Chapter 5 The Skeletal System

The Skeletal System Parts of the skeletal system Two divisions: Bones (skeleton) – 206 in Human Joints Cartilages Ligaments Two divisions: Axial skeleton Appendicular skeleton

Functions of Bones 1) Support of the body 2) Protection of soft organs 3) Movement due to attached skeletal muscles 4) Storage of minerals and fats 5) Blood cell formation

Tissues in the Skeletal System A. Bone Mainly consists of hard calcium compounds and flexible collagen fibers, and is innervated and vascular Two basic types of bone tissue 1) Compact bone Homogeneous Solid and smooth Outer part of the bone 2) Spongy bone Small needle-like pieces of bone Many open spaces Inner part of the bone

Tissues in the Skeletal System B. Cartilage Mainly consists of firm gel with embedded fibers, and is avascular Three basic types of cartilage 1) Hyaline Cartilage – tough and flexible, found in ribs, joints, and respiratory tract 2) Elastic Cartilage – very flexible, found in out ear and epiglottis 3) Fibrocartilage – very dense, durable, and tough, found in spinal column, knee, and pubic bone

Types of Cartilage

Types of Cells in Skeletal System A. Bone Osteocytes: mature bone cells, maintain bones Osteoblasts: bone-forming cells, repair bones Osteoclasts: bone-destroying cells, remodel bones and release calcium into the blood B. Cartilage Chondrocytes: mature cartilage cells

Microscopic Anatomy of Bone Osteon (Haversian System) – individual unit of bone Contain: canals that run parallel and perpendicular canals carry blood vessels and nerves within each osteon and between osteons cavities that contain mature bone cells Run parallel along the shaft so they are resistant to damage from mechanical stress

Microscopic Anatomy of Bone Figure 5.3

Classification of Bones a. Long bones Typically longer than wide Have a shaft with heads at both ends Contain mostly compact bone, ends and inside the shaft contain spongy bone Examples: Femur, humerus

Structures of a Long Bone Arteries - supply bone cells with nutrients Articular cartilage - covers the external surface of the ends of the bone to decrease friction at the joint Medullary cavity – cavity in the shaft that contains marrow yellow marrow (mostly fat) in adults red marrow (for blood cell formation) in infants Figure 5.2c

Classification of Bones b. Short bones Generally cube-shaped Contain mostly spongy bone Examples: Carpals, tarsals

Classification of Bones c. Flat bones Thin and flattened Usually curved Thin layers of compact bone around a layer of spongy bone Examples: Skull, ribs, sternum

Classification of Bones d. Irregular bones Irregular shape Do not fit into other bone classification categories Examples: Vertebrae and hip

Classification of Bones on the Basis of Shape

Bone Growth In embryos, the skeleton is mainly cartilage During development, much of this cartilage is replaced by bone (some areas of the skeleton don’t stop growing until the age of 25) Cartilaginous space between shaft and the ends is called “epiphyseal plate” allows for growth during childhood (cartilage gets replaced by bone) Bones not only change in length, but in width (cavity gets larger) Cartilage remains in isolated areas – nose, ribs, joints

Bone Remodeling Every year, 18% of protein and minerals in skeleton are replaced Osteocytes maintain the bone by removing and replacing calcium Osteoblasts build osteons, and osteoclasts destroy osteons Mineral turnover enables the bone to adapt to new stresses

Bone Remodeling Maintains homeostasis by regulating calcium concentrations Parathyroid hormone and calcitrol increase calcium in blood (osteoclasts are active) Calcitonin decreases calcium in blood (osteoblasts are active) Large fluctuations in calcium ion concentration can cause problems with nerve and muscle cells: unresponsiveness if too low convulsions if too high

Bone Formation and Growth

Bone Fractures A break in a bone Types of bone fractures Closed (simple) fracture – break that does not penetrate the skin Open (compound) fracture – broken bone penetrates through the skin Bone fractures are treated by reduction and immobilization Realignment of the bone

Common Types of Fractures Table 5.2

Repair of Bone Fractures Hematoma (blood-filled swelling) is formed Break is splinted by fibrocartilage to form a callus Fibrocartilage callus is replaced by a bony callus Bony callus is remodeled to form a permanent patch

Stages in the Healing of a Bone Fracture

Bone Markings Surface features of bones Sites of attachments for muscles, tendons, and ligaments Passages for nerves and blood vessels Categories of bone markings Projections and processes – grow out from the bone surface Depressions or cavities – indentations

The Axial Skeleton Forms the longitudinal part of the body Divided into three parts Skull Vertebral column Bony thorax

The Axial Skeleton

The Skull Two sets of bones Bones are joined by sutures Cranium Facial bones Bones are joined by sutures Only the mandible is attached by a freely movable joint

The Skull

Bones of the Skull

Human Skull, Superior View Figure 5.8

Human Skull, Inferior View

The Hyoid Bone The only bone that does not articulate with another bone Serves as a moveable base for the tongue

The Fetal Skull The fetal skull is large compared to the infants total body length

The Fetal Skull Fontanelles – fibrous membranes connecting the cranial bones Allow the brain to grow Convert to bone within 24 months after birth

The Vertebral Column Vertebrae separated by intervertebral discs The spine has a normal curvature Each vertebrae is given a name according to its location

Structure of a Typical Vertebrae

Regional Characteristics of Vertebrae Figure 5.17a, b

Regional Characteristics of Vertebrae

The Bony Thorax Forms a cage to protect major organs

The Bony Thorax Made-up of three parts Sternum Ribs Thoracic vertebrae

The Appendicular Skeleton Limbs (appendages) Pectoral girdle Pelvic girdle

The Appendicular Skeleton

The Pectoral (Shoulder) Girdle Composed of two bones Clavicle – collarbone Scapula – shoulder blade These bones allow the upper limb to have exceptionally free movement

Bones of the Shoulder Girdle

Bones of the Shoulder Girdle Figure 5.20c, d

Bones of the Upper Limb The arm is formed by a single bone Humerus Figure 5.21a, b

Bones of the Upper Limb The forearm has two bones Ulna Radius

Bones of the Upper Limb The hand Carpals – wrist Metacarpals – palm Phalanges – fingers Figure 5.22

Bones of the Pelvic Girdle Hip bones Composed of three pair of fused bones Ilium Ischium Pubic bone The total weight of the upper body rests on the pelvis Protects several organs Reproductive organs Urinary bladder Part of the large intestine

The Pelvis Figure 5.23a

The Pelvis

Gender Differences of the Pelvis

Bones of the Lower Limbs The thigh has one bone Femur – thigh bone

Bones of the Lower Limbs The leg has two bones Tibia Fibula

Bones of the Lower Limbs The foot Tarsus – ankle Metatarsals – sole Phalanges – toes

Joints Articulations of bones Functions of joints Hold bones together Allow for mobility Ways joints are classified Functionally Structurally

Functional Classification of Joints Synarthroses – immovable joints Amphiarthroses – slightly moveable joints Diarthroses – freely moveable joints

Structural Classification of Joints Fibrous joints Generally immovable, joined by fibrous tissue Example: distal end of tibia and fibula Cartilaginous joints Immovable or slightly moveable, joined by cartilage Example: pubic symphysis, intervertebral joints Synovial joints Freely moveable, separated by a fluid-filled cavity

The Synovial Joint

Types of Synovial Joints Based on Shape

Types of Synovial Joints Based on Shape

Effects of Aging on the Skeletal System Less calcium is deposited in bones and they lose density and become more porous Higher incidence of bone fractures Decrease in height as the intervertebral disks become compressed Increased incidence of osteoarthritis, or degeneration of joint tissues