The skeletal system is a framework of bones, cartilage, and ligaments that provides the underlying structure for the human body. This internal scaffold gives the body its shape and protects soft tissues. The skeletal system performs five functions.
Providing Structural Support
The skeleton serves as the body’s main scaffold, resisting gravity and preventing soft tissues from collapsing inward. This foundational support allows humans to maintain an upright posture and bear their own weight. The vertebral column, or spine, forms the central axis of the body, providing vertical support for the head and torso. The skeleton also provides attachment surfaces for muscles, which are necessary for the structure to remain stable and functional.
Protecting Internal Organs
The skeletal system forms passive barriers that shield internal organs from external trauma. This protective function is evident in the skull, a fused bony case that surrounds the brain. The rib cage forms a semi-rigid enclosure around the thoracic cavity, protecting the heart and lungs. The vertebrae that form the spinal column encase the spinal cord. The pelvis also offers protection to the organs of the lower abdomen.
Facilitating Motion
The skeletal system enables movement by working in coordination with the muscular system. Bones function as a system of levers that muscles pull upon. Skeletal muscles attach to bones via tendons, and when these muscles contract, they transmit force to the bone, causing it to pivot at a joint. Joints, where two or more bones meet, provide the points of articulation necessary for a wide range of movements. Cartilage covers the ends of bones at the joints, reducing friction and acting as a shock absorber.
Storing Essential Minerals
Bone tissue is a dynamic reservoir for several inorganic minerals, most notably calcium and phosphate. Approximately 99% of the body’s calcium and 85% of its phosphorus are stored within the bones, incorporated into the hard matrix that gives bone its rigidity. This storage function is not static; the body constantly deposits and withdraws these minerals to maintain a tightly controlled balance in the bloodstream, a process called mineral homeostasis.
When blood calcium levels drop below a certain threshold, hormones like parathyroid hormone (PTH) signal specialized bone cells called osteoclasts to break down bone tissue. This process releases stored calcium back into the circulation, where it is needed for muscle contraction, nerve impulse transmission, and blood clotting. Conversely, when calcium levels are too high, hormones like calcitonin promote the storage of excess calcium back into the bone matrix.
Producing Blood Components
The fifth function of the skeletal system is a physiological process called hematopoiesis, which is the formation of blood cellular components. This vital process occurs within the red bone marrow, a specialized connective tissue found in the internal cavities of certain bones. The red marrow is densely packed with hematopoietic stem cells, which are the precursor cells for all mature blood cells.
These stem cells undergo differentiation to produce red blood cells, which are responsible for oxygen transport throughout the body. They also generate various types of white blood cells, the core components of the immune system that fight off infection. Furthermore, the bone marrow produces platelets, which are cell fragments necessary for blood clotting. In adults, active red marrow is primarily located in flat bones such as the pelvis, sternum, ribs, and vertebrae.