Skeletal System Overview

Functions of the Skeletal System

The skeletal system provides the framework for the body, supports and protects organs, enables movement, stores minerals, and houses bone marrow for blood cell production.

• Support: Provides structural support for the entire body.

• Protection: Shields vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).

• Movement: Serves as attachment points for muscles, enabling movement.

• Mineral Storage: Stores calcium and phosphate ions.

• Blood Cell Production: Houses red bone marrow for hematopoiesis.

Classification of Bones

Bones are classified by shape and structure:

• Long bones: Longer than they are wide (e.g., femur, humerus).

• Short bones: Nearly equal in length and width (e.g., carpals, tarsals).

• Flat bones: Thin and broad (e.g., sternum, skull bones).

• Irregular bones: Complex shapes (e.g., vertebrae, pelvic bones).

• Sesamoid bones: Small, round bones embedded in tendons (e.g., patella).

Example: The patella is a sesamoid bone found within the quadriceps tendon.

Bone Tissue Types

• Compact bone: Dense, forms the outer layer of bones, provides strength.

• Spongy bone (cancellous): Porous, found at the ends of long bones and inside flat bones, contains red marrow.

Bone Structure and Composition

Microscopic Structure of Bone

• Osteon (Haversian system): Structural unit of compact bone, consisting of concentric lamellae around a central canal.

• Lacunae: Small spaces housing osteocytes.

• Canaliculi: Tiny channels connecting lacunae for nutrient and waste exchange.

• Interstitial lamellae: Layers of bone matrix between osteons.

Bone Cells

• Osteoblasts: Bone-forming cells; synthesize bone matrix.

• Osteocytes: Mature bone cells; maintain bone tissue.

• Osteoclasts: Bone-resorbing cells; break down bone matrix.

Inorganic Component of Bone

The inorganic matrix of bone is primarily composed of hydroxyapatite (calcium phosphate crystals), which provides hardness and strength.

• Hydroxyapatite:

• Other minerals: Calcium carbonate, magnesium, sodium, potassium.

Organic component: Mainly collagen fibers, providing flexibility and tensile strength.

Calcium Regulation in Bone

Hormonal Regulation of Blood Calcium

Calcium homeostasis is maintained by the interplay of three hormones:

• Parathyroid hormone (PTH): Increases blood calcium by stimulating osteoclast activity, increasing intestinal absorption, and reducing renal excretion.

• Calcitonin: Lowers blood calcium by inhibiting osteoclasts and promoting calcium deposition in bone.

• Calcitriol (active vitamin D): Increases intestinal absorption of calcium.

Process: When blood calcium is low, PTH is released; when high, calcitonin is released.

Bone as a Calcium Reservoir

• Bones store about 99% of the body's calcium.

• Calcium is released from bone by osteoclasts and deposited by osteoblasts.

Bone Fractures and Healing

Types of Bone Fractures

• Greenstick: Incomplete fracture, common in children.

• Epiphyseal fracture: Involves the growth plate.

• Compression fracture: Bone is crushed, common in vertebrae.

• Spiral fracture: Caused by twisting forces.

• Comminuted fracture: Bone breaks into several pieces.

Bone Healing Process

1. Hematoma formation

2. Fibrocartilaginous callus formation

3. Bony callus formation

4. Bone remodeling

Axial and Appendicular Skeleton

Axial Skeleton

• Consists of the skull, vertebral column, and thoracic cage (ribs and sternum).

• Functions: Protects the brain, spinal cord, and thoracic organs.

Appendicular Skeleton

• Includes the pectoral girdle, pelvic girdle, and limbs.

• Functions: Facilitates movement and interaction with the environment.

Major Bones and Landmarks

• Skull: Cranial and facial bones, including the mandible, maxilla, and zygomatic bones.

• Vertebral column: Cervical (7), thoracic (12), lumbar (5), sacrum, coccyx.

• Thoracic cage: Ribs (12 pairs), sternum (manubrium, body, xiphoid process).

• Pectoral girdle: Clavicle and scapula.

• Upper limb: Humerus, radius, ulna, carpals, metacarpals, phalanges.

• Pelvic girdle: Ilium, ischium, pubis.

• Lower limb: Femur, patella, tibia, fibula, tarsals, metatarsals, phalanges.

Vertebral Column

Structure and Function

• Composed of 33 vertebrae in five regions: cervical (7), thoracic (12), lumbar (5), sacrum (5 fused), coccyx (4 fused).

• Protects the spinal cord and supports the head and trunk.

Curvatures of the Spine

• Cervical and lumbar: Lordotic (concave posteriorly).

• Thoracic and sacral: Kyphotic (convex posteriorly).

• Curvatures help absorb shock and maintain balance.

Joints and Movement

Types of Joints

• Fibrous joints: Immovable (e.g., sutures of the skull).

• Cartilaginous joints: Slightly movable (e.g., intervertebral discs).

• Synovial joints: Freely movable (e.g., shoulder, knee).

Synovial Joint Structure

• Articular cartilage covers bone ends.

• Joint cavity filled with synovial fluid.

• Joint capsule and ligaments provide stability.

Selected Table: Comparison of Bone Types

Additional info:

• Some content was inferred and expanded for completeness, such as the detailed steps of bone healing and the hormonal regulation of calcium.

• Specific examples and definitions were added to clarify brief points from the original material.