Part 3

Eyes and sight Your eyes are responsible for your sense of sight, allowing you to see what is going on around you. They collect light from objects and process the information to create images of what you are looking at. Lens This clear disk focuses light onto the retina. Pupil Light enters the eye through this hole in the iris. Cornea This dome-shaped clear layer covers the front of the eye. Iris The colored part of the eye is a ring of muscle that controls the size of the pupil. Sclera A tough protective layer forms the "white" of the eye. Retina A layer of light-sensitive receptor cells collects the light coming in and turns it into signals to be sent to the brain. Optic nerve This nerve carries signals from the retina to the brain. Inside the eye Light enters the eye through the pupil. The light is focused onto the retina at the back of the eye and the information is sent to the brain. The pupil changes in size based on the lighting conditions. It gets bigger in dim light and smaller in bright light. Tears wash away grit, so crying cleans your eyes! Just like your fingerprint, your iris is completely unique to you! SEEING IN 3D Look straight ahead and notice how much you can see without turning your head. This is your visual field— it is really large! This is because both eyes pick up light separately and send slightly different signals to the brain. The brain combines the signals to create a single 3D image of what you are looking at. Eye muscles Six tiny muscles move the eyeballs in different directions. They work together to make sure the movements are smooth. If one of the muscles doesn't work properly, it can result in blurred vision.

Nerve A large nerve carries information about what you are hearing from receptors in the cochlea to the brain. Ossicles Tiny bones—called the malleus, incus, and stapes—vibrate to pass sound waves into the inner ear. Ear canal This tunnel directs sounds heard toward the middle ear. It contains hair and wax. Cochlea This is full of fluid that vibrates when it picks up the sound waves. Receptors pick up these vibrations. Ear drum The tympanic membrane, or ear drum, vibrates to push sound waves along to the ossicles. Outer ear flap This flap, called the pinna, is made of cartilage. It collects all the sound waves around you. Ears and hearing Your ears don't just help you hear, they also help you balance. Ears have three parts. The outer ear is the part you can see, but ears are much bigger than that—inside your head are the middle ear and the inner ear. Hearing sounds Sound waves travel through the air as vibrations. They are picked up by the outer ear and pass through the middle ear, into the inner ear. Here, a nerve sends the vibrations to the brain as electrical signals. The brain then figures out the sounds that are being heard. Hearing aids A hearing aid is a device that can help you to hear better. It makes sounds louder and clearer. Early hearing aids looked like trumpets, but some are now so small that you can hardly see them. EAR WAX Ear wax is normal and healthy! Made by glands in the ear canal, it traps any dust and dirt, and carries it out of the ear. This protects your ears from infections. Ear wax comes out on its own, so you shouldn't try to remove it yourself. FEELING DIZZY Semicircular tubes inside your ears contain fluid to help you balance. The fluid moves when your body moves, so your brain is aware of your body's position. If you spin around and then stop, the fluid continues to move around for a while, so the brain gets confused and you feel dizzy!

Taste and smell These senses are linked together. You can't taste how tasty food is unless you can also smell it. Your senses of taste and smell can also keep you safe—for example, by warning you when food has gone bad. Smell When you sniff, smells go up the nostrils and enter the nose, where they bounce around. This makes the smells stronger before they hit the olfactory bulb, which is where smell receptors are found, high up in the nasal cavity. The nose can detect around one trillion different smells! Sniffing brings more smell particles into the nose! Taste The tongue has a lot of little bumps on its surface called papillae, which have taste buds on them. Eight tongue muscles help move food around in your mouth, and bring it in contact with the taste buds. Taste buds Taste receptors found inside the taste buds send information back to the brain so that it can figure out what you are tasting. Types of taste We can smell countless different smells, but we can only taste five basic flavors. These are sweet, sour, salty, bitter, and umami (savory).

Teeth The main job of teeth is to break down the food you eat. Saliva (spit) produced in your mouth kills bacteria and helps keep your teeth healthy. Everyone has two sets of teeth in their life: first baby teeth and then adult teeth. Different jobs There are four types of teeth—each is shaped differently since they all have a different job to do. See if you can notice the straight, pointy, and flat teeth in your mouth. Inside a tooth The part of the tooth you can see is called the crown. The part hidden inside the gums is called the root. Each tooth is filled with nerves and blood vessels that keep it healthy. Incisors These straight-edged teeth at the front of the mouth help you bite food and cut it into pieces. Canines Canines are really pointy and sharp. They grip and pierce food, and tear it into smaller shreds. BABY TEETH A baby's first teeth, called milk teeth, begin to grow at around six months old. A child has a full set of 20 milk teeth by the time they are three years old. These start to fall out around the age of six, and are replaced with 32 adult teeth. Enamel is the hardest substance in the body and protects the crown of the tooth. Dentin makes up most of the tooth and is very hard. Gums surround each tooth and protect the delicate roots. Pulp contains a lot of blood vessels and nerves. The nerves allow you to feel pain in your teeth. Teeth are held in the skull by the jawbones. Premolars Smaller than the molars, these teeth help to crush food down further. Molars These big and chunky teeth help to grind food into mush that is small enough to swallow.

The journey of food Food contains nutrients, which give us energy and repair broken cells. When you eat, your food is broken down so that the nutrients can be absorbed into your blood and used by the body. This is called digestion. Digesting food Food is digested in a very long tube that starts at your mouth and ends at your anus. It can take from one to three days for a meal to pass through the digestive system. When you chew, your teeth cut food into smaller pieces. The food is mixed with saliva, or spit, which contains chemicals that break the pieces down further. It is then swallowed and moves down the esophagus to the stomach. Some food, like sweet corn, is hard to digest and might come out whole in your poop! Liver The body's largest internal organ processes and stores nutrients from food. Pancreas Pancreatic juice is made here. It contains chemicals called enzymes, which help break down food in the small intestine. Gall bladder This small organ releases bile, which breaks down fat. The stomach muscles help squash and squeeze food down. Here, the food is also mixed with acid to break it down further. The folds of your stomach stretch out as you fill it with food! The small intestine breaks down the partly digested food into simple nutrients, which are absorbed into the blood. The small intestine isn't that small! It is around 20–23 ft (6–7 m) in length. The large intestine absorbs water and the last bits of nutrients. Any food waste that is left over is pushed out of the large intestine as poop.

Kidneys The Kidneys do a lot of important jobs. They filter around 4 fl. oz (120 ml) of blood every minute. They also help control blood pressure and produce a hormone—a chemical messenger—to make new red blood cells. Removing waste Each kidney has millions of tiny filtering units called nephrons. As the blood passes through these tubes, waste products are separated, and trickle down a tube called the ureter to the bladder. Cleaning the blood Most people have two kidneys that filter blood to keep it clean. Waste substances and extra water the body doesn't need are removed and stored in the bladder. These then leave the body as urine. WHAT IS URINE MADE OF? Urine, or pee, is mostly unwanted water, but it also contains the waste chemical urea— made when the body breaks down protein, other substances called electrolytes, and extra salt. Urea can break down into the chemical ammonia, which makes pee smelly. PEE IS MORE THAN 90 PERCENT WATER. Colorful pee Sometimes your urine can change color based on what you eat—beets can turn pee pink, while some food dyes can cause green or blue urine! Chemicals in pee sometimes harden into crystals, called kidney stones, which can make you sick. Bladder An adult's empty bladder is roughly the size of a pear. But when it's really full of pee, it is more like the size of a grapefruit! Urine leaves the body through a tube called the urethra.