Skeletal System
Big Picture Overview of the Skeletal System
Framework of the Body: The skeletal system serves as the body's primary structural framework. It is not composed of dead structures, but rather of living tissue that is constantly remodeling and responding to physical stress.
Major Divisions: The system is divided into two primary sections:
Axial Skeleton: Forms the central axis of the body.
Appendicular Skeleton: Includes the limbs and the girdles that attach them to the axial skeleton.
High-Yield Importance: This topic is frequently tested in both lecture exams and lab practicals. Understanding the balance between structural rigidity and metabolic activity is key.
Functions of the Skeletal System
Support: The skeleton provides the structural integrity necessary to keep the body upright.
Protection: Bones act as a physical shield for vital organs. Specific examples include:
The Skull: Protects the brain.
The Ribs: Protect the heart and the lungs.
Movement: Muscles are attached to bones; movement is created when muscles pull on bones, using them as levers.
Hematopoiesis: This is the process of blood cell production. It occurs within the red marrow of bones.
Electrolyte Balance: Bones serve as a storage reservoir for essential minerals, specifically calcium and phosphate.
Acid-Base Balance: Bone tissue helps buffer the body's pH levels against excessive changes.
Bone Classification by Shape
Long Bones: These bones are significantly longer than they are wide. A primary example is the femur.
Short Bones: These possess a cube-like shape. Examples include the carpals of the wrist.
Flat Bones: These are thin, curved bones that specialize in protecting internal organs. An example is the sternum.
Irregular Bones: These have complex, varied shapes that do not fit other categories. A primary example is the vertebrae.
Sesamoid Bones: These are unique bones that form within tendons. The most prominent example is the patella (kneecap).
Long Bone Anatomy and Marrow Types
Epiphyses: These are the expanded ends of a long bone. They primarily contain spongy bone.
Diaphysis: This is the main shaft of the bone, composed primarily of compact bone.
Periosteum: A specialized connective tissue membrane that covers the exterior surface of the bone.
Medullary Cavity: The central hollow space within the diaphysis that contains bone marrow.
Bone Marrow Types:
Red Marrow: Responsible for hematopoiesis (the production of red blood cells, white blood cells, and platelets).
Yellow Marrow: Functions as a site for fat storage.
Division Details: Axial and Appendicular Skeleton
Axial Skeleton
Forms the central vertical axis.
Components: Includes the skull (cranial and facial bones), the vertebral column (cervical, thoracic, and lumbar vertebrae), the ribs, the sternum, the sacrum, and the coccyx.
Appendicular Skeleton
Includes the limbs and the support structures (girdles).
Pectoral Girdle: Consists of the clavicle and the scapula.
Pelvic Girdle: Consists of the ilium, ischium, and pubis.
Upper Limbs: Includes the humerus, radius, ulna, carpals, metacarpals, and phalanges.
Lower Limbs: Includes the femur, tibia, fibula, tarsals, metatarsals, and phalanges.
Bone Tissue, Histology, and Cells
Compact Bone: A dense, hard outer layer. It is organized into structural units called osteons (also known as Haversian systems).
Spongy Bone: A porous inner layer that contains a lattice-like structure called trabeculae and spaces filled with red marrow.
Bone Cells:
Osteoblasts: These cells are responsible for building bone tissue and depositing the bone matrix.
Osteoclasts: These cells are responsible for breaking down bone tissue (resorption).
Osteocytes: These are mature bone cells that maintain the tissue.
Cartilage Types and Functions
Hyaline Cartilage: Smooth and provides support. It is found in the nose and at the ends of bones in joints.
Elastic Cartilage: Highly flexible. It is the primary structural component of the external ear.
Fibrocartilage: Characterized by its ability to absorb shock. It is found in high-pressure areas such as the intervertebral discs and the menisci of the knee.
Joints: Classification and Movement
Fibrous Joints: Connected by dense connective tissue; they allow for little to no movement. An example is the sutures of the skull.
Cartilaginous Joints: Connected by cartilage; they allow for slight movement. An example is the pubic symphysis.
Synovial Joints: These are freely movable joints that contain synovial fluid. Examples include the shoulder.
Specific Synovial Types:
Hinge Joints: Move in a single direction (planar movement).
Ball-and-Socket Joints: Move in multiple directions (multiaxial movement).
Bone Physiology and Calcium Homeostasis
Remodeling: A continuous process involving the coordinated activity of osteoblasts and osteoclasts.
Hormonal Regulation of Calcium:
Parathyroid Hormone (PTH): Increases blood calcium levels by stimulating bone resorption (osteoclast activity).
Calcitonin: Lowers blood calcium levels by promoting bone deposition (osteoblast activity).
Vitamin D: Essential for the absorption of calcium from the digestive tract.
Ossification and Bone Growth
Intramembranous Ossification: The process that forms flat bones, such as those in the skull.
Endochondral Ossification: The process where long bones are formed from a pre-existing cartilage model.
Appositional Growth: This refers to the increase in the thickness or diameter of a bone.
Epiphyseal Plates: Often called growth plates, these allow bones to increase in length.
Fractures and Bone Healing
Fracture Types:
Closed Fracture: The bone breaks but does not penetrate the skin surface.
Open Fracture: The broken bone pierces through the skin.
The Healing Process:
Hematoma formation: A blood clot forms at the break site.
Fibrocartilage callus formation: A soft callus forms to bridge the break.
Bony callus formation: A hard callus of bone replaces the fibrocartilage.
Remodeling: The bone is reshaped over time to return to its original form.
Bone Disorders
Osteoarthritis (OA): A degenerative condition caused by physical wear and tear on the joints.
Rheumatoid Arthritis (RA): An autoimmune disorder characterized by inflammation of the joints.
Osteoporosis: A metabolic condition where bone density decreases, making bones fragile and prone to fracture.
Herniated Discs: Occurs when the padding between vertebrae protrusions, potentially compressing spinal nerves.
Lab Practical Review and Identification Tips
Radius: Located on the lateral (thumb) side of the forearm.
Ulna: Located on the medial (pinky) side of the forearm.
Fibula: The thin bone located on the lateral side of the lower leg.
Tibia: The larger weight-bearing bone of the lower leg.
Temporal Bone: Located on the side of the skull, near the ear canal.
Occipital Bone: Located at the back of the skull.
Focus Areas for Lab: Practice identifying bone landmarks, joint movements, vertebrae types, and differentiating between compact and spongy bone on slides or models.
Questions & Discussion
Practice Test Questions
Which bone cell builds bone tissue?
Which cartilage is found in intervertebral discs?
Which hormone raises blood calcium levels?
Which type of joint allows the most movement?
Which ossification process forms long bones?
What is the function of red marrow?
Which bone is a sesamoid bone?
What is found inside compact bone?
Difference between axial and appendicular skeleton?
Which structure increases bone thickness?
Answer Key
Osteoblasts: They build bone tissue and deposit bone matrix.
Fibrocartilage: It acts as a shock absorber in discs and menisci.
PTH: Parathyroid hormone raises blood calcium by increasing bone resorption.
Synovial joints: These are freely movable and contain synovial fluid.
Endochondral ossification: Long bones begin as cartilage models before turning to bone.
Blood cell production: Red marrow produces RBCs, WBCs, and platelets (Hematopoiesis).
Patella: Sesamoid bones are those that develop inside tendons.
Osteons: Also called Haversian systems, these are the structural units of compact bone.
Axial vs. Appendicular: Axial refers to the central axis (skull, spine, ribs); Appendicular refers to the limbs and girdles.
Appositional growth: This is the specific process that increases bone thickness.
Active Recall Check
What are the functions of the skeletal system? Support, protection, movement, hematopoiesis, electrolyte balance, and acid-base balance.
Difference between compact and spongy bone? Compact is dense and made of osteons; spongy is porous, made of trabeculae, and contains red marrow.
What does PTH do? Raises blood calcium via bone resorption.
What does calcitonin do? Lowers blood calcium via bone deposition.
Difference between OA and RA? OA is wear-and-tear; RA is autoimmune inflammation.
What type of cartilage absorbs shock? Fibrocartilage.
What is hematopoiesis? The production of blood cells in the red marrow.
What forms long bones? Endochondral ossification.
What is appositional growth? Growth in bone thickness/width.
What are osteons? The structural units of compact bone tissue.