The human skeleton serves as the body’s fundamental support structure, providing a complex framework that gives the body its shape. Composed of approximately 206 bones in adulthood, along with cartilage, ligaments, and tendons, the skeletal system is a dynamic and active tissue. It performs several sophisticated functions, continuously working within the body.
Providing Structure and Protection
The skeleton acts as the body’s internal framework, providing a solid structure that supports the body against gravity and helps maintain posture. This framework gives the human body its characteristic shape and serves as an anchor for connecting tissues.
Beyond providing shape, the skeletal system offers protection for many internal organs. The skull forms a rigid enclosure that safeguards the brain from external forces. The rib cage creates a protective barrier around the heart and lungs. The vertebrae, which make up the spinal column, encase and protect the spinal cord.
Enabling Movement
Bones collaborate with muscles, joints, ligaments, and tendons to form the musculoskeletal system. Bones function as levers, with muscles pulling on them to create motion. This coordinated action enables everything from walking and running to fine motor skills like writing.
Joints, where two or more bones meet, facilitate movement and flexibility. Different types of joints, such as hinge joints in the knees and elbows or ball-and-socket joints in the hips and shoulders, permit various degrees and directions of motion. Tendons attach muscles to bones, while ligaments connect bones to other bones, providing stability to the joints during movement.
Producing Blood Cells
The skeleton produces blood cells, a process known as hematopoiesis. This occurs within the red bone marrow, a spongy tissue found inside certain bones. In adults, red bone marrow is primarily located in flat bones such as the skull, ribs, sternum, and pelvis, as well as at the ends of long bones like the femur and humerus.
Within the red bone marrow, specialized stem cells produce new blood cells. These include red blood cells, which transport oxygen throughout the body. White blood cells, which fight infections, are also generated here. Platelets, which are important for blood clotting, originate in the bone marrow.
Storing Vital Minerals
The skeletal system serves as a reservoir for minerals, primarily calcium and phosphorus. Approximately 99% of the body’s calcium and about 85% of its phosphorus are stored within the bones. These minerals are integrated into the bone matrix, providing hardness and strength to the skeletal structure.
When needed, these stored minerals can be released from the bones into the bloodstream to maintain levels for various bodily functions. Calcium, for example, is important for nerve function, muscle contraction, and blood clotting. Bone remodeling involves the continuous release and storage of these minerals, ensuring their availability for metabolic needs.
Metabolic and Endocrine Roles
Beyond its more widely recognized functions, the skeleton also performs metabolic and endocrine roles, acting as an active organ in regulating body processes. Bone cells produce hormones, notably osteocalcin, which is secreted by bone-forming cells called osteoblasts.
Osteocalcin plays a role in glucose metabolism and fat deposition. Specifically, a form of osteocalcin promotes insulin secretion and stimulates the growth of pancreatic beta-cells. It also contributes to reducing the accumulation of visceral fat and decreasing fat storage in the liver. The yellow bone marrow, found in the central cavities of long bones, serves as an energy reserve by storing fat.