The human skeletal system provides the structure for the body, and a major component is compact bone, also known as cortical bone. This tissue forms the dense, hard outer layer of nearly every bone, accounting for approximately 80% of the total bone mass in an adult skeleton. Compact bone’s unique composition and highly organized structure allow it to perform several functions, ranging from providing mechanical strength to serving as a reservoir for essential minerals.
Structural Support and Mechanical Strength
Compact bone provides structural support and mechanical strength, particularly against bending and compressive forces. This tissue has low porosity and high density, making it rigid, which is why it forms the shafts of long bones like the femur and tibia. It is built to resist the immense loads placed on the skeleton during movement and daily activities.
The microscopic organization of compact bone allows it to achieve this strength. Its fundamental structural unit is the osteon, or Haversian system, a tiny, cylindrical column packed tightly together. Osteons are composed of concentric layers of calcified matrix, called lamellae, which surround a central Haversian canal containing blood vessels and nerves.
The osteons are generally aligned parallel to the long axis of the bone, which is the direction of the greatest stress. This parallel alignment allows the bone to resist bending and fracturing when forces are applied in a single direction, such as during walking or running. The layers within the osteons contain collagen fibers that alternate in direction, providing resistance to twisting, or torsional, forces. This architecture ensures the skeleton can withstand complex, multi-directional stresses.
Physical Protection of Internal Organs
Beyond supporting the body’s weight, compact bone provides physical protection for internal organs. The hard, dense nature of the cortical bone acts as a shield against external impact. This protective role is evident in flat bones and the outer regions of others.
The cranium, for instance, is a shell of dense compact bone that safeguards the brain from trauma. Similarly, the rib cage protects the heart and lungs. In long bones, the compact bone forms a thick wall around the medullary cavity, protecting the bone marrow housed within.
Role in Mineral Storage and Homeostasis
Compact bone is the body’s largest reservoir for minerals, primarily storing 99% of the body’s calcium and a significant amount of phosphate. This function is dynamic, constantly undergoing remodeling to maintain systemic mineral balance, a process known as homeostasis. The controlled release of these minerals is necessary for countless physiological processes outside of the skeletal system.
The endocrine system regulates this storage function through hormones like parathyroid hormone (PTH) and calcitonin. When blood calcium levels fall, PTH is released, stimulating the release of calcium from the bone matrix into the bloodstream. Conversely, calcitonin is released when blood calcium is too high and inhibits the release of calcium from the bone.
This exchange ensures a stable concentration of calcium in the blood, which is necessary for muscle contraction, nerve signaling, blood clotting, and hormone release. Through continuous bone resorption by osteoclasts and bone formation by osteoblasts, compact bone actively participates in a metabolic process vital for the nervous and muscular systems.