What is the skeleton and what does it do?
The skeleton is all the bones of the body joined together. A newborn baby has about 300 bones; many join as we grow, so an adult has 206.
The skeleton has five main jobs:
- Support and shape: it holds the body up and gives it its shape.
- Protection: the skull guards the brain, the ribs guard the heart and lungs, the backbone guards the spinal cord.
- Movement: muscles are fixed to bones and pull them.
- Making blood cells: red marrow inside some bones makes red cells, white cells and platelets.
- Storing minerals: bones store calcium and phosphorus and release them when the body needs them.
Main parts
The axial skeleton is the middle line: skull, backbone (vertebral column), ribs and breastbone. The appendicular skeleton is the limbs plus the shoulder and hip girdles that join them on.
Types of bones and what bone is made of
Bones come in different shapes for different jobs:
- Long bones (femur, humerus): act as levers for big movements.
- Short bones (wrist, ankle): small and strong, for small careful movements.
- Flat bones (skull, ribs, shoulder blade): wide plates that protect and give room for muscles to attach.
- Irregular bones (vertebrae): odd shapes for special jobs.
Bone is living tissue with blood vessels and cells. It is made of a protein called collagen (makes it a little bendy) and calcium salts (make it hard). The outside is hard compact bone; inside is light, honeycomb spongy bone and soft marrow.
Joints, ligaments, tendons and cartilage
A joint is a place where two or more bones meet.
- Fixed joints: no movement, e.g. between skull bones.
- Slightly movable joints: a little movement, e.g. between vertebrae.
- Freely movable (synovial) joints: lots of movement. They have a capsule filled with slippery synovial fluid.
Kinds of movable joints
- Hinge: bends one way, like a door (elbow, knee).
- Ball and socket: a round end sits in a cup, moves in all directions and rotates (shoulder, hip).
- Pivot: turns round (neck, turning the head "no").
- Gliding: flat bones slide (wrist, ankle bones).
Cartilage covers bone ends so they do not rub. Ligaments join bone to bone and hold the joint together. Tendons join muscle to bone.
Muscles and movement
Muscles can only pull (contract, get shorter). They cannot push. So they work in antagonistic pairs: when one contracts (the agonist), the other relaxes (the antagonist).
- Biceps contracts, triceps relaxes: the elbow bends (flexion).
- Triceps contracts, biceps relaxes: the elbow straightens (extension).
- Hamstrings and quadriceps do the same at the knee.
Three kinds of muscle
Skeletal muscle moves bones and is under our control. Smooth muscle is in the gut and blood vessels and works on its own. Cardiac muscle is only in the heart and never tires.
Keeping bones healthy
Eat calcium (milk, curd, ragi, green leafy vegetables) and get vitamin D (sunlight). Weight-bearing exercise like running and skipping makes bones denser. Good posture protects the back. Weak, thin bones in old age is called osteoporosis; a broken bone is a fracture; swelling of joints is arthritis.
Key formulas and definitions
- Adult skeleton: 206 bones
- Ligament: bone ↔ bone
- Tendon: muscle ↔ bone
- Cartilage: cushion on bone ends
- Antagonistic pair: one contracts, the other relaxes
- Hinge joint: one plane (elbow, knee)
- Ball and socket: all directions (shoulder, hip)
Worked examples
1. A footballer kicks a ball. Name the joint at the knee and the muscle pair that straightens the leg.
The knee is a hinge joint. To straighten (extend) the leg, the quadriceps (front of thigh) contracts and the hamstrings (back of thigh) relax.
2. Why can the arm swing in a full circle at the shoulder but not at the elbow?
The shoulder is a ball-and-socket joint: the round head of the humerus turns in a cup, so it moves in all directions. The elbow is a hinge, so it only bends and straightens in one plane.
3. A person's blood test shows very few red blood cells. Which part of the skeleton could be involved, and why?
Red bone marrow, found inside bones such as the femur, ribs, breastbone and pelvis. It makes red blood cells, so if marrow is not working well, fewer red cells are made.
Common mistakes
- Thinking muscles can push. Muscles only pull; a second muscle pulls the other way.
- Mixing up ligaments and tendons. Ligament = bone to bone; tendon = muscle to bone.
- Thinking bones are dead and dry. Living bone has blood vessels and cells and can heal.
- Saying everyone has 206 bones from birth. Babies have about 300; some fuse as they grow.