B7

Small fragments of cells that help the blood to clot at the site of a wound. 

Help to fight infection by protecting your body against attack from microorganisms. 

Transport oxygen from the lungs to the rest of the body. They don't have a nucleus so they can be packed full with haemoglobin- a substance that bind with oxygen. Red blood cells have a biconcave shape to give them a large surface area for exchanging oxygen. 

The liquid that carries nutrients, antibodies, hormones and waste. 

Support the body.

Allow the body to move.

Protect vital organs. 

The bones at a joint are held together by ligaments. Ligaments have a high tenisle strength but they are also slightly elastic. This means they help to stabilise joints but still allow movement. 

The ends of bones are covered with a smooth layer of cartilage to reduce friction between the bones. Cartilage can be slightly compressed so it acts as a shock absorber.

Membranes at some joints release oily synovial fluid to lubricate the joints, allowing them to move more easily by reducing friction. 

Bones are attached to muscles by tendons.

Muscles move bones at a joint by contracting.

Tendons can't stretch much so when a muscle contracts a tendon pulls on the bone, transmitting the force from the muscle to the bone. 

Muscles can only pull on bones to move a joint- they can't push. This is why muscles usually come in pairs (antagonistic pairs).

When one muscle in the pair contracts, the join moves in one direction. When the other muscle contracts, it moves in the opposite direction.

Being fit is a measure of how well you can do physical activities. 

Exercise increases fitness, and it's most effective when it's done in a structured way by following a regime. Fitness practitioners can design fitness regimes for people. 

• Health problems
• Current medication
• Previous fitness treatments
• Family medical history
• Physical activity