Skeletal System Functions

Primary Functions of the Skeletal System

• The skeletal system provides structural support for the entire body
• It acts as a framework for the attachment of soft organs and tissues.
• Bones store calcium salts and are a source of calcium and phosphate ions
• Yellow marrow cavities in long bones serve as storage sites for energy reserves, like fats
• Red marrow within the internal cavities of bones produces red blood cells (RBCs), white blood cells (WBCs), and other blood elements
• Bones protect delicate tissues and organs
• Ribs protect the heart and lungs
• The skull encloses the brain
• The vertebrae shield the spinal cord
• The pelvis cradles delicate digestive and reproductive organs
• Bones function as levers to modify the magnitude and direction of forces generated by skeletal muscles
• A lever is a rigid structure that moves on a fixed point called a fulcrum
• Each bone is a lever and each joint is a fulcrum in the body
• First-class levers have the arrangement: Resistance-Fulcrum-Applied Force, like a seesaw with few examples in the body, such as the muscle arrangement in the neck
• Second-class levers have the arrangement: Fulcrum-Resistance-Applied Force, like a wheelbarrow, with few examples in the body, such as calf muscles undergoing plantar flexion
• Third-class levers have the arrangement: Fulcrum-Applied Force-Resistance, like forceps which is the most common type of lever in the body, such as the biceps brachii flexing the elbow

Classification of Bones

• The human body has 206 bones which are divided into 6 categories based on shape
• Long bones are relatively long and slender and are found in the arm, forearm, thigh, leg, palms, soles, fingers, and toes; the femur (thigh bone) is the largest and heaviest
• Short bones are boxlike in appearance, as wide as they are long, and are found in the wrists (carpal bones) and ankles (tarsal bones)
• Flat bones have thin, roughly parallel surfaces of compact bone sandwiching a thin layer of spongy bone, found in the roof of the skull, sternum, ribs, and scapula
• Irregular bones have complex shapes with short, flat, notched, or ridged surfaces, and are found in the vertebra and the ethmoid and sphenoid bones of the skull
• Sesamoid bones are generally small and nodular, develop within tendons, and are located near joints at the knees, hands, and feet
• Sutural or Wormian bones are small, flat, irregular-shaped bones found between flat bones in the skull, and fill gaps in sutures
• Each bone contains two types of osseous tissue, compact and spongy
• Compact bone is relatively solid, found only on the surface and its functional unit is the osteon
• Compact bone provides a sturdy protective layer and withstands extreme forces applied to either end
• Spongy bone is a brittle bone with an open network of struts and plates (trabeculae) in the interior
• Spongy bone has no osteons present
• Nutrients diffuse into the osteocytes within each trabeculae via canaliculi
• Spongy bone reduces the weight of bone and houses the red marrow where blood cells are produced

Parts of a Long Bone

• The diaphysis is the long tubular shaft that houses the medullary cavity
• The marrow cavity is a hollow chamber in the innermost part of long bones that contains yellow marrow
• The epiphysis consist of expanded areas at each end of the bone and is usually covered with hyaline cartilage
• Articular cartilage is hyaline cartilage that covers the articular ends of epiphyses
• The periosteum is a fibrous, vascular covering of the bone, excluding joint areas
• Isolation of bone from surrounding tissues, provides a route for blood and nervous supply, and participates in bone growth/repair (remodeling) are the functions of the periosteum
• The endosteum is a thin layer of squamous epithelial cells lining the inner portion of bone and functions in bone remodeling

Bone Development and Growth

• The growth of the skeleton determines overall body size and proportions
• The bony skeleton begins to develop about 6 weeks after fertilization when the embryo is ½ inch long, and continues until around 25 years old
• Ossification is the replacement of other tissues with bone
• Intramembranous and endochondral ossification are the two types of ossification
• Intramembranous ossification occurs when bone develops within fibrous connective tissue
• Sheets of primitive connective tissue appear where flat bones will form
• Primitive cells arrange around blood vessels in layers
• Cells differentiate into osteoblasts when bony matrix surrounds them
• The periosteum develops from connective tissue
• Osteoblasts deposit compact bone over spongy bone and become osteocytes when completely surrounded by bony matrix
• Flat bones of the skull are examples of intramembranous bones
• Endochondral ossification occurs when bone replaces existing cartilage
• Masses of hyaline cartilage serve as a model for future bones
• Cartilage breaks down, while a periosteum develops around the structure
• Blood vessels and differentiating osteoblasts invade tissue, disintegrating cartilaginous tissue
• Osteoblasts become osteocytes when bony matrix surrounds them
• Osteoblasts in periosteum deposit compact bone around spongy bone
• Types of endochondral bones include nearly all bones
• The primary ossification center is where hyaline cartilage in the center of the diaphysis is replaced by bone
• Osteoblasts from the periosteum deposit a thin layer of compact bone around the ossification center
• Epiphyses remain cartilaginous and continue to grow
• The secondary ossification center, located at the center of each epiphysis, leads to spongy bone laid down in all directions
• Ossification continues until spongy bone meets the spongy bone of the diaphysis
• As long as the cartilaginous cells are active in the disk, the bone is long and continues to grow
• An epiphyseal disk (thin band of cartilage) is formed
• Bone growth stops when the disk fully calcifies

Fracture Types and Repair

• Types of fractures include: Greenstick, Fissured, Comminuted, Transverse, Oblique and Spiral
• Bone healing begins with hematoma formation (blood-filled swelling)
• Blood vessels are ruptured during the break and osteocytes die because lack of oxygen
• Fibrocartilage forms to fill the gap
• Osteocytes clean debris while osteoblasts lay down new spongy bone and blood vessels reform
• The bony callus forms as more osteoblasts enter the area
• More spongy bone is laid to replace the fibrocartilage splint
• Bone remodeling occurs when osteoblasts rebuild new bone (spongy and compact) making it stronger
• Osteoclasts remove excess bone to reshape the bone

Factors Affecting Bone Development, Growth, and Repair

• Vitamin D₃ is needed for absorption of calcium and phosphate ions in the intestines
• Lack of Vitamin D₃ prevents calcium absorption, resulting in weak and deformed bones like rickets in children and osteomalacia in adults
• Vitamin A stimulates resorption activity; its deficiency retards growth
• Vitamin C is required for collagen synthesis; its deficiency results in fragile-slender bones
• Growth hormone, produced by the pituitary gland, stimulates bone growth at the epiphyseal disks, and over-secretion results in acromegaly
• Sex hormones stimulate increased cellular activity at the epiphyseal disks during puberty
• Calcitonin is secreted when blood calcium levels are high
• Calcitonin triggers increases in osteoblast activity and increased kidney excretion of calcium ions
• Parathyroid glands secrete when blood calcium levels are low
• The parathyroid glands' secretions trigger increases in osteoclast activity and decreases in kidney excretion of calcium ions
• Bones adapt to the forces applied to them during physical stress
• Heavily stressed bones become thicker and larger
• Unstressed bones become thin and brittle
• More powerful muscles result in larger bumps and ridges (attachment points) on bones

Skeletal Organization

• The human skeleton contains two divisions: axial and appendicular
• The axial skeleton has 80 bones; skull (cranium: 8, and facial bones: 14), hyoid bone (1) / 6 auditory ossicles, vertebral column (24 vertebrae/1 sacrum/1coccyx), and thorax (24 ribs/1 sternum)
• The appendicular skeleton has 126 bones; pectoral girdle (scapula-2, clavicle-2), upper limbs (humerus-2, ulna-2, radius 2, carpals-16, metacarpals-10, phalanges 28), pelvic girdle (2 coxal bones) fused ilium, ischium and pubis, and lower limbs (femur-2, tibia-2, fibula-2, patella-2, tarsals-14, metatarsals-10, phalanges-28)