The Skeletal System
The skeletal system is the framework for the body. It is made of bones and joints. The skeletal system is support for the body, support for muscle movement, and produces new blood and immune cells. The bone has four layers, the periosteum, spongy bone, compact bone, and bone marrow. The bone can be remodeled by osteoclasts or osteoblasts, depending on mineral needs. Bones are connected to other bones by different types of joints: immovable, slightly movable, and freely movable.
The Muscular System
The muscular system is made of muscle tissues. It allows body movement, powers the organs, and regulates temperature. The three types of muscle tissue are skeletal muscle, smooth muscle, and cardiac muscle. Muscles are joined to bone via tendons. Muscles are organized into muscle fibers which are themselves organized into sarcomeres, the basic unit of muscle. Sarcomeres are made of thin and thick filaments, which are actin and myosin, respectively. Muscle contraction occurs when the thick filaments pull the thin filaments, shortening the actin, and making the muscle shorter, and thus contraction.
The Integumentary System
The integumentary system is made of skin, hair, and nails. Its basic function is protection of the entire body, as well as exchange and secretion with the external environment. The skin has three layers, the epidermis, dermis, and hypodermis. Hair grows from follicles, and while the hair cell dies it fills with keratin. Nails grow from the tips of toes and fingers, and are also filled with keratin.