Compact bone, also known as cortical bone, is the dense outer layer forming the exterior of most bones. It serves as the primary load-bearing tissue for the skeletal system. This tissue provides structural integrity, rigidity, and substantial support. Its presence is pronounced in the shafts of long bones, such as the femur and tibia, where its strength withstands daily stresses.
Unique Structure
Compact bone’s functions stem from its organized microscopic architecture. The fundamental structural unit is the osteon, also known as the Haversian system. This roughly cylindrical structure consists of concentric layers of calcified bone matrix, called lamellae, arranged around a central canal. Within these lamellae are small spaces, lacunae, which house mature bone cells called osteocytes.
Tiny channels, called canaliculi, radiate from the lacunae, connecting osteocytes to each other and to the central Haversian canal. These canaliculi transport nutrients to osteocytes and remove waste products, as osteocytes are embedded within the dense matrix. The central Haversian canal contains blood vessels, nerves, and lymphatic vessels, supplying the living bone tissue. This arrangement of osteons, often aligned parallel to the bone’s long axis, gives compact bone its strength and rigidity, enabling it to withstand mechanical stress and resist bending.
Mechanical Roles
Compact bone provides structural support, forming the rigid framework that upholds the body. Its density allows it to bear weight, enabling upright posture and resisting compression forces. This characteristic makes it the primary tissue for strength in the skeleton, particularly in long bones like the femur.
The strength of compact bone also safeguards internal organs from physical trauma. For instance, the skull, composed primarily of this dense tissue, encases and protects the brain. Similarly, the ribs, sternum, and vertebrae form a protective cage around the heart and lungs, while the pelvis shields abdominal organs.
Beyond support and protection, compact bone enables movement. It functions as a system of levers upon which muscles exert force. When muscles contract and pull on these bony levers, they facilitate a range of motions, from locomotion to fine motor skills. The density and strength of compact bone are adapted for these weight-bearing, protective, and movement-related functions.
Mineral Storage
Compact bone serves as a reservoir for minerals, primarily calcium and phosphate. The bone matrix stores most of the body’s calcium. This mineral storage is not static; the bone actively participates in maintaining mineral balance, or homeostasis, throughout the body. When systemic levels of these minerals fall, compact bone can release them into the bloodstream to restore equilibrium.
Maintaining stable concentrations of calcium and phosphate in the blood is important for numerous physiological processes. Calcium, for example, is necessary for nerve impulse transmission. It is also important for muscle contraction and plays a role in blood clotting mechanisms. This mineral exchange highlights compact bone’s physiological role, complementing its mechanical functions and supporting overall bodily health.
Dynamic Nature
Compact bone is not static but a living, dynamic tissue that undergoes continuous remodeling throughout life. This process involves a balanced activity between two primary cell types: osteoblasts and osteoclasts. Osteoblasts form new bone tissue by synthesizing and secreting matrix that becomes mineralized. Conversely, osteoclasts break down and resorb old or damaged bone tissue, creating microscopic pits that are subsequently refilled.
This ongoing remodeling allows compact bone to repair microscopic damage from daily stresses, maintaining skeletal integrity. It also enables the bone to adapt its strength and density in response to mechanical stresses. Bone remodeling contributes to the body’s overall mineral homeostasis by regulating the release and uptake of calcium and phosphate from the bone reservoir. The entire adult skeleton is replaced about every 10 years through this continuous cycle of breakdown and renewal.