The Human Skeleton

Most of an adult's skeleton consists of bone, a tough, rigid tissue. Cartilage is more flexible. It helps joints move and allows the ribcage to bend.

Your skeleton gives your body shape and support. It also shields delicate organs. The brain sits inside the skull, while ribs surround the heart and lungs.

Movement happens when muscles pull on the bones they attach to. Those bones act as levers when you walk, jump or wave.

Inside many bones, red bone marrow produces fresh blood cells. Bones also hold mineral reserves, including calcium, and store fat for later use. Support, protection, movement, storage and blood cell production are all jobs of your skeleton.

Bones, cartilage and ligaments together form the skeletal system, which supports and shapes your body. The usual adult bone count is 206, divided into two groups.

Along the body's central axis are 80 bones called the axial skeleton. These include the skull, ribcage and spine, also known as the vertebral column. They shield organs: the brain lies within the skull, and ribs surround the lungs and heart.

The remaining 126 bones belong to the appendicular skeleton. These are the arm, hand, leg and foot bones, along with the shoulder and hip bones that connect the limbs to the central skeleton. They help you run, throw, climb and dance.

Your skeleton provides support, protects organs and helps movement. It stores fat and minerals too. Certain bones contain red bone marrow, where blood cells are made. Scientists call this production hematopoiesis. The skeleton and muscles together form the musculoskeletal system.

A bone's shape suits its work. The femur, or thigh bone, is a long bone that helps you move. Wrist bones are examples of short bones, with compact, chunky shapes. Thin, often curved flat bones include ribs and skull plates that shield organs. Vertebrae belong to the irregular bones: their shapes do not fit the other groups.

Blood contains tiny cells and cell fragments that need regular replacement. Red cells survive for months. Platelets and many white cells last for days, while some white cells survive only hours.

Red bone marrow makes their replacements. This soft tissue lies inside bones such as the ribs, vertebrae and hip bones. It contains blood-forming stem cells that can develop into different blood cell types.

Chemical messages called growth factors help these cells divide and mature. Red cells transport oxygen, and white cells fight germs. Other marrow cells release platelets, the fragments that help a cut stop bleeding by forming a clot. Your marrow produces billions of fresh blood cells each day.

Bones, ligaments and cartilage make up your skeletal system. Besides supporting you and shielding organs, it gives muscles something firm to pull against. It also stores fat and contains blood-making tissue.

Its central group, the axial skeleton, consists of the ribcage, skull and spine. These bones protect the lungs, heart, brain and spinal cord.

The appendicular skeleton contains limb bones and the shoulder and hip bones that join them to the body's center. It helps your arms and legs move. Think of the axial group as the central protective framework and the appendicular group as the bones that move your limbs.