A&P CH 6

Bone Function

• Support: Provides structural framework for the body.
• Mineral Storage: Stores minerals like calcium, phosphorus, sodium, and magnesium, crucial for physiological processes.
• Blood Cell Production: Red bone marrow within bones produces blood cells.
• Protection: Encases and protects vital organs.
• Leverage: Bones act as levers for muscle action.

Compact vs Spongy Bone

• Compact Bone: Dense and strong, forming the outer layer of most bones.
  • Osteon: Functional unit of compact bone, characterized by concentric rings of bone matrix providing strength and support.
• Spongy Bone: Lighter and less dense, found in the inner layer of bones.
  • Red Bone Marrow: Produces blood cells and contains blood vessels for nutrient supply.
  • Yellow Bone Marrow: Stores fat.

### Long Bone

• Mineral Storage: Long bones serve as a reservoir for critical minerals like calcium, phosphorus, sodium, and magnesium.
• Structure and Support: Provide strength, structure, and stability to the skeletal system.

Periosteum

• Membrane: Covers the outer surface of bones (except within joints).
• Layers: Consists of an outer fibrous layer and an inner cellular layer.
• Fibers: Intertwined with tendons, providing robust attachment.
• Perforating Fibers: Fibers embedded within bone tissue, reinforcing attachments.

Endosteum

• Cellular Layer: Lines the medullary (marrow) cavity and covers trabeculae (spongy bone lattice).
• Role: Active in bone growth, repair, and remodeling processes.
• Composition: Consists of flattened osteogenic cells.

Intramembranous Ossification

• Bone Development: Formation of bone from sheets of connective tissue membranes.
• Bone Types: Forms certain flat bones of the skull, clavicles, and some irregular bones.

Exercise and Bone

• Mineral Recycling: Allows bones to adapt to the stress of exercise.
• Strength Enhancement: Heavier stress on bone leads to increased density and strength.
• Osteoblast Stimulation: Exercise, particularly weight-bearing activities, stimulates osteoblasts (bone-building cells).
• Bone Degeneration: Rapid loss of bone mass can occur within weeks of inactivity.

Hormones and Bone

• Growth Hormone and Thyroxine: Stimulate bone growth.
• Sex Hormones (Estrogen and Testosterone): Promote osteoblast activity, contributing to bone growth.
• Parathyroid Hormone (PTH): Increases blood calcium levels.
  • Stimulates osteoclast activity (indirectly).
  • Enhances intestinal calcium absorption by increasing calcitriol (active vitamin D) secretion by kidneys.
  • Decreases calcium excretion by kidneys.
• Calcitonin: Decreases blood calcium levels.
  • Inhibits osteoclast activity.
  • Increases calcium deposition in bone.

Calcium

• Bone Matrix: Calcium salts deposit in bone matrix, contributing to its density.
• Osteocyte Organization: Bone cells (osteocytes) reside in lacunae (small spaces) around blood vessels.
• Canaliculi: Narrow passageways that facilitate nutrient, waste, and gas exchange between osteocytes.

Bone Development

• Ossification (Osteogenesis): Bone formation.
• Calcification: Deposition of calcium salts during ossification.
• Endochondral Ossification: Bone formation from cartilage models.
• Intramembranous Ossification: Bone formation from connective tissue membranes.
• Bone Growth: Continues until about age 25 in humans.

Parathyroid Hormone and Calcitonin

• Parathyroid hormone (PTH) and calcitonin are hormones that work in opposition to regulate calcium levels in the blood.
• Parathyroid Hormone (PTH): Increases blood calcium levels
  • Activates osteoclasts to break down bone and release calcium.
• Calcitonin: Decreases blood calcium levels
  • Inhibits osteoclast activity.
  • Increases calcium deposition in bone.

Bone Function

• The skeletal system provides support, stores minerals and lipids, produces blood cells, offers protection, and acts as leverage.
• Compact bone is denser and stronger than spongy bone, containing osteons as its functional unit.
• Spongy bone lacks osteons, but houses red bone marrow for blood cell production and yellow bone marrow for fat storage.

Long Bone

• Long bones store minerals like calcium, phosphorus, sodium, and magnesium, essential for regulating physiological activities.
• Long bones also provide strength, structure, and stability to the skeleton.

Periosteum

• Periosteum covers the outer surface of bones except within joint cavities.
• It comprises an outer fibrous layer and an inner cellular layer.
• Perforating fibers integrate with bone tissue, enhancing its strength and attachments.

Endosteum

• Endosteum lines the medullary cavity, a thin layer of cells active in bone growth, repair, and remodeling.
• It covers the trabeculae of spongy bone and lines the central canals of compact bone.
• It is composed of a flattened layer of osteogenic cells.

Intramembranous Ossification

• Intramembranous ossification develops bones from sheets of connective tissue membranes.
• This process is involved in forming certain flat bones of the skull, clavicles, and some irregular bones.

Exercise and Bone

• Exercise, particularly weight-bearing exercise, stimulates osteoblasts, leading to thicker and stronger bones.
• Mineral recycling allows bones to adapt to stress.
• Inactivity can lead to bone degeneration, with up to one-third of bone mass lost in a few weeks.

Hormones and Bone

• Growth hormone and thyroxine stimulate bone growth.
• Sex hormones like estrogen and testosterone stimulate osteoblasts, promoting bone growth.

Calcium Homeostasis

• Parathyroid hormone (PTH) and calcitonin maintain calcium ion homeostasis.
• PTH, produced by the parathyroid glands, increases blood calcium levels by:
  • Stimulating osteoclast activity.
  • Increasing intestinal absorption of calcium.
  • Decreasing calcium excretion by kidneys.

Bone Development

• Ossification (osteogenesis) is the process of bone formation, involving calcification, the deposition of calcium salts.
• There are two forms of ossification: endochondral ossification and intramembranous ossification.
• Some human bones continue to grow until approximately age 25.

Parathyroid Hormone vs Calcitonin

• Parathyroid hormone (PTH) and calcitonin regulate calcium levels in the blood.
• PTH, secreted by the parathyroid glands, increases blood calcium by activating osteoclasts, which break down bone to release calcium.
• Calcitonin, secreted by the thyroid gland, decreases blood calcium levels by inhibiting osteoclast activity and promoting calcium deposition in bones.