Chapter 6: Bones and Bone Structure – Study Guide for Anatomy & Physiology I

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The adult human skeleton consists of 206 bones. These bones are organized into two main divisions: the axial and appendicular skeleton.

Axial Skeleton: 80 bones; includes the skull, thorax, and vertebral column.
Appendicular Skeleton: 126 bones; includes the limbs and the pectoral and pelvic girdles.

Axial Skeleton:
- Forms the longitudinal axis of the body.
- Provides support and protection for the brain, spinal cord, heart, and lungs.
- Attachment sites for muscles.
Appendicular Skeleton:
- Includes bones of the limbs and girdles.
- Enables movement and locomotion.
- Acts to help muscles move.

Flat Bones: Thin, parallel surfaces; protect underlying tissues and provide surface area for muscle attachment. Example: Parietal bone, sternum, ribs, scapulae.
Sutural Bones: Small, flat, irregularly shaped bones found between flat bones of the skull; variable in number and size.
Long Bones: Elongated and slender; found in arms, legs, palms, soles, fingers, toes. Example: Femur (largest bone).
Irregular Bones: Complex shapes with short, flat, notched, or ridged surfaces. Example: Vertebrae, pelvis, some skull bones.
Sesamoid Bones: Small, flat, sesame seed-shaped; develop inside tendons near joints (knees, hands, feet). Example: Patella.
Short Bones: Small and boxy. Example: Carpal (wrist) and tarsal (ankle) bones.

Epiphysis: Expanded ends of the bone; contains spongy (trabecular) bone with a thin covering of compact bone.
Metaphysis: Narrow zone connecting epiphysis to diaphysis.
Diaphysis: Shaft; long and tubular, composed mainly of compact bone forming a protective layer around the medullary cavity.
Medullary Cavity: Central space within the diaphysis; contains bone marrow.
Red Bone Marrow: Highly vascular; produces red blood cells.
Yellow Bone Marrow: Contains adipose tissue; serves as energy reserve.

Osteon (Haversian System): Basic functional unit of mature compact bone.
Central Canal: Contains blood vessels and nerves.
Concentric Lamellae: Layers of bone matrix around the central canal.
Lacunae: Small spaces containing osteocytes.
Osteocytes: Mature bone cells.
Canaliculi: Tiny channels connecting lacunae for nutrient/waste exchange.
Perforating Canals: Run perpendicular to osteons; supply blood to deeper osteons and medullary cavity.
Interstitial Lamellae: Fill spaces between osteons; remnants of recycled osteons.
Circumferential Lamellae: Found at outer and inner surfaces; produced during bone growth and maintenance.

Compact Bone: Outer layer of entire bone; makes up most of the diaphysis; thin covering of epiphyses.
Spongy Bone: Mainly in epiphyses; inside the bone.

Osteogenic Cells: Mesenchymal cells found in endosteum; divide to produce osteoblasts; important in fracture repair.
Osteoblasts: Produce new bone matrix (ossification/osteogenesis); make and release proteins and organic matrix (osteoid); help deposit calcium salts.
Osteocytes: Mature bone cells; maintain bone matrix.
Osteoclasts: Not directly mentioned, but implied; break down bone matrix for remodeling and calcium release.

Growth at the outer surface of bone, increasing thickness or diameter.

Growth in length, occurring at the epiphyseal plate in long bones.

Bones serve as a reservoir for calcium ions, which are regulated by hormones to maintain homeostasis.

Parathyroid Hormone (PTH): Released when blood calcium is low; stimulates osteoclast activity via RANKL, increasing calcium release from bone.
Calcitonin: (Note: The text refers to calcitonin as an active form of vitamin D, but this is incorrect. Additional info: Calcitriol is the active form of vitamin D; calcitonin is a hormone that lowers blood calcium.)
Calcitriol: Increases calcium absorption in intestines and reabsorption in kidneys; works with PTH to regulate calcium levels.

Bone Response: PTH stimulates osteoclasts to break down bone matrix, releasing calcium.
Intestinal Response: PTH enhances calcitriol's effect, increasing calcium absorption from food.
Kidney Response: PTH increases calcitriol production, stimulating calcium reabsorption in kidneys.

Simple/Closed Fracture: Bone pierces the skin; increased risk of infection and uncontrolled bleeding.
Compound/Open Fracture: Completely internal; visible only via X-ray.

Bone Type	Shape/Features	Location/Example
Flat	Thin, parallel surfaces	Skull, sternum, ribs, scapulae
Sutural	Small, flat, irregular	Between skull bones
Long	Elongated, slender	Femur, humerus, fingers
Irregular	Complex shapes	Vertebrae, pelvis
Sesamoid	Small, flat, sesame seed-shaped	Patella, near joints
Short	Small, boxy	Carpals, tarsals

Calcitonin is a hormone produced by the thyroid gland that lowers blood calcium levels by inhibiting osteoclast activity and increasing calcium excretion by the kidneys.
Calcitriol is the active form of vitamin D, produced by the kidneys, and increases calcium absorption in the intestines.
Osteoclasts are large, multinucleated cells responsible for bone resorption.