The skeletal system, composed of bones, cartilage, ligaments, and tendons, is a dynamic and metabolically active organ system. The skeleton is constantly remodeling and interacting with other bodily systems to maintain overall health and function. This complex network provides the body with mechanical support while simultaneously performing several life-sustaining tasks.
Providing Structural Support
The skeleton functions as the internal framework that anchors all the body’s soft tissues and organs against the force of gravity. This rigid structure gives the human body its characteristic shape and maintains upright posture. The long bones of the legs, such as the femur and tibia, are engineered to bear and transmit the weight of the upper body during standing and movement.
The vertebral column, or spine, serves as the main central pillar, supporting the head and torso while also providing attachment points. Muscles, tendons, and ligaments attach to the bone surfaces, allowing the skeleton to hold the body’s form and resist collapse.
Shielding Vital Organs
A primary function of the skeleton is to offer physical protection to vital organs from external trauma. Specific bony enclosures have evolved to shield these sensitive structures. The cranium, a fused dome of bone, completely surrounds and protects the brain from impact.
The thoracic cage, formed by the ribs, sternum (breastbone), and thoracic vertebrae, creates a protective barrier for the heart and lungs. The ribs possess flexibility that allows the chest cavity to expand during respiration while acting as a shock absorber. The vertebral column encases the entire length of the spinal cord, providing a segmented shield for the central nervous system’s main communication pathway.
Serving as Levers for Movement
Bones interact with skeletal muscles to produce all forms of bodily motion, from walking to fine motor skills. The bones act as rigid levers, with the joints serving as the fixed points, or fulcrums, around which movement occurs. Muscles attach to the bones via tendons and provide the force necessary to move a load, often the weight of the body part itself.
This biomechanical arrangement allows a relatively small muscle contraction to generate significant movement at the end of a limb. For instance, the contraction of the biceps muscle in the arm causes the forearm bones to pivot at the elbow joint. Most movements utilize a third-class lever system, which favors a wide range of motion and speed at the expense of force.
Maintaining Mineral Balance
The skeletal system functions as the body’s primary reservoir for minerals, particularly calcium and phosphate. Over 99% of the body’s calcium is stored within the bone matrix in the form of hydroxyapatite crystals. This mineral storage is actively regulated to maintain a stable balance in the bloodstream.
When blood calcium levels drop, hormones like parathyroid hormone (PTH) signal specialized bone cells called osteoclasts to break down small amounts of bone tissue. This process releases stored calcium back into the blood for use in functions like nerve impulse transmission and muscle contraction. Conversely, when blood calcium levels are high, the hormone calcitonin promotes the deposition of excess calcium back into the bone matrix by cells called osteoblasts.
Manufacturing Blood Cells
A critical function of the skeletal system is hematopoiesis, the process of producing blood cells. This process occurs within the red bone marrow, a spongy tissue found inside the cavities of certain bones. In adults, red marrow is concentrated in flat bones like the sternum, ribs, and pelvis, and the ends of long bones.
Hematopoietic stem cells within the red marrow constantly differentiate to form all the components of blood. These include red blood cells, which transport oxygen throughout the body, white blood cells important for immune function, and platelets necessary for blood clotting. The skeletal system thus ensures a constant supply of blood cells daily.