We know a lot about the human body, partly because we all have one. Yet the more we know about this amazing living machine, the more we realize that there is still a huge amount left for us to find out.
BILLIONS OF BODIES
There are more than 6 billion human bodies in the world. These bodies belong to human beings and make up the group of living things called the human species. Our scientific name is Homo sapiens. Every human being is an individual person with his or her own appearance, personality, likes, dislikes and ways of behaving. But, inside, all human bodies are very similar. The study of the body’s parts and how they fit together is called human anatomy. The study of how these parts work, with thousands of substances and chemicals changing all the time, is human physiology.
PARTS OF THE BODY
All living things are made of tiny building bricks called cells. These can be seen only under a microscope. The human body contains more than 50 trillion (50 million million) cells. There are about 200 different kinds, with different shapes. Muscle cells are long and thin, like hairs. Red blood cells are rounded like doughnuts. Nerve cells have lots of thin wire-like parts for carrying nerve signals.
Many cells of the same kind together are called a tissue. For example, many muscle cells together make muscle tissue, lots of nerve cells make nerve tissue and so on. Several kinds of tissue make up a main part of the body, called an organ. The brain, heart, and stomach are organs. A group of organs works together as a body system. Each system carries out a major task that keeps the body alive and working well. There are about 12 main systems.
The most obvious main body part is the skin. Together with the hair and nails, it forms a system that protects the body and helps to keep in warmth. The skin has another job—it provides our sense of touch. So the skin is also part of the sensory system, along with the eyes, ears, nose, and tongue. Senses send information to the brain about what is happening around the body. This information is sent to the brain along nerves. The nerves and brain make up the nervous system. The nervous system controls many processes that go on inside the body, like breathing and heartbeat, as well as the movements we make with our muscles.
MUSCLES AND BONES
The muscular system has more than 640 muscles. They are joined to bones and shorten, or contract, to pull on the bones and so move the body. All the bones together are known as the skeletal system. Bones are linked to each other at joints, which can bend. Sometimes the muscles, bones, and joints are all included in one system, the musculo-skeletal system.
FOOD AND DIGESTION
Taking in food and breaking it into tiny pieces to use around the body is the task of the digestive system. This system includes the mouth, gullet, stomach and intestines or guts. The liver is also part of the digestive system. It stores and releases digested foods according to the body’s needs. Next to the liver and stomach is another digestive part, the pancreas. This makes strong digestive juices, which are used to break down the food we eat.
HORMONES AND KIDNEYS
The pancreas also makes hormones. These are natural substances made by body parts called glands. Hormones travel around the body in the blood and control many processes, such as growth and the way cells use energy. All the hormones and their glands are known as the endocrine system.
Hormone glands include the thyroid in the front of the neck and the adrenals, one on top of each kidney. The two kidneys are the main parts of the waste-removal, or excretory, system. They filter unwanted substances from the blood to form the waste liquid called urine. This is stored in the bladder before it leaves the body.
BLOOD, HEART, AND LUNGS
Blood is part of another system, the circulatory system. Blood flows around the body in tubes called blood vessels, and it is pumped non-stop by the heart in the chest. On either side of the heart are the lungs. These are the main parts of the breathing or respiratory system. The task of this system is to take in oxygen from the air because the body needs oxygen in order to get energy from food.
THE BODY’S DEFENCE SYSTEM
Sometimes germs enter the body through the lungs, in food or through a skin cut. The germs are quickly attacked by the body’s self-defense, or immune, system. This has many small glands, which are called lymph nodes. They are all around the body but mainly in the neck, armpits, and groin (where the legs join the torso). These lymph nodes become bigger during illness when they are called swollen glands.
One body system is different in females (girls and women) and males (boys and men). This is the reproductive system. Its parts are responsible for producing more human bodies or making babies. The female reproductive system produces tiny egg cells, each slightly smaller than this full stop. The male reproductive system makes sperm cells, which are much tinier. Both of these are needed to create a new human.
When an egg and sperm join, they start a new human body. It grows and develops inside the womb of its mother for nine months, during the time called pregnancy. The baby comes out at birth. It grows quickly and learns quickly too. It learns many skills such as walking and talking, and becomes a child. The child’s growth slows down slightly. But by the teenage years, it speeds up again, during the time known as puberty. This is when the body becomes able to produce its own babies.
THE HUMAN LIFE CYCLE
In adulthood, the body stops growing in height. But the brain continues to learn and gain experience. Gradually the body passes through middle age to old age, when parts start to wear out. The whole process of being born as a baby, growing up and then producing more babies is the human life cycle. It means that human bodies continue generation after generation, as a child becomes a parent, then a grandparent, and so on.
Some human bodies are less healthy than others. They may catch germs, develop diseases or suffer from injuries. But there are many ways that each of us can keep our bodies as well as possible. These include eating healthy foods, taking plenty of exercise, not smoking, staying safe and avoiding serious risks—and of course, enjoying our lives too.
Did you know?
• There are 206 bones in an adult human’s body. The smallest is the stirrup bone in the ear, and the biggest is the femur, in the leg.
• An average adult human has about 5.5 litres of blood.
• Human bodies are about 75 per cent water.
• No two people have exactly the same fingerprint. Even identical twins have different fingerprints. These patterns are formed before we are born.
• People are about 2 centimetres taller in the morning than they are in the evening. During the day the spine is slightly squashed by the weight of the body. During the night, while you are asleep, it stretches out again.