What Are Bones and What Do They Do? Bones provide support for our bodies and help form our shape. Although they're very light, bones are strong enough to support our entire weight. Bones also protect the body's organs. The skull protects the brain and forms the shape of the face. The spinal cord, a pathway for messages between the brain and the body, is protected by the backbone, or spinal column. The ribs form a cage that shelters the heart and lungs, and the pelvis helps protect the bladder, part of the intestines, and in women, the reproductive organs. Bones are made up of a framework of a protein called collagen, with a mineral called calcium phosphate that makes the framework hard and strong. Bones store calcium and release some into the bloodstream when it's needed by other parts of the body. The amounts of some vitamins and minerals that you eat, especially vitamin D and calcium, directly affect how much calcium is stored in the bones. Bones are made up of two types of bone tissues: Compact bone is the solid, hard outside part of the bone. It looks like ivory and is extremely strong. Holes and channels run through it, carrying blood vessels and nerves. Cancellous (KAN-suh-lus) bone, which looks like a sponge, is inside compact bone. It is made up of a mesh-like network of tiny pieces of bone called trabeculae (truh-BEH-kyoo-lee). This is where bone marrow is found. In this soft bone is where most of the body's blood cells are made. The bone marrow contains stem cells, which produce the body's red blood cells and platelets, and some types of white blood cells. Red blood cells carry oxygen to the body's tissues, and platelets help with blood clotting when someone has a cut or wound. White blood cells help the body fight infection. Bones are fastened to other bones by long, fibrous straps called ligaments (LIG-uh-mentz). Cartilage (KAR-tul-ij), a flexible, rubbery substance in our joints, supports bones and protects them where they rub against each other. How Do Bones Grow? The bones of kids and young teens are smaller than those of adults and contain "growing zones" called growth plates. These plates consist of multiplying cartilage cells that grow in length, and then change into hard, mineralized bone. These growth plates are easy to spot on an X-ray. Because girls mature at an earlier age than boys, their growth plates change into hard bone at an earlier age. Bone-building continues throughout life, as a body constantly renews and reshapes the bones' living tissue. Bone contains three types of cells: osteoblasts (AHS-tee-uh-blastz), which make new bone and help repair damage osteocytes (AHS-tee-o-sites), mature bone cells which help continue new born formation osteoclasts (AHS-tee-o-klasts), which break down bone and help to sculpt and shape it What Are Muscles and What Do They Do? Muscles pull on the joints, allowing us to move. They also help the body do such things as chewing food and then moving it through the digestive system. Even when we sit perfectly still, muscles throughout the body are constantly moving. Muscles help the heart beat, the chest rise and fall during breathing, and blood vessels regulate the pressure and flow of blood. When we smile and talk, muscles help us communicate, and when we exercise, they help us stay physically fit and healthy. Humans have three different kinds of muscle: Skeletal muscle is attached by cord-like tendons to bone, such as in the legs, arms, and face. Skeletal muscles are called striated (STRY-ay-ted) because they are made up of fibers that have horizontal stripes when viewed under a microscope. These muscles help hold the skeleton together, give the body shape, and help it with everyday movements (known as voluntary muscles because you can control them). They can contract (shorten or tighten) quickly and powerfully, but they tire easily. Smooth, or involuntary, muscle is also made of fibers, but this type of muscle looks smooth, not striated. We can't consciously control our smooth muscles; rather, they're controlled by the nervous system automatically (which is why they're also called involuntary). Examples of smooth muscles are the walls of the stomach and intestines, which help break up food and move it through the digestive system. Smooth muscle is also found in the walls of blood vessels, where it squeezes the stream of blood flowing through the vessels to help maintain blood pressure. Smooth muscles take longer to contract than skeletal muscles do, but they can stay contracted for a long time because they don't tire easily. Cardiac muscle is found in the heart. The walls of the heart's chambers are composed almost entirely of muscle fibers. Cardiac muscle is also an involuntary type of muscle. Its rhythmic, powerful contractions force blood out of the heart as it beats. How Do Muscles Work? The movements that muscles make are coordinated and controlled by the brain and nervous system. The involuntary muscles are controlled by structures deep within the brain and the upper part of the spinal cord called the brain stem. The voluntary muscles are regulated by the parts of the brain known as the cerebral motor cortex and the cerebellum (ser-uh-BEL-um). When you decide to move, the motor cortex sends an electrical signal through the spinal cord and peripheral nerves to the muscles, making them contract. The motor cortex on the right side of the brain controls the muscles on the left side of the body and vice versa. The cerebellum coordinates the muscle movements ordered by the motor cortex. Sensors in the muscles and joints send messages back through peripheral nerves to tell the cerebellum and other parts of the brain where and how the arm or leg is moving and what position it's in. This feedback results in smooth, coordinated motion. If you want to lift your arm, your brain sends a message to the muscles in your arm and you move it. When you run, the messages to the brain are more involved, because many muscles have to work in rhythm. Muscles move body parts by contracting and then relaxing. Muscles can pull bones, but they can't push them back to the original position. So they work in pairs of flexors and extensors. The flexor contracts to bend a limb at a joint. Then, when the movement is completed, the flexor relaxes and the extensor contracts to extend or straighten the limb at the same joint. For example, the biceps muscle, in the front of the upper arm, is a flexor, and the triceps, at the back of the upper arm, is an extensor. When you bend at your elbow, the biceps contracts. Then the biceps relaxes and the triceps contracts to straighten the elbow. What Are Joints and What Do They Do? Joints are where two bones meet. They make the skeleton flexible — without them, movement would be impossible. Joints allow our bodies to move in many ways. Some joints open and close like a hinge (such as knees and elbows), whereas others allow for more complicated movement — a shoulder or hip joint, for example, allows for backward, forward, sideways, and rotating movement. Joints are classified by their range of movement: Immovable, or fibrous, joints don't move. The dome of the skull, for example, is made of bony plates, which move slightly during birth and then fuse together as the skull finishes growing. Between the edges of these plates are links, or joints, of fibrous tissue. Fibrous joints also hold the teeth in the jawbone. Partially movable, or cartilaginous (kar-tuh-LAH-juh-nus), joints move a little. They are linked by cartilage, as in the spine. Each of the vertebrae in the spine moves in relation to the one above and below it, and together these movements give the spine its flexibility. Freely movable, or synovial (sih-NO-vee-ul), joints move in many directions. The main joints of the body — such as those found at the hip, shoulders, elbows, knees, wrists, and ankles — are freely movable. They are filled with synovial fluid, which acts as a lubricant to help the joints move easily. Three kinds of freely movable joints play a big part in voluntary movement: Hinge joints allow movement in one direction, as seen in the knees and elbows. Pivot joints allow a rotating or twisting motion, like that of the head moving from side to side. Ball-and-socket joints allow the greatest freedom of movement. The hips and shoulders have this type of joint, in which the round end of a long bone fits into the hollow of another bone. Back to Articles Related Articles Growth Plate Fractures Injuries to growth plates, which produce new bone tissue and determine the final length and shape of bones in adulthood, must be treated so that bones heal properly. Read More Hamstring Strain A hamstring strain happens when one or more of the muscles in the back of the leg gets stretched too far and starts to tear. Find out how to treat hamstring strains in this article for teens. Read More First Aid: Strains and Sprains Here's what to do if you think your child has pulled or torn a muscle, ligament, or tendon. Read More First Aid: Broken Bones A broken bone needs emergency medical care. Here's what to do if you think your child just broke a bone. Read More Stress Fractures It's not always easy to tell if you have a stress fracture, and stress fractures can get worse quickly. This article explains how to prevent and treat them. Read More Movie: Bones & Skeletal System Watch a movie about your bones. Read More Growing Pains Does your child sometimes wake up crying in the middle of the night complaining of throbbing leg pain? It could be growing pains. Read More What a Pain! Kids and Growing Pains Growing pains are for real. Usually they happen when kids are between the ages of 3 and 5 or 8 and 12. Read More Your Muscles You have more than 600 muscles in your body! They do everything from pumping blood throughout your body to helping you lifting your heavy backpack. Find out more. Read More Broken Bones What happens when you break a bone? Read More Scoliosis Scoliosis makes a person’s spine curve from side to side. Large curves can cause health problems like pain or breathing trouble. Health care providers treat scoliosis with back braces or surgery when needed. Read More Strains and Sprains Sprains and strains are common injuries, especially for people who play hard or are into sports. Find out what they are and how to recuperate from one. Read More Your Bones Where would you be without your bones? Learn more about the skeletal system in this article for kids. Read More Broken Bones Bones are tough stuff - but even tough stuff can break. Find out what happens when a bone fractures. Read More Broken Bones Many kids will have a broken bone at some point. Here's what to expect. Read More Casts Some injuries will heal best if a cast is used. Find out how they work and how to take care of them in this article for kids. Read More Bones, Muscles, and Joints Our bones, muscles, and joints form our musculoskeletal system and enable us to do everyday physical activities. Read More Achilles Tendonitis If the tendon just above your heel becomes swollen or irritated due to overuse, it can lead to a painful condition called Achilles tendonitis. Find out how to treat it - and prevent it. Read More 3 Ways to Build Strong Bones We build almost all our bone density when we're kids and teens. Kids with strong bones have a better chance of avoiding bone weakness later in life. Here's how parents can help. Read More How Broken Bones Heal Broken bones have an amazing ability to heal, especially in kids. Here's how. Read More How Broken Bones Heal Broken bones have an amazing ability to heal. New bone forms within a few weeks of the injury, although full healing can take longer. Read More How Broken Bones Heal Broken bones have an amazing ability to heal, especially in kids. Full healing can take time, but new bone usually forms a few weeks after an injury. Read More Buckle Fractures A buckle or torus fracture is a type of broken bone. One side of the bone bends, raising a little buckle, without breaking the other side of the bone. Teens don't usually get this type of fracture. Read More Buckle Fractures A buckle or torus fracture is a type of broken bone. One side of the bone bends, raising a little buckle, without breaking the other side of the bone. Read More Casts Casts keep bones and other tissues in place while they heal. Here's what to expect, and how to care for casts. Read More Casts This article for teens has tips on taking care of a cast so it keeps working as it should. Read More Note: All information is for educational purposes only. For specific medical advice, diagnoses, and treatment, consult your doctor. © 1995-2021 KidsHealth®. All rights reserved. Images provided by The Nemours Foundation, iStock, Getty Images, Veer, Shutterstock, and Clipart.com.