Category Human Body

          The body needs continual supplies of oxygen. This invisible gas makes up about one-fifth of the air around us. It is needed for chemical processes inside the body’s cells that release energy from food. Breathing draws air into the body so that oxygen can be absorbed.

RESPIRATORY SYSTEM

          The respiratory system draws fresh air into the body, absorbs the vital oxygen from it into the blood, and then passes the stale air out again. The main parts of the system where oxygen is absorbed are the lungs. Breathing muscles stretch the lungs to make them larger and suck in air. These muscles are the diaphragm below the lungs, and the intercostal muscles between the ribs. Fresh air passes in through the nose and mouth, down the pharynx (throat) and trachea (windpipe), into the lungs. The lung airways, called bronchi, divide many times and become thinner, ending in terminal bronchioles, narrower than human hairs.

          Each terminal bronchiole ends in a cluster of microscopic air bubbles, called alveoli. There are about 300 million alveoli in each lung, giving the whole lung a spongy texture. Besides fresh air, the lungs also receive low-oxygen blood from the heart along the pulmonary arteries. These divide and form networks of microscopic blood vessels (capillaries) around the alveoli. Oxygen from the air inside the alveoli passes easily through the thin walls of the alveoli and capillaries into the blood. This high-oxygen blood returns along pulmonary veins to the heart.

CLEANING THE LUNGS

          The lungs are delicate and easily damaged. Hairs in the nose filter bits of floating dust and other particles from air as it is breathed in. The airways are lined by sticky mucus which traps dirt and dust. Microscopic hairs, called cilia, line the smaller airways. They sweep mucus and trapped dirt into the throat, where it can be swallowed.

SPEECH

          Air emerging from the lungs not only carries waste carbon dioxide. It has another use—speech. At the top of the trachea is the larynx (voice box). This has a shelf-like fold of cartilage projecting from each side, known as the vocal cords. To speak, muscles pull the vocal cords together so that there is only a very narrow slit between them. Air rushing through the slit makes the cords shake or vibrate, which produces sounds. These sounds are shaped into clear words by movements of the mouth, cheeks, teeth, tongue and lips.

BREATHING RATE

          As oxygen passes from the air in the alveoli into the blood, the waste substance carbon dioxide passes the opposite way, from the blood into the air. This stale air is then pushed out of the lungs when the breathing muscles relax and the stretched lungs spring back to their smaller size. At rest, an adult person breathes in and out about 12 times each minute. Each breath is around half a litre of air. After running a race, a person may breathe 60 times each minute and take in more than two litres of air each time, to obtain extra oxygen for the active muscles.

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          The entire digestive system, from the mouth to the anus, is about nine metres long. Looped and coiled into the lower abdomen, the small intestine makes up two-thirds of this length. Digestive juices from the small intestine’s lining are added to the food to complete its chemical breakdown. The resulting nutrients are so small that they can pass through the lining into the blood, to be carried away to the liver. The liver acts as a kind of food processor, making new chemicals from the nutrients it receives and storing them until they are required. Those substances the body does not need, including impurities in the blood, it sends on to the kidneys.

THE KIDNEYS

          The two kidneys receive a very large flow of blood—more than one litre per minute. It passes through about one million microscopic filtering units, called nephrons, packed into the outer layer of each kidney. The nephrons remove waste substances and excess water from the blood. These flow through the kidney’s inner layer, where some water is taken back into the blood according to the body’s needs. The resulting liquid waste is called urine. It dribbles down a tube, the ureter, to a stretchy bag in the lower abdomen, the bladder. It is stored here until it can be passed to the outside.

NUTRIENTS

          The body needs a wide range of nutrients to stay healthy. There are six main groups of nutrients—proteins, fats, carbohydrates, vitamins, minerals and fibre. Different kinds of foods are rich in different groups. Proteins are found in meat, poultry, fish, milk, beans and green vegetables. They help to build and maintain muscles and other body parts, so they are important for growth. Fats (lipids) are found in meat, dairy products, pies, and some oily fruits and vegetables like avocado, olive and sunflower seeds. Small amounts are needed to build the walls around the body’s microscopic cells, and also for healthy nerves. Carbohydrates such as starches and sugars are found in bread, pasta, rice and other grains, and potatoes. They are the main source of energy for movement, digestion and other life processes.

VITAMINS AND MINERALS

          Many vitamins and minerals are needed for good health, but usually in small amounts. Vitamins have letters such as A, B and C. Lack of a vitamin may cause illness. For example, lack of vitamin A from tomatoes, carrots, cheese, fish and liver may result in poor eyesight. Minerals include calcium and iron. Iron is found in meat, green vegetables and nuts. It is needed for healthy blood. Its lack causes a type of anaemia.

FIBRE

          Fibre is found only in plant foods, chiefly in breads, pastas and other products made from whole meal grains or cereals, and also in many fresh fruits and vegetables. Fibre is not actually digested and absorbed by the body, but it helps the digestive system to work effectively and stay healthy. It adds bulk to the food so that the stomach and intestines can grip and squeeze the food along.

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                    The body needs energy to power its chemical life processes. It also needs raw materials for maintenance, growth and repair. The energy and raw materials are in our food. Digestion is the process of taking in, or eating, food and breaking it down into tiny pieces, small enough to pass into the blood and be carried all around the body. The parts that take in and break down food are known as the digestive system.

INSIDE THE MOUTH

                     The teeth cut off and chew pieces of food into a soft pulp. Saliva (spit) makes the food moist and slippery, for easy swallowing. The tongue tastes the food, to make sure it is not bad or rotten, and moves it around in the mouth, for thorough chewing. The lips seal at the front of the mouth to stop food and drink dribbling out during chewing.

TEETH

                     There are four main kinds of teeth. The sharp-edged, chisel-like incisors at the front of the mouth slice and cut pieces from large food items. The taller, pointed canines tear and rip tough food. The premolars and molars at the back of the mouth squeeze and crush the food. Each tooth has a long root that fixes it firmly in the jaw bone, and a crown that sticks up above the soft, pink gum. The whitish enamel covering the crown is the hardest substance in the body.

OESOPHAGUS AND STOMACH

               Swallowed food is squeezed down the oesophagus by wave-like muscular contractions of its wall, called peristalsis. The food enters the stomach, a J-shaped muscular bag. This expands like a balloon to hold about three litres of food and drink. It churns up the food, mixing in its strong digestive juices to break it into smaller and smaller particles. An average meal takes between three and six hours to be digested in the stomach. If the food is bad or unsuitable in some way, peristalsis works in reverse and pushes it up and out of the mouth, a process called vomiting.

                  Two large organs aid the process of digestion. The pancreas gland is wedge-shaped and lies behind the stomach on the left. It makes strong digestive juices that flow along a tube, the pancreatic duct, into the small intestine. These juices dissolve the food further. The other organ is the liver, in front of the stomach on the right. It makes a yellow-green liquid, bile. This is stored in the gall bladder and then added to the food in the small intestine, to help digest fatty foods.

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          The Human Body is the most studied object in all of science. Yet every year we learn even more about its most detailed structures and its innermost workings. Even in ancient times people have known basic facts—for example, that there are 206 bones in its skeleton. Since the invention of the microscope nearly 400 years ago, people have studied the body’s billions of tiny building blocks, known as cells. In more recent years we have learned about the instructions or “blueprint” for making the body—its genes.

ORGANS AND SYSTEMS  

         The body’s main parts, like the brain, heart, lungs and stomach, are called organs. Different groups of organs work together as systems. Each system has a vital job to keep the whole body alive and healthy. For example, the heart, the body-wide network of tubes, called blood vessels, and the red liquid called blood, together form the circulatory system. This carries essential nutrients and oxygen to all body parts and collects waste materials for disposal. There are about a dozen major systems.

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Each tooth has two main parts. The root anchors it firmly in the gum, to withstand the tremendous pressures that are exerted when you bite and chew hard foods like nuts. The crown is the visible part above the gum. It is covered with whitish enamel, which is the hardest substance in the entire body. Under the enamel is a layer of dentine, which is not quite so hard, and absorbs shocks and knocks. In the middle of the tooth are blood vessels, providing nourishment to the tooth’s parts and layers, and nerves, to detect pressure and pain.

Regular visits to the dentist are important for healthy teeth. If you don’t look after your teeth, they may go bad and decay. That means that they may have to be filled or even taken out by the dentist.

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The average person has two sets of teeth, one after the other. The first is the baby, milk, or deciduous set. Even before birth, teeth appear as tiny buds below the gums. They begin to erupt, or show above the gum from the age of a few months. By the age of about three all 20 first teeth have usually appeared. In each half (left and right) of each jaw (upper and lower), there are two incisors, one canine, and two molars.

From about the age of six years, the first teeth start to fall out. These are replaced by the adult, second, or permanent set. First are usually the front incisors and the first molars, at around seven to eight years. Last are the rear-most molars, or wisdom teeth. They appear at 18-20 years of age in some people, while in others they erupt at 40 or 50 years of age – and sometimes they never appear. In each half of each jaw, there are typically two incisors, one canine, two premolars, and three molars, making a full set of 32 teeth.

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