stops
• Bones are remodeled in response to blood calcium levels and the pull of gravity and muscles on the skeleton
• Bones grow in width through appositional growth

Types of Bone Cells

• Osteocytes are mature bone cells
• Osteoblasts are bone-forming cells
• Osteoclasts are giant bone-destroying cells that break down bone matrix for remodeling and release of calcium in response to parathyroid hormone
• Bone remodeling is performed by both osteoblasts and osteoclasts

Bone Fractures

• Fracture is a break in a bone
• Types of bone fractures:
  • Closed (simple) fracture is a break that does not penetrate the skin
  • Open (compound) fracture is a broken bone that penetrates through the skin
• Bone fractures are treated by reduction and immobilization

Common Types of Fractures

• Comminuted - bone breaks into many fragments
• Compression - bone is crushed
• Depressed - broken bone portion is pressed inward
• Impacted - broken bone ends are forced into each other
• Spiral - ragged break occurs when excessive twisting forces are applied to a bone
• Greenstick - bone breaks incompletely

Repair of Bone Fractures

• Hematoma (blood-filled swelling) is formed.
• The break is splinted by fibrocartilage to form a callus.
• The fibrocartilage callus is replaced by a bony callus
• The bony callus is remodeled to form a permanent patch

The Axial Skeleton

• Forms the longitudinal axis of the body
• Divided into three parts: skull, vertebral column, and bony thorax

The Skull

• The skull includes two sets of bones: cranium and facial bones
• Bones are joined by sutures
• Only the mandible is attached by a freely movable joint

Paranasal Sinuses

• Hollow portions of bones surrounding the nasal cavity
• Functions to lighten the skull, and give resonance and amplification to voice

The Hyoid Bone

• The only bone that does not articulate with another bone
• Serves as a moveable base for the tongue
• Aids in swallowing and speech

The Fetal Skull

• The fetal skull is large compared to the infant’s total body length with the fetal skull equaling 1/4 body length compared to adult which equals 1/8 body length
• Fontanels consist of fibrous membranes, that allow skull compression during birth and allow the brain to grow during later pregnancy and infancy
• Fontanels convert to bone within 24 months after birth

The Vertebral Column

• Each is given a name according to its location
• There are 24 single vertebral bones separated by intervertebral discs
• Seven cervical vertebrae are in the neck designated C₁- C7
• Twelve thoracic vertebrae are in the chest region designated T1-T12
• Five lumbar vertebrae are associated with the lower back designated L₁-L5
• Nine vertebrae fuse to form two composite bones:
• Sacrum
• Coccyx

The Vertebral Column Curvatures

• Primary curvatures are the spinal curvatures of the thoracic and sacral regions
  • Present from birth
  • Form a C-shaped curvature as in newborns
• Secondary curvatures are the spinal curvatures of the cervical and lumbar regions
  • Develop after birth
  • Form an S-shaped curvature as in adults

A Typical Vertebrae

• Body
• Vertebral arch
  • Pedicle
  • Lamina
• Vertebral foramen
• Transverse processes
• Spinous process
• Superior and inferior articular processes

Sacrum

• Formed by fusion of five vertebrae

Coccyx

• Formed from the fusion of between three and five vertebrae
• Also known as the tailbone, or remnant of a tail that other vertebrates have

The Bony Thorax

• Forms a cage to protect major organs and consists of three parts:
  • Sternum
  • Ribs:
    • True ribs (pairs 1–7)
    • False ribs (pairs 8–12)
    • Floating ribs (pairs 11–12)
  • Thoracic vertebrae

The Appendicular Skeleton

• The appendicular skeleton is composed of 126 bones divided into:
  • Limbs (appendages)
  • Pectoral girdle
  • Pelvic girdle

The Pectoral (Shoulder) Girdle

• Composed of two bones:
  • Clavicle—collarbone: Articulates with the sternum medially, and with the scapula laterally
  • Scapula—shoulder blade: Articulates with the clavicle at the acromioclavicular joint, and with the arm bone at the glenoid cavity
• Helps allow greater upper limb mobility

Bones of the Upper Limbs

• Humerus
  • Forms the arm, is a single bone
  • Head articulates with the glenoid cavity of the scapula
  • Proximal end articulates at the
  • Trochlea and capitulum articulate with the bones of the forearm as the distal end articulation

Bones of the Upper Limbs - Forearm and Wrist

• The forearm has two bones:
  • Ulna which the medial bone in the anatomical position, that has the Coronoid process and olecranon articulate with the humerus.
• Radius which is the lateral bone in anatomical position having Head which articulates with capitulum of the humerus
• Wrist is made of carpals with Eight bones arranged in two rows of four bones in each hand; Metacarpals consisting of 5 five per hand; and Phalanges that include Fourteen per hand In each finger, there are three bones In the thumb, there only two bones

Bones of the Pelvic Girdle

• Formed by two coxal (ossa coxae) bones composed of three pairs of fused bones including the following:
  • Ilium
  • Ischium
  • Pubis
• Pelvic girdle = 2 coxal bones, sacrum
• Bony pelvis = 2 coxal bones, sacrum, coccyx
• The total weight of the upper body rests on the pelvis and protects reproductive organs, the urinary bladder, and part of the large intestine

Gender Differences of the Pelvis

• The female inlet is larger and more circular and a female pelvis as a whole is shallower
• The bones are lighter and thinner
• Female ilia flare more laterally
• Female sacrum is shorter and less curved, female ischial spines are shorter and farther apart; thus the outlet is larger.
• The female pubic arch is more rounded because the angle of the pubic arch is greater.

Bones of the Lower Limbs

• Femur-thigh bone which is the heaviest, strongest bone in the body and articulates with head articulates with the acetabulum of the coxal (hip) bone. Lateral and medial condyles articulate with the tibia in the lower leg

Bones of the Lower Limbs - Legs

• There are two bones of the lower leg with the Tibia being the Shinbone, larger and medially oriented as the Medial and lateral condyles articulate with the femur to form the knee joint
• The fibula is Thin and sticklike; lateral to the tibia and:Has no role in forming the knee joint
• The foot includes Tarsals with seven bones comprised of the two largest including the Calcaneus (heel bone) and Talus as well as Metatarsals with five bones that form the sole of the foot; Phalanges -tourteen bones that form the toes

Arches of the Foot

• Three strong bones are used to create the arches of the feet, specifically: Two longitudinal and One transverse

Joints

• Articulations of bones
• Provide function to: Hold bones together and Allow for mobility, and classified through being Structurally and Functionally

Functional Classification of Joints

• Synarthroses - Immovables
• Amphiarthroses - Slightly Moveable
• Diarthroses - Freely Moveable

Structural Classification of Joints

• Fibrous – Generally immovable
• Cartilaginous – Immovable or slightly moveable
• Synovial – Freely moveable

Fibrous Joints

• Bones united by collagenic fibers
• Sutures
  • Immobile Syndesmoses
  • Allows more movement than sutures but still immobile Example Distal end of tibia and fibula Gomphosis Immobile

Cartilaginous Joints

• Bones connected by cartilage
• Types
  • Synchrondrosis - Immobile
    • Symphysis
      • Slightly Movable Example Public Symphysis and Intervertebral Joints

Synovial Joints

• Articulating bones are separated by a joint cavity and contains synovial fluid

Features of Synovial Joints

• Articular cartilage (hyaline cartilage) covers the ends of bones
• Articular capsule encloses joint surfaces and lined with synovial membrane
• Joint cavity is filled with synovial fluid
• Reinforcing ligaments Structures Associated with the Synovial Joint Bursae—flattened fibrous sacs and lined with synovial membranes, filled with synovial fluid Tendon sheath is Elongated bursa that wraps around a tendon

Inflammatory Conditions Associated with Joints

• Bursitis - Inflammation of bursa cause by burt or friction
• Tendonitis - Inflammation of tendon sheaths
• Arthritis - Inflammatory or degenerative diseases of joints, with over 100 dfferent types and considered, Most widespread crippling diease in the U.S. With initial symptoms: pain, stifiness, swelling of joint

Clinical Forms of Arthritis

• Osteoarthritis which is the Most common chronic arthritis probably related to normal aging processes
• Rheumatoid arthritis is An autoimmune disease that the immune system attacks the joints and Symptoms that Begin with bilateral inflammation of certain joints which often leads to Deformities
• Gouty or arthritis, An Inflammation of joints is caused by a deposition of uric acid crystals from the blood and Can usually be controlled with diet with occurrences being More common in men.

Developmental Aspects of the Skeletal System

• At birth, the skull bones are incomplete
• Bones are joined by fibrous membranes called fontanels
• Fontanels are completely replaced with bone within two years after birth Curves of the spine also

Skeletal Changes Throughout life:

• The fetus has long bone made of hyaline cartilage, and that flat. Long bones are as fibrous membranes
• At birth, fontanels may remain until age 2
• During adolescence, epiphyseal plates will become ossified and long bone growth has ends
• Size of cranium in relationship to the body where - 2 years old Skull is larger in proportion to the body compared to that of an adult, and. 8 or 9 years old skull is near adult size and proportion, or Between ages 6 and 11, the face grows out from the skull Curves are set up in the. Spine and abnormalities appear in spinal curvatures (scoliosis and lordosis), and vertebrae collapse causing kyphosis Osteoporosis increases a with bone thinning disease. Most common with women over age percent and 20 percent over age 70. Reduced with estrogen and bone aid for a healthy female skeleton