4.1 Introduction to the Musculoskeletal System
Fig.4-1 The Skeleton
Task 1 Fill in the numbered blanks according to the video.
The musculoskeletal system provides mechanical support for the body，protects internal organs and permits movement.It is 1.________of bones，cartilage，skeletal muscles，2.________and collective tissues such as tendons and ligaments.Bones also serve as the body’s main mineral reservoir.They store calcium and phosphate and release them according to the body’s needs.Red bone marrow is the body’s production center for blood cells.
The central nervous system controls body movements by stimulating skeletal muscles to 3.________.Contraction of skeletal muscles moves bones，which act as levers.Bones 4.________with each other through joints.Cartilage provides padding for the ends of bones within joints.Muscles are collected to bones by tendons，while bones are held together by ligaments.(https://www.daowen.com)
Bones are classified according to their shapes and corresponding functions.Long bones are responsible for most body movements，short bones provide some limited motion，while flat bones and irregular bones are mainly protective and supportive.The bone tissue or osseous tissue is composed of bone cells and a characteristic extracellular matrix.
Bone matrix is made of 5.________component，mainly collagen and inorganic component of minerals，mainly calcium.Collagen gives bones 6.________while calcium provides stiffness.
Without calcium，bones would be soft and bent easily.On the other hand，without collagen，bones would be brittle like chalk.Bones renew and remodel throughout a person’s life in a process known as bone remodeling，which constantly removes old bone tissue and adds new bone tissue.Bone remodeling serves to re-shape bones to 7.________to changing mechanical needs，and to repair everyday micro-damages as well as fractures following 8.________.This process also underlies the mechanism by which the constant levels of plasma calcium and phosphate are maintained.
Bone remodeling is performed by two types of cells：osteoclasts，which dissolve bony matrix，and osteoblasts，which 9.________new matrix around themselves to form new bone tissue.Bone remodeling is under control of complex signaling 10.________.Major regulators include parathyroid hormone，vitamin D，growth hormones，glucocorticoids，thyroid hormones，estrogen and testosterone.
