Regular exercise benefits both the body and the mind. It helps keep bones, joints, and muscles healthy and strong. It’s especially good for the heart and circulatory
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THE HUMAN BODY is an introduction to the amazing and complicated machines we live in. This guide o˜ers facts, interaction tips, and prompts for conversation as you and your child explore eight of the body™s systems: NERVOUS SKELETAL RESPIRATORY CIRCULATORY DIGESTIVE MUSCULAR UROGENITAL IMMUNEOpen up THE HUMAN BODY and see how we work.

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Nervous systemGET STARTED Laughing, tasting, seeing, hearing, smelling, daydreaming, and singing are just a few of the amazing things we can do because of the nervous system. The nervous system includes the spinal cord, nerves, neurons, and sensory organs: the ears, nose, eyes, tongue, and skin. At the center of it all is the brain. A brilliant scientist, James D. Watson (he co-discovered the structure of DNA), said the brain is fithe most complex thing we have yet discovered in our universe.fl The brain is constantly sending and receiving signals from tiny cells called neurons. Neurons pass messages to other neurons through synapses. These messages are transmitted along the spinal cord which, along with the skull and vertebrae, connects the brain to the rest of the body. Neurons send and receive signals to and from the brain (at speeds faster than 150 miles per hour!) to tell the body to do things like contract muscles, pump blood, breathe, blink, regulate its temperature, and stay balanced. Through neurons, our brains process messages from the skin, eyes, ears, nose, and tongue about how things feel, look, sound, smell, and taste. This is how we know how sharp a pin is, how hot a cup of hot chocolate is, and how to respond. When the skin feels something sharp or hot, the brain sends a message for us to move away from it.

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DISCOVER Tap the mosquito and send it ˜ying. What happens when the mosquito bites the body? Ouch! We feel mosquito bites because the skin reacts to touch. Skin is an organ that constantly sends messages about the stimuli it feels to the brain. Often, it™s sending messages to keep us from doing things that might hurt, like touching a hot or sharp object. It might also tell you to slap that mosquito so the little bugger doesn™t bite you again. Drag the feather and tickle the body. What happens? Skin feels the tickle of a feather the same way it feels the bite of a mosquito: by sending a message to the brain. But why we laugh when we™re tickled is a bigger mystery, even scientists haven™t quite ˜gured it out. Many parts of the brain work together when we laugh. We do know that laughter is a way we communicate and that it can be triggered by thoughts and memories. Drag the ˜owers to the nose. How does smell travel to the brain? Smell starts inside the nose where special cells, called olfactory receptors, ˜rst detect a scent and send signals to the olfactory bulb in the brain. Located at the bottom of the brain, the olfactory bulb processes and identi˜es unique odors. Swipe the legs. How does the nervous system respond to exercise? The nervous system is super snappy during exercise: the brain is working with the muscular, circulatory, and respiratory systems. The cerebellum helps voluntary muscles with posture, balance, and equilibrium. The brainstem controls the involuntary muscles found in the circulatory and respiratory systems that are also working to keep you breathing and your blood moving. Regular exercise bene˜ts both the body and the mind. It helps keep bones, joints, and muscles healthy and strong. It™s especially good for the heart and circulatory system, keeping blood vessels ˚exible and open and improving blood pressure. Exercise also releases chemicals, called endorphins, in the brain. Endorphins make you feel happy, alert, and help you concentrate. Take a closer look: tap the nose. Drag different objects under it. How does the brain react to smell? Everything you smell, from fresh baked bread to a stinky trash can, puts molecules into the air. When we smell, the inside of the nose, the nasal cavity, detects these chemical molecules. The nasal cavity is lined with a wet, sticky liquid (mucus) and tiny hair cells (cilia, which you™ll also ˜nd in the ear). The mucus helps capture dust and germs, preventing them from entering the body. The cilia detect microscopic molecules and tell the nerves to send a message to the brain. The brain responds and identi˜es what you™re smelling. You might sneeze to get rid of unwanted germs, or your mouth might water at the thought of– mmm–bacon. When the brain recognizes the smell of something tasty, it tells the mouth to create saliva in anticipation of eating and digesting food. Smell also helps the sense of taste tell the difference among foods that are sweet, salty, sour, bitter, and spicy.

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Take a closer look: tap the brain. Move the slider. What do you see? While the brain is so complicated that scientists are still ˜guring out exactly how everything works, there are some things we do know about it, starting with what we can easily see: all those wrinkles. The outer surface of the brain, the cerebral cortex, has crevices, called sulci, and ridges, called gyri, to increase the surface area of the brain without making it too big for the skull. The larger surface area allows for more neurons. We need all of the room for neurons we can get: adults have about 90,000 miles of neural pathways in their brains. Tap different parts of the brain. What processes happen in each part? Most thoughts, language, movement, and memories are processed in the part of the brain called the cerebrum. Four different parts of the cerebrum, called lobes, manage different tasks. The frontal lobe processes speech, thought, learning, emotions, and movement. The occipital lobes process visual images. The parietal lobes process touch, temperature, and pain. And the temporal lobes process sounds, along with some memories and vision. MEMORY Memory allows us to retain what we™ve learned over time. As we learn, neurons connect to one another; these connections are called synapses. As we learn more and more, the number of syn -apses between neurons increases, creating pathways in our brains. When we remember some -thing, the brain reconstructs those pathways to recall the information. This sounds simple enough, but our thoughts and knowledge aren™t neatly ˜led away in one place like books on a shelf: those neural pathways wind all over. As the brain (constantly!) processes new information, neural pathways change and connect in different ways. We tend to remember things better if we link them to other things we already know and understand, so to retrieve memo -ries, the brain traces back through all of those paths. This is why memories seem to change some – times. The way that you think of something now may not be the way you think of it, or remember it, in the future. MUSICSound is processed in the brain™s temporal lobes, which also help process memory and vision. We know that listening to and enjoying music involves memory, learning, and emotions. But just how and why this happens remains a mystery. VISIONImages are processed in the brain™s occipital lobes with a little help from the temporal lobes. Signals from the eyes are translated by the visual cortex in the occipital lobe, which processes information about shape, color, texture, size, distance, depth, movement, and location. It also turns what you see right side up (learn more about that in the eye).

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Take a closer look: tap the ear. Speak or drag an instrument to the ear. How does sound travel? When somethingŠyour voice or a violinŠmakes a noise, it sends out vibrations, also called sound waves. Hearing is the ability to detect those vibrations in the air. Sound waves travel from the air into your ear and through the auditory canal. When they reach the eardrum, the waves turn into vibrations that travel into the inner ear, then to the cochlea which is ˜lled with ˚uid and lined with tiny hairs called cilia (just like in the nose). Neurons attached to the cilia detect the sound waves and send a signal to the brain. Your brain tells you what you hear. Take a closer look: tap the eye. What and how does it see? Yep, what you see is actually upside down in your brain. The camera on your device is acting in the same way your eye reads light. In the eye, light travels through the lens and onto the retina, which ˜rst displays what you see upside down. Cells called photoreceptorsŠeach eye has almost 125 million of them!Šon your retina detect the light in this upside down image and send signals to the brain along the optic nerve. The occipital lobes receive the signals and process what you™re seeing, right side up. Experiment with the eye. Cover the camera and tap the eye. What happens? When you block or let light into the eye, the size of the pupil changes. It gets bigger to allow more light to pass through to the retina when it™s dark, and shrinks to let less light in when it™s bright. Your eyes adjust according to the amount of light so you can see regardless of whether it™s day or night. The magic part: your eyes do all of this automatically. When you blink, the eyelid and its eyelashes help protect the eye by keeping tiny particles of dust and dirt from entering it. The eyelid also helps to keep the eye moist, spreading a layer of tears, oil, and mucus over the cornea every time we blink. An average person blinks 15 to 20 times per minute!Eyes can be many different colors, from brown, blue or green, to hazel or gray. Our genes determine eye color. But, that doesn™t mean you have the same color eyes as your parents, it can still be a surprise. Eye color is one of those things that™s so complicated scientists are still trying to ˜gure it out.

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Skeletal system GET STARTED The skeleton gets a bum rap as a creepy creature that lurks in graveyards (and closets); but the truth is, we wouldn™t be able to stand upright, sit, or move, without our bones. A total of 206 bones provide a framework for our bodies, protect important organs, and support movements big and smallŠfrom running to tying our shoes. Teeny-tiny bones in our hands enable precise movements making them the most useful tools in the world. While bones are strongŠpound for pound, stronger than concreteŠthey™re not totally solid. Our bones have tiny holes, like a sponge. And, they™ve got a lot going on inside: new blood cells are being made. The role that the skeletal system plays in our health and growth is anything but bare.

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DISCOVER Drag the bones apart and reassemble them. How do bones ˚t together? Like a puzzle, bones ˜t together to do speci˜c jobs in the body. They work so well together that you might not even realize you have 206 of them. For example, your skull: that hollow ball comprises eight bones that create a case to protect the brain. Twenty-four ribsŠpoke your side (you can feel them)Šform a cage to protect the lungs, heart, and other internal organs. Twenty-four small bones, called vertebrae, stack on top of each other to create the spinal column, for short, the spine. It helps us sit up straight, twist, or bend up and down. Vertebrae also protect the spinal cord, the main passageway for messages going to the brain. Take a closer look: tap the hand or pelvis. Swipe to move the bones. How do joints help them move? Bones swing and slide, bend and straighten from the places they™re connected by joints. Different types of joints allow for different movements; here are a few examples: In the elbow and knee, HINGE JOINTS allow the arms and legs to bend and straighten. They™re used every time you take a step. In the shoulder and hip, BALL-AND-SOCKET JOINTS allow for even greater movementŠ rotationŠof arms and legs. Ball-and-socket joints are used when you throw a ball. In the thumb, the SADDLE JOINT enables side-to-side and backwards-and-forwards movement. You use the saddle joint to hold a pencil. In ankles and wrists, GLIDING JOINTS connect ˚at or slightly curved bones, letting them slide back and forth. You use gliding joints when you shake a new friend™s hand. Also found in wrists, CONDYLOID JOINTS permit all movement except rotation. You use condy -loid joints when you make your hand into a ˜st and curl it in. You can™t see it in the app (it can™t be shown in two dimensions!) but the PIVOT JOINT in the neck enables the head to turn. Look to the left or right and you™re using the pivot joint.

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Take a closer look: tap the magnifying glass or a bone. What are bones made of, and what do they make? Ranging from solid to spongy, the composition of bone varies as you move from the outside in. The strong, outermost part of the bone is called the compact bone. Inside the compact bone is the cancellous, or spongy, bone; it™s softer and has tiny holes for blood vessels and nerves to weave through. In the core of the bone, you™ll ˜nd marrow. Bone marrow is made up of tissue, fat, veins, and arteries: this is where all those new red blood cells are produced. About 500 billion blood cells are made inside your bones every day. (Learn more about blood in the circula -tory system.) Bones also add new cells and get bigger and stronger as we grow. DISCUSSWhy do you have bones?How are your bones connected? Why do your bones have di˜erent shapes? What are your bones made of? What™s inside of a bone? What do your bones do? How do your bones change as you grow? What allows the spine to move?fifi

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Respiratory system GET STARTED Breathing is essential to our survival. Thankfully, it happens automatically so we don™t have to think twice about it. With the help of an involuntary muscle, the diaphragm, air enters through the nose or mouth, travels down the windpipe, through the bronchi and an intricate network of airways, and ˚nally, into the lungs. We take in the oxygen needed for energy and growth and exhale to expel carbon dioxide. This process continues as we breathe in and out, and in and out, about 20,000 times a day.

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