Muscular system explained to kids

The human body is made up of many different kinds of cells. Cells are the basic units of life. Different types of cells do different jobs in the body. Muscles are cells that help the body move. There are three types of muscle cells: skeletal muscle, smooth muscle, and cardiac muscle. Skeletal muscle cells are the type of muscle cells that people use when they move their arms or legs. These cells are also called “striated muscle cells” because they have stripes when you look at them under a microscope. Smooth muscle cells are the type of muscle cells that people use to move their internal organs, such as the stomach and the intestines. These cells are also called “visceral muscle cells”. Cardiac muscle cells are the type of muscle cells that people use to pump blood through their bodies. These cells are also called “striated muscle cells” because they have stripes when you look at them under a microscope. All muscle cells are made up of smaller units called “myofibrils”. Myofibrils are made up of even smaller units called “myofilaments”. Myofilaments are made up of proteins called “actin” and “myosin”. The proteins actin and myosin work together to make muscles move. When a muscle cell contracts, the myofilaments slide past each other. This makes the muscle cell shorter and thicker. The muscle cells are attached to bones by tendons. Tendons are made up of a protein called “collagen”. When a muscle cell contracts, it pulls on the tendon, which then pulls on the bone. This is how muscles make the body move. Muscles can only contract. They cannot push. This is why muscles always work in pairs. For example, when you lift your arm, one muscle contracts to lift the arm up, and another muscle contracts to lower the arm down. Muscles get their energy from a molecule called “ATP”. ATP is made up of three parts: adenosine, ribose, and three phosphate groups. The muscle cells use ATP to power the contraction of the myofilaments. When ATP is used, it is broken down into ADP + Pi. The muscle cells store ATP in small organelles called “mitochondria”. Mitochondria are often called the “powerhouses of the cell” because they produce ATP. Muscles need oxygen to produce ATP. This is why people need to breathe when they exercise. Oxygen enters the body through the lungs and is carried to the muscles by the blood. The blood carries carbon dioxide away from the muscles. Carbon dioxide is a waste product that is produced when ATP is made. The blood also carries nutrients to the muscles. The muscles need these nutrients to produce ATP.