Part 2

What is blood made of? Blood contains three types of cells—red blood cells, white blood cells, and platelets. These float around in a yellowish liquid called plasma. Blood is 55 percent plasma and 44 percent red blood cells. The remaining 1 percent is made up of white blood cells and platelets. Blood Blood travels to every part of your body through a network of tubes called blood vessels. It delivers oxygen to the body to keep it working, and removes carbon dioxide by carrying it to the lungs to be breathed out. Blood also fights infections. Plasma Plasma is mostly made of water. The rest is a mix of substances, including antibodies—which fight infections—as well as protein and waste matter. White blood cells and platelets White blood cells patrol the body to kill germs. Platelets, found in plasma, help to heal cuts and wounds. Red blood cells Red blood cells contain a protein called hemoglobin, which carries oxygen. Hemoglobin also gives blood its red color. Some animals, such as crabs and lobsters, have blue blood! BLOOD TYPES There are four main blood groups: A, B, O, and AB. The antigen, or protein, on the red blood cell determines which type you are. Type A The A antigen lives on the surface of the red blood cell. Type B The B antigen lives on the surface of the red blood cell. Type AB Both A and B antigens are found on the cell's surface. Type O There are no antigens on the cell's surface.

The story of blood People's knowledge of blood and how it circulates around the body has changed greatly since ancient times. Over the years, careful study of the human body has helped our understanding of how blood works. Ancient Egyptians In Ancient Egypt, it was believed that different vessels carried fluids, such as blood, snot, and tears, from organs in the body to the heart. Four humors Ancient Greek doctors believed the body had four "humors": blood, phlegm, black bile, and yellow bile. A transfusion is when blood is transferred from one animal to another. Knowing the different blood groups made blood transfusions much safer. White blood cells In 1843, white particles were seen in blood along with red ones— this was the discovery of white blood cells. Blood groups The different blood groups were discovered by Austrian scientist Karl Landsteiner in 1901. Blood transfusions Scientists successfully carried out blood transfusions in dogs and sheep before the first successful human-to-human blood transfusion in 1818. Platelets Scientists had seen oddly shaped granules in blood, which they had thought were damaged blood cells. In 1882, it was discovered that they were in fact platelets. Blood makes up around 8 percent of your body weight. Ancient India Around 600BCE, Indian doctors believed that by feeling the pulse, they could tell if the humors were balanced correctly. Blood vessels Ancient Romans were the first to suggest that blood was carried in blood vessels. However, they also thought that the liver made blood, and from there it flowed to the body. Plugging blood At the start of the 20th century, experiments showed that platelets in blood flock to the site of an injury, creating a blood clot to stop the bleeding. William Harvey This British doctor discovered the connection between the heart and blood vessels, and showed how blood was transported to and from the rest of the body. Today When blood is donated today, the white blood cells are removed. The red blood cells and platelets may be separated so they can be used where needed.

Inside the lungs Air travels to the lungs through the nose and mouth, and down the trachea (windpipe). The trachea splits into two branches, called bronchi, one going to each lung. The bronchi split into smaller branches—the smallest of which are called bronchioles. These end in small air sacs called alveoli. You breathe around 20–25 times per minute. Breathe in When you breathe in, your lungs fill with fresh oxygen gas, which is needed to make your body's cells work. Carbon dioxide, a waste gas, is removed from your body when you breathe out. A HELPING HAND The "iron lung" was a machine used to help people with breathing difficulties. The patient laid inside a large metal tube, and air was pumped in and out to keep their lungs working. Exchanging gases A lot of tiny blood vessels called capillaries wrap around each alveolus, allowing the gases to move in and out. Oxygen passes through the walls of the alveoli into the blood, whilst carbon dioxide moves into the alveoli to be breathed out. Arteries (red) carry oxygen-rich blood away Veins (blue) bring oxygen- poor blood in Oxygen goes in Carbon dioxide comes out Blood cells change from being oxygen-poor (blue) to oxygen-rich (red) In and out The diaphragm is a large sheet of muscle under your ribcage. It contracts and relaxes when you breathe to pull air into your lungs and push it out. Muscles attached to the ribcage move it up and out when you breathe in, to fit your expanding lungs. Breathing in The diaphragm flattens, and the lungs and ribcage become larger. Breathing out The lungs and ribcage shrink, and the diaphragm relaxes upward. Your lungs are like balloons that inflate when you breathe in and deflate when you breathe out.

Your brilliant brain Everything you see, hear, do, feel, and think is controlled by different parts of your brain. It contains billions of cells, called neurons, that communicate with the rest of the body through a large network of nerves. Personality The frontal lobe is home to your personality. This affects how you behave, and what you like and dislike. Cerebrum The largest part of the brain is the cerebrum. Its outer layer is split into four different "lobes." Hearing The temporal lobe makes sense of the sounds picked up by the ears. It helps you to hear and talk. Awareness External information, such as temperature, pain, and pressure, are sent to the parietal lobe. This makes you more aware of your surroundings. Seeing Everything the eye sees is sent as signals to the occipital lobe. It then processes the information to form an image. One-fifth of your blood supply goes to the brain. Cerebellum The cerebellum helps the muscles of your body work together to make smooth movements. LEFT AND RIGHT SIDE The cerebrum is made up of two halves, called hemispheres. The left hemisphere tells the right side of the body what to do and the right hemisphere tells the left side of the body what to do. Control center Just under the cerebellum is the brain stem. It joins the brain to the spinal cord and controls important functions, such as breathing and heart rate.

Network of nerves Every part of the body needs a nerve supply in order to work properly. Nerves are like electrical wires, carrying information in the form of signals all around the body. The nervous system controls everything you do. Your brain has around 86 billion neurons! Neurons Nerves are made up of bundles of cells called neurons. Electrical signals travel along the neuron's length—from the dendrites to the nucleus, and then along the axon. A chemical is released at the synapse, passing the signal onto the next neuron. Axon The electrical impulse travels down the axon, a long, thread-like fiber, to the end of the neuron. Myelin sheath Many axons have a protective covering, which helps information travel faster Nucleus The nucleus is found in the cell body. It controls the neuron. Spinal cord This large bundle of nerve tissues links the brain to the rest of the body. Signals travel from nerves in the body along the spinal cord to the brain, where the information is processed. Synapse Information is passed along at the gap where two neurons meet. Dendrites These thread-like branches coming off the cell body gather information from connecting neurons. Ouch! If you touch something hot, nerves in your hand send a signal to the brain along the spinal cord. The brain processes the information and you feel pain. WHERE DO NERVES GO? Together, the brain and spinal cord form the central nervous system (CNS). All other nerves make up the peripheral nervous system (PNS), which links the CNS to your organs and muscles. Sense organs Sensory nerves take signals from your sense organs, such as the eyes, to your brain. Muscles Motor nerves carry information from the brain to your muscles, to tell them when to move. Internal organs Nerves also control your internal organs—for example, they tell your heart to keep beating.

The secrets of skin Skin is the largest organ in the body. It keeps your body wrapped up and protects your insides. Sensory receptors in the skin detect what is going on around you and keep you safe from danger. Skin cells only live for 30 days. Under the skin Skin is made up of two layers. The thin outer layer is the epidermis. Below this is the dermis, a thick layer full of nerves, blood vessels, and glands. Pores Tiny holes on the skin allow sweat to leave the body. Epidermis This layer is waterproof to keep you dry. The epidermis grows new skin cells every month. Blood vessels These get bigger when the body is too warm, allowing heat to escape from the blood through the skin. Dermis Elastic fibers located here make the skin stretchy. Hair follicle New hairs grow from tubes called hair follicles. Sweat glands These glands make salty sweat to help cool you down. Nerves These send signals from receptors to the brain. LEECHES Leeches are worms that have been used in medicine since the late 1700s. They suck on blood and release substances that numb the skin, stop blood clotting, and make blood vessels get bigger. Fresh blood then flows to the skin, helping it heal. Soggy skin Glands in the skin make an oily substance called sebum to keep it soft. When you spend too long in the water, the sebum is washed away. Without it, water is absorbed into the skin, making it wrinkly. Your skin is thickest on the soles of your feet. AMAZING SKIN Skin tones Skin contains a chemical called melanin—the more melanin you have, the darker your skin is. Sometimes, your skin makes extra melanin in the sun to protect you from its harmful rays. This is why you get a suntan. Healing Tiny cells in the skin called fibroblasts help to heal cuts and wounds. They arrive at the injury and start to make new skin. This includes scar tissue, which is 80 percent as strong as the original skin. Growing old As we age, our skin becomes less elastic. We produce fewer elastic fibers and less sebum, and we also lose water from our bodies. This makes skin stiffer and wrinkly.