Functions and Structures of the Skeletal System

Protective Functions of the Skeletal System

• The skeletal system serves several protective functions:

  • Skull protects the brain.

  • Vertebral Column (spine) protects the spinal cord.

  • Rib Cage protects the heart and lungs.

• These lesser-known functions of the skeletal system broaden its significance beyond mere structural support.

Blood Cell Formation in the Skeleton

• Key Point: Blood cell types are produced within bones.

  • Types of Blood Cells:

  • Red Blood Cells (Erythrocytes)

  • White Blood Cells (Leukocytes)

  • Platelets (Thrombocytes)

  • Red Bone Marrow: Primary site for blood cell synthesis.

  • Located within the skeleton.

  • Functions crucially to form all the different blood cell types.

Storage Functions of the Skeleton

• The skeleton acts as a reservoir for:

  • Minerals:

  • Primarily calcium and phosphate ions which contribute to the hardness of the bones.

  • Bone is recognized as a dynamic tissue that can adapt and accumulate bone mass throughout life.

  • Calcium Regulation:

    • The body requires a constant level of calcium in the blood for various physiological functions.

    • Excess calcium is stored in the bones; if dietary intake is insufficient, the body withdraws calcium from the skeleton to maintain blood calcium levels.

    • Approximately one gram of bone mass can be gained or lost daily based on the body’s calcium needs.

  • Fat Storage:

  • In adults, bones can also store yellow bone marrow, which is composed of adipose (fat) tissue.

  • This storage capacity develops mainly after age 20.

Types of Bone Marrow

• Two key types of bone marrow:

  • Red Bone Marrow:

  • Responsible for the formation of blood cells (hematopoiesis).

  • Found in specific areas of the skeleton, particularly in the epiphyses of long bones.

  • Yellow Bone Marrow:

  • Serves mainly as a fat storage area.

  • More prevalent in adults.

Structure of Long Bones

• Long bones have distinct structural characteristics:

  • Diaphysis:

  • Long central shaft of the bone.

  • Medullary cavity (also called marrow cavity) runs through this region, typically containing yellow bone marrow in adults.

  • Epiphysis:

  • The ends of the long bone, typically wider than the shaft.

  • Contains red bone marrow and is where blood cell production occurs.

  • The interior is known as spongy (trabecular) bone, which has a honeycomb structure filled with red bone marrow.

  • Articular Cartilage:

  • Covers the epiphyses at joints, acts as a cushion to reduce friction and absorb shock during movement.

Growth and Development of Long Bones

• Epiphyseal Plate:

  • A growth plate that allows long bones to lengthen during childhood and adolescence.

  • Composed primarily of cartilage during growth; eventually replaces by bone, resulting in epiphyseal lines when growth ceases.

  • Growth typically completes around age 18 for females and up to 25 for males.

• Cartilage Healing:

  • Healing of damaged cartilage is significantly less efficient than that of bone due to a lack of blood supply and stem cells.

• Periosteum:

  • A dense layer of connective tissue surrounding the bone, which aids in attachment for muscles and ligaments and provides a pathway for blood vessels.

Composition of Bone Tissue

• Bone Structure:

  • Spongy Bone: Composed of a porous structure that houses red bone marrow.

  • Compact Bone: Denser layer that forms the outer layer of bones, providing strength and structure.

  • Osteons: The structural units of compact bone, consisting of:

  • Central Canal: Contains blood vessels and nerves.

  • Lamellae: Concentric rings of bone matrix surrounding the central canal.

  • Lacunae: Small cavities that hold osteocytes (bone cells).

• Nerve Presence: Bone tissue is vascular and innervated; thus, it responds to stress and injury quickly compared to cartilage.

Functions and Divisions of the Skeleton

• 206 Bones: Adult human skeleton typically consists of 206 bones, but born with approximately 300, many of which fuse over time.

• Axial Skeleton: Comprises the bones of the skull, vertebral column, rib cage, and hyoid bone.

• Appendicular Skeleton: Includes the limb bones as well as the bones that form the girdles connecting the limbs to the axial skeleton.

Components of the Skull

• Cranium: Houses and protects the brain; made up of 8 bones.

  • Fontanelles: Soft spots on a baby’s skull that allow for growth and are typically closed by age 2.

• Facial Bones: Provide structure for the face and serve as attachment sites for muscles facilitating expression.

  • Notable bones include the mandible (the only movable bone), and the hyoid bone (a floating bone aiding in tongue movement and speech).

Vertebral Column Structure

• Vertebrae: Composed of cervical, thoracic, lumbar, sacral, and coccygeal regions; various types serve supportive or functional roles in the skeleton.

• Curvatures: Natural curves provide strength and flexibility.

  • Conditions affecting these curvatures include:

  • Scoliosis: Lateral curvature of the spine.

  • Kyphosis: Excessive outward curvature (hunchback).

  • Lordosis: Excessive inward curvature (swayback).

• Intervertebral Discs: Pads of fibrocartilage between vertebrae, acting as shock absorbers and allowing for movement.

  • Herniated Discs: Occurs when the inner core of the disc bulges out due to excessive pressure, leading to nerve compression and pain.

Thoracic Cage Functions

• Protection: Encloses vital organs, notably the heart and lungs.

• Respiration: The rib cage changes shape during breathing, aiding in lung expansion and contraction.

  • Floating ribs (ribs 11 and 12) do not attach to the sternum, providing flexibility and protection to the kidneys.

Appendicular Skeleton and Girdles

• Pectoral Girdle: Comprised of

  • Clavicles (collarbones) and Scapulas (shoulder blades).

  • Supports the upper limbs and facilitates a wide range of motion.

• Pelvic Girdle: Facilitates support and attachment for the lower limbs.