65 Red blood cells carry oxygen throughout the body, white blood cells help the body fight infection and other diseases. THE SKELETAL AND MUSCULAR SYSTEMS The skeleton The human skeleton consists of 206 bones made of calcium and other minerals that make them strong and slightly elastic structures. The skeleton has four main functions: supporting the body; Any physical activity can stimulate the production of synovial fluid (i.e. lubrication) in the joints. protecting the organs; helping the body move some bones are rigidly joined together and cannot move (for example the bones of the skull), but others are joined to each other by flexible joints to let certain parts of the body move with the help of muscles; making blood cells: red blood cells and white blood cells are produced in the bone marrow, which is a soft tissue inside our larger bones. Joints Bones are connected together by joints. The ends of the bones in a joint are covered with a smooth substance called cartilage which is kept slippery and lubricated by a liquid called synovial fluid . Muscles The human body has over 650 muscles, which are pieces of soft tissue that work by contraction: when a muscle contracts, it pulls on the bone, and the bone can move if it is part of a joint. Muscles are attached to the bones by tendons, cords of strong, flexible tissue. Muscles work in pairs: when one contracts, its antagonist relaxes. There are three types of muscles: skeletal muscles are attached to bones and allow movement. They are under our conscious control, which is why they are also known as voluntary muscles; smooth muscles are located in various internal structures including the digestive tract, uterus, and blood vessels such as arteries. They facilitate the passage of nutrients or blood and are also called involuntary muscles because they are not under our control; the cardiac muscle is specific to the heart. It contracts and relaxes without our conscious awareness. ankle: caviglia blood vessel: vaso sanguigno joint: articolazione marrow: midollo rib: costola slightly: lievemente slippery: scivoloso squat: tozzo wrist: polso 132 SCIENCE AND PHYSICS Types of bones There are four different types of bones in the human body: long bones have a long, thin shape, e.g. the bones of arms and legs; short bones have a squat, cubed shape, e.g. the bones of wrists and ankles; flat bones have a flattened surface, e.g. ribs, shoulders, breastbone, and skull bones; irregular bones have a shape that does not conform to the above three types, e.g. the vertebrae.