Structure and Function

Core Concepts

Bone: the body's framework

The adult skeleton has 206 bones, divided into five classes by shape:

• Long bones (femur, humerus) — act as levers for movement
• Short bones (carpals of the wrist, tarsals of the ankle) — provide stability with limited motion
• Flat bones (skull bones, sternum, scapulae) — protect organs and offer broad surfaces for muscle attachment
• Irregular bones (vertebrae) — shaped for specialized jobs
• Sesamoid bones (patella) — small bones embedded in tendons that protect them from wear

A typical long bone has a dense outer layer of compact bone, an inner lattice of spongy (trabecular) bone, and a tough outer membrane called the periosteum, which carries blood vessels and nerves and anchors ligaments and tendons. Inside the spongy bone lies the marrow: red marrow produces blood cells (hematopoiesis), and yellow marrow stores fat. Bone is living tissue — it remodels in response to stress, stores calcium and phosphorus, and releases them into the blood as needed. That is why weight-bearing activity and nutrition affect bone health across the lifespan.

Joints: where bones meet

A joint (articulation) is any place where two bones meet, and joints are classified by how much movement they allow:

• Fibrous joints — essentially immovable (e.g., the sutures of the skull)
• Cartilaginous joints — slightly movable (e.g., between the vertebrae of the spine)
• Synovial joints — freely movable (e.g., shoulder, hip, knee)

Synovial joints are the ones a nurse assesses most often. Their bone ends are capped with articular cartilage so they glide rather than grind, and the joint cavity is filled with synovial fluid, which lubricates and absorbs shock. Most joints that hurt, swell, or lose range of motion are synovial joints.

Ligaments, tendons, and cartilage: the connective team

Three types of connective tissue hold the system together:

• Ligaments connect bone to bone and stabilize joints.
• Tendons connect muscle to bone and transmit the force of muscle contraction.
• Cartilage cushions joint surfaces and gives shape to structures like the nose and ears.

A key structural fact: cartilage has no direct blood supply — it receives nutrients by diffusion from surrounding fluid. This is why cartilage injuries and joint wear heal slowly compared with well-vascularized tissues like muscle.

Skeletal muscle: the engine

Skeletal muscle is voluntary (you decide to move it) and striated (banded under the microscope). Muscles cross joints via tendons; when a muscle contracts, it shortens and pulls on the bone, producing movement. Muscles work in pairs: the agonist (prime mover) contracts while the antagonist relaxes — the biceps and triceps at the elbow are the classic example. Even at rest, muscles maintain muscle tone, a slight constant tension that holds posture and keeps joints stable. Muscle mass and strength tend to decline with age (sarcopenia), which is one reason mobility and fall risk change across the lifespan.

Posture, alignment, and body mechanics

Nurses practice body mechanics every time they lift, turn, or reposition a patient. The principles are applied anatomy: keep a wide base of support, lower your center of gravity, keep the load close, and use your large leg muscles rather than your back. Understanding how the spine's natural curves distribute weight explains why sustained poor posture strains joints and why regular repositioning matters for immobile patients — and why the nurse's own back needs the same protection the patient's does.

Common Confusions