Part 1

Introduction Your body is simply amazing! It carries out countless different activities each day, such as walking, eating, and thinking—and many more without you even realizing it. Do you ever wonder why you don't forget to breathe? Or how many times your heart needs to pump each minute? Your body does this all for you on its own! All the body's organs work together, a little bit like an orchestra, to keep every part of you going strong. In this book, we'll take a closer look at the inner workings of the human body, and you'll discover how our understanding of the body has changed since ancient times. I hope you will come to appreciate just how unique and wonderful your body is! Dr. Bip Choudhury

Your amazing body Your body is an amazing machine, more complicated than a computer. It has a lot of different parts, some that you can see and many others that are hidden inside you. Body parts often come in pairs—this means you have a spare in case one gets damaged! What makes you, you? Although all bodies work in the same way, there is only one you. No one looks, thinks, or feels exactly like you. Many of your features are decided by your genes, but the experiences you have during your life also shape your appearance and personality. Each person has around 20,000 genes. Genes Each of your body's cells contains genes—a set of instructions that builds every part of you. You have two versions of each gene, one from your mother and one from your father. Your genes are unique to you, but twins that look the same share most of their genes. DNA Chromosomes are made of tightly coiled strands of a chemical called DNA. When unwound, DNA looks like a twisted ladder. Genes Sections of DNA are called genes. There are thousands of genes in each chromosome. Chromosomes There are 23 pairs of chromosomes in each cell. Half are from your mother and half from your father. Nucleus This is the instruction center of a cell. Your chromosomes, which contain your genes, are found every cell with a nucleus. Genetics Children look like their parents because of the genes they inherit from them. You inherit some genes from one parent and some from the other parent. This is why you might have eyes like your mother's but hair the same color as your father's. This child has inherited brown eyes from her father.

Groups of organs form organ systems. The mouth, throat, stomach, and intestines make up the digestive system, which takes energy and nutrients from the food you eat. Organs are composed of two or more tissues that work together to carry out a particular function. The stomach lining works with muscle tissue to digest food. Inside out Together, all the organ systems make one complete organism. Each part of the body, from a cell to a system, has to function properly to keep us healthy. Bone is a type of connective tissue—it connects different body parts. A lot of cells that do similar jobs combine to form tissues. The cells here are from the lining of the stomach and their job is to produce stomach acid. Red blood cell These cells have no nucleus and are a special shape so they can fit through vessels. White blood cell There are different types of white blood cells. They fight off infections. Fat cell Fat cells are found throughout the body and are an important store of energy. Muscle cell These cells make up muscles that contract and relax to help you move. Nerve cell Nerve cells send electrical messages around the body to tell it what to do. What are we made of? The smallest building block of the body is the cell. Groups of cells make tissues, which combine to make organs. All the organs put together form an organism, like you! Cells are held together by a skin, called a membrane. Inside is a soup-like liquid called cytoplasm in which organelles, or "mini organs," float around. The most important organelle is the nucleus—it contains instructions that tell the cell what to do.

Body systems Organ systems are groups of organs that work together to do a particular job. Our bodies have a lot of organ systems and some of the most important ones are shown here. Musculoskeletal system The musculosketetal system is made up of muscles attached to bones. Together, they help us move. Muscles are stretchy, while bones are very hard. Nervous system The brain, spinal cord, and nerves make up the nervous sytem. The brain stores memories and sends out instructions. The spinal cord and nerves carry messages between the brain and the whole body. Respiratory system This system is made up of the lungs and airways. It helps us breathe in air full of oxygen and gets rid of the waste gas carbon dioxide. Other body systems There are several more organ systems not shown here. These include the renal system, which makes urine, and the immune system, which fights infections. Every organ is made from cells a little like me! Cardiovascular system The cardiovascular system is made up of the heart and blood vessels. One of its main functions is to supply all parts of the body with oxygen, which is carried by red blood cells. Digestive system This system starts at the mouth and ends at the anus. Its job is to digest food so that the body has plenty of energy. Strong teeth start the process by breaking down food. Reproductive system Babies wouldn't be born without this system. Male reproductive organs make sperm, and female organs make eggs. These join together inside the woman's uterus, where the baby grows.

Your super skeleton Your skeleton acts as a frame for your body—it keeps everything together and your muscles attach to it. The bones in your skeleton are strong and some protect your organs. Joints A joint is formed where two or more bones meet. The bones are held together by tough tissues called ligaments. Joints let us bend, stretch, and dance! A joint that can move in almost any direction, such as the hip joint, looks like a ball in a socket. The funny bone is not a bone at all— it's a nerve behind your elbow. Cartilage A flexible tissue called cartilage is found where bones meet, protecting their surface to stop it from wearing away. Your ears and the front of your nose are also made of cartilage. That is why they feel a little bendable. Inside the bone The outside of a bone is hard, but the inside is soft, spongy, and full of holes—this makes it light. Bones are filled with a jelly-like substance called bone marrow, and the holes contain many blood vessels and nerves. X-RAYS X-ray images help doctors to see if bones are healthy. The "X" in X-ray stands for "unknown" because X-rays were discovered by accident! Broken bones Bones are so dense, or solid, that they appear white on an X-ray image. Can you spot the broken bone in this X-ray? Some bones are stronger than concrete!