Comprehensive Study Guide on Skeletal System Structure and Function
The Five Essential Functions of the Skeletal System
Support: The skeleton provides structural support for the body. Without bones, muscles would have nowhere to attach. The skeletal system also supports body weight. Bones that bear weight, such as the femur and tibia, are significantly thicker than those that do not support weight, such as the humerus or ulna.
Storage: * Minerals: Approximately of the body's calcium and phosphate are stored within the bones. Phosphorus is the elemental form, which binds to oxygen to be stored as phosphate. Calcium is specifically crucial for muscle contractions, including cardiac and skeletal muscle. * Lipids: Adult long bones contain a hollow shaft called the medullary cavity, which stores yellow bone marrow. This yellow bone marrow is composed primarily of adipose tissue (lipids) and serves as an energy reserve.
Blood Cell Production (Hematopoiesis): All blood cells are produced within the red bone marrow. In adults, this marrow is found in the ends of long bones (within spongy bone) and in flat bones.
Protection: Major organs are encased in bone for safety. Examples include: * The skull protecting the brain. * Vertebrae protecting the spinal cord. * The rib cage protecting the heart, lungs, and liver.
Leverage: Bones do not move on their own; they act as levers. Muscles attach to bones via tendons. When muscles contract and pull on the bones, movement occurs. A misconception often held by students is that bones move themselves; however, a skeleton (like one hanging at Halloween) cannot move without muscles to pull on the attachment points.
Hormonal Regulation of Bone Storage
If blood calcium levels are low, the body triggers a homeostatic mechanism to release calcium from the bone into the bloodstream.
If blood calcium levels are high, bone cells pick up the calcium to produce new bone.
Regulatory Hormones: Two primary hormones regulate this process: * Parathyroid Hormone (PTH) * Calcitonin
Red Bone Marrow and Hematopoiesis
Erythrocytes (Red Blood Cells): These cells circulate throughout the body to carry gases, specifically oxygen and carbon dioxide, and facilitate exchange with the lungs.
Leukocytes (White Blood Cells): These are immune cells categorized into several types: * Lymphocytes: Specific immune cells, including B cells and T cells, which must be activated for a high-intensity response. * Monocytes: Immature macrophages that mature into phagocytic cells. * Neutrophils: Phagocytic cells involved in immune response. * Eosinophils: Cells involved in allergy responses and defense against large parasite infections, such as tapeworms. Elevated eosinophil counts in blood or respiratory samples can help dictate medical treatment. * Basophils: Generally involved in allergy responses.
Platelets: These are not whole cells but fragments of cells derived from mature cells called thrombocytes. Platelets are essential for starting blood clot formation.
Classification of Bones by Shape
Long Bones: These are longer than they are wide. Examples include: * The humerus, ulna, and radius (arm). * The femur, tibia, and fibula (leg). * The metacarpals (hand/palm) and metatarsals (feet). * The phalanges (fingers and toes).
Short Bones: Roughly cube-like and equal on all sides. Examples include: * The carpals (wrist). * The tarsals (ankle).
Flat Bones: Thin, often curved bones. Examples include: * The ribs and sternum. * Cranial bones. * The scapula.
Irregular Bones: Bones with complex or unusual shapes that do not fit other categories. Examples include: * The vertebrae and sacrum. * The hip bone (formed by the fusion of the pubis, ilium, and ischium). * Certain cranial bones like the sphenoid and ethmoid bones (the maxilla is also an example).
Sesamoid Bones: Small bones that develop inside tendons due to friction or stress, shaped like a sesame seed. The one universal example found in almost everyone is the patella (kneecap), though they can also form in hands and feet.
Gross Anatomy of the Long Bone
Diaphysis: The central shaft of the long bone. It is hollow and contains the medullary cavity.
Epiphysis: The expanded ends of the long bone. They are covered in a thin layer of compact bone with spongy bone (containing red marrow) in the center.
Metaphysis: The region where the diaphysis and epiphysis join.
Medullary Cavity: Also known as the marrow cavity. In adults, it contains yellow bone marrow; in children, it may contain red bone marrow.
Articular Cartilage: A layer of hyaline cartilage covering the joint surface at the ends of the epiphysis. It is highly smooth and slippery to reduce friction between bones.
Growth Components: * Epiphyseal Plate: Known as the growth plate, it consists of hyaline cartilage and allows the bone to grow in length. * Epiphyseal Line: Formed once the epiphyseal plate has fused or calcified in adulthood, indicating the bone can no longer grow in length. On an X-ray, this appears as a distinct line.
Bone Markings: These are bumps, ridges, and openings (formina). Openings like the nutrient foramen allow blood vessels and nerves to enter the bone and reach the medullary cavity.
Bone Membranes
Periosteum: Membrane covering the outer surface of the bone. It consists of two layers: * Fibrous Layer: The outermost layer made of dense irregular connective tissue. * Cellular Layer: The inner layer containing osteogenic cells (stem cells) that differentiate into osteoblasts (matrix-producing cells).
Endosteum: A connective tissue membrane that lines the internal surfaces of the bone, including the medullary cavity and the surfaces of the trabeculae in spongy bone. It also contains osteogenic cells, osteoblasts, and osteoclasts (bone-destroying cells).
Chemical Composition and Ratios of Bone
Bone is a calcified connective tissue containing cells, ground substance, and fibers.
The Ratio of Organic to Inorganic Material: * Collagen (Protein Fibers): Accounts for approximately (or )) of the bone composition. Collagen provides flexibility. * Calcium Salts (Calcium Phosphate): Accounts for approximately (or )) of the bone composition. This provides hardness.
Homeostatic Imbalance: * High Collagen / Low Calcium: Bones become "bendy" and cannot support weight, leading to conditions like osteomalacia or rickets (often due to Vitamin D deficiency preventing calcium absorption). * High Calcium / Low Collagen: Bones become overly hard and brittle, making them susceptible to breaking easily because they lack flexibility.
Microscopic Anatomy: Compact vs. Spongy Bone
Compact Bone: * Osteon: The structural and functional unit, also known as a Haversian system. Osteons are arranged like bundles of pencils (columns) to provide strength in one direction (parallel stress). * Central (Haversian) Canal: The center of the osteon containing blood vessels (arteries/veins), nerves, and lymphatic vessels. * Perforating (Volkmann) Canal: Horizontal channels that connect central canals of different osteons. * Lacunae: Small spaces where mature bone cells (osteocytes) reside. * Canaliculi: Tiny channels connecting lacunae to allow for communication and nutrient transport between osteocytes. * Concentric Lamellae: Circular layers of matrix that form the osteon. * Circumferential Lamellae: Layers that wrap around the entire circumference of the bone. * Interstitial Lamellae: Layers of matrix that fill the gaps between circular osteons.
Spongy Bone: * Trabeculae: A network of rods and extensions. Though it looks less substantial than compact bone, the random pattern makes it very strong and capable of withstanding multidirectional mechanical stress. * Spongy bone lacks osteons. The spaces between trabeculae are filled with red bone marrow.
Clinical Implications and Learning Strategies
Epiphyseal Fractures: A break near the epiphyseal plate in children is a major concern as it can stunt growth or cause one limb to be shorter than the other. Doctors monitor these breaks every six months to ensure the plate stays open; if it closes prematurely, it cannot be reopened, though growth hormone is sometimes used as a treatment while the plate is still open.
Study Methods: The unit on bones is closely related to upcoming units on joints and muscles.
Hands-on Learning: * Cooking chicken bones in bleach or vinegar can demonstrate the roles of calcium and collagen. * Using raw chicken wings or legs allows students to see articular cartilage (smooth and white), joints, tendons, and muscles, which are identical in structure to human components (humerus, radius, ulna).
Clarification on Bone Movement: The instructor clarified that bones do not move on their own; muscles must move across a joint (e.g., from the humerus over the elbow joint to the radius) to pull on the bone to create movement.