Human Body

1. What is inside a tooth?

A tooth consists of four kinds of tissues. They are (1) pulp (2) dentine (3) enamel and (4)cementum.
Pulp is the innermost layer of a ooth. It consists of connective tissue, blood vessels and nerves. The blood vessels nourish the tooth. The nerves transmit sensation of pain to the brain. Dentine is a hard, yellow substance that surronds the pulp. It makes up most of a tooth. Dentine is harder than bone. It consists mainly of mineral salts and water but also has some living cells. Enamel overlays the dentine in the crown of the tooth. It forms the outermost covering of the crown. Enamel is the hardest tissue in the body. It enables a tooth to withstand the pressure placed on it during chewing. Cementum overlays the dentine in the root of the tooth. In most cases, the cementum and enamel meet where the root ends and the crown begins. As the surface of the tooth wears awaay, the tooth grows farther out of its socket, exposing the root.

*Sometimes teeth grom crookedly or become overcroweded in the mouth. This can be put right by wearing teeth braces. Braces consist of metal ot clear ceramic brackets that are bonded on the front surface of each tooth and connected by wires.

2. How do kidneys work?

The kidneys work by effectively removing the majority of waste products from our blood, and are vital to our health. We each have two kidneys, which lie on the back wall of the abdomen. From the inner side of each kidney a tube called the ureter runs down the abdominal cavity entering the bladder.

Blood is pumped through groups of tiny tubes inside the kidneys, and harmful waste material passes out through the walls of these vessels and down the ureter into the bladder. Here it is ready to be discharged from the body as urine. The two kidneys also work by producing certain hormones which help to regulate blood pressure.

*The adrenal glands are attached to the kidneys. They help create energy ehich stimulates the body to prepare it for instant action. Any extreme sport or violent activity, such as ski-jumping, causes a sudden ruch of adrenaline.

3. How do people age?

Ageing is a result of the gradual failure of the body's cells and organs to replace and repair themselves. This is because there is a limit to the number of times that each cell can divide. As the body's cells begin to near this limit, the rate at which they divide slow down. Sometimes the new cells that are produced have defects or do not carry out their usual task effectively. Organs can then begin to fail, tissues change in structure, and the chemical reactions that power the body become less efficient. Sometimes the blood supply to the brain is not effective. The brain cells become straved of oxygen and nutrients, leading to forgetfulness. For most old people memories bring great pleasure. Strangely, even though recent events may be forgotten, old people often remember events that took place in their childhood.

*The skin becomes looser as people age. As skin sags it forms into wrinkles and creases because the fibres of collagen that normally provide support to the skin become weaker.

4. How do we digest food?
Taking food into our bodies is not enough to keep us alive and growing. The food must be changed so that it can be used by the body, this process is called digestion. Ih the mouth, the saliva helps break down straches. When food has been moistened and crushed in the mouth, it travels to the stomach. Here, the juices from the stomach wall are mixed with the food, helping to break down proteins into simpler forms to aid digestion. The straches continue to break down until the material in the stomach becomes too acid.

The materials in the stomach are churned about to mix digestive jucies well throughout the food. When the food becomes liquified it enters the small intestine. In the first part of the small intestine, the duodenum, digestion continues. Jucies from the pancrease and liver help further break down the foods. The breakdown of proteins is finished here, fats are split into finer parts, and strach digestion is completed here. It is also in the small intestine that digested food is absorbed into the blood and lymph. Finally, in the large intestine, water is absorbed and the contents become more solid, so they leave the body as waste material.

*Cholesterol is a fatty substance found in blood and some fatty foods. It can be deposited on the walls of arteries, making them more narrow. This reduces blood flow in the arteries and can cause blockages.

5. What makes us hungry?
When we need food, our body begins to crave for it. But how do we know that we are feeling 'hungry' ? How does our mind receive the massage and make us feel 'hungry' ? Hunger has nothing to do with an empty stomach, it begins when certain nutritive materials are missing in the blood. When the blood vessels lack these materials, a message is sent to a part of the brain called the hunger centre. This hunger centre works like a brake on the stomach and intestine. As long as the blood has sufficient food, the hunger centre slows up the action of the stomach and intestine. When the food is missing from the blood. hunger centre makes the stomach and intestine more active. That is why a hungry person often hears his stomach rumbling.

When we are hungry, our body doesn't crave any special kind of food it just wants nourishment. It depends on the individual how long we can actually live without food. A very calm person can live longer than an excitable one because the protein stored up in his body id used up more slowy.

*The thyroid is a large butterfly-shaped gland located in the throat. Its hormones accelerate the release of energy from food and help control our metabolism.

6. How does the immune system work?
Unlike most of the other body systems, the immune system is scattered throughout the body. The main defences against the invaders such as bacteria and viruses are white blood cells called lymphocytes which are stored in the body's lymphatic system. This is a network of thin tubes running throughout the body. It contains a watery liquid called lymph, which ir drains from the tissue and return to the blood.

At intervals along the length of the lymph vessels are small lumps caled lymph nodes. Lymphocytes are stored in these lymph nodes. Waves of lymphocytes are released when the body is injured, or when invaders are detected, and the lymphocytes swarm to the damage area to protect and repair it.

*Newborn babies are fed on their mother's milk which contains special antibodies, helping to boost the baby's immune system.

7. How do antibodies work?
Antibodies are chemicals. When these chemicals are put into the body, they help your body fight disease. Many antibodies are made from microbes. These tiny living things are engaged in a constant struggle for survival. In the course of this struggle microbes produce chemicals, many of which kill disease germs by interfering with the way in which bacteria reproduce.

The use of antibodies has meant that many diseases that used to be fatal can now be cured. Antibodies work in different ways against different germs. They may kill the germ in one case. In another, they may only weaken the germs and let the body's natural defences take over damaging bacterial production rather than killing the bacteria.

*As viruses invade a cell (1) they shed their outer layer (2) and take over the genetic material in the host cell in order to reproduce themselves. (3) They begin to construct protein coats around the new viruses (4) and eventually burst out of the host cell (5) or leave it in an envelope (6) ready to infect new cells.

8. How does the heart work?

The heart is a fist-sized muscular organ that pumps blood around the body. It is actually two pumps that are joined together. At the top of each side of the heart is a thin walled chamber called the atrium which receives the blood that returns to the heart through the veins. Once the atrium is filled, it contracts and squeezes its blood into a much more muscular chamber called the ventricle. The ventricle contracts in turn and forces blood at high pressure along the arteries and off to the lungs or the rest of the body. A system of the one way values stops the blood from leaking back into the heart. The left side of the heart pumps blood to the lungs to collect more oxygen.

*An electrocardiogram, or ECG, measures the electrical signals that the heart produces as it beats. These singals change when a person suffers from certain medical conditions that affect the heart.They are measured by attaching wires to the chest near the heart. A doctor can study results as printed information.

9. How does our blood circulate?
Blood is pumped in a continuous flow from heart. Blood flows inside a network of tubes called blood vessels - arteries, veins and capplillaries.

The blood in arteries comes straight from the heart and is pumped under pressure, so the artery walls are thick and muscular (to withstand the pressure). Blood moves from arteries to veins through tiny capillaries, which are about one-tenth the thickness of a hair. Cappillaries are so narrow that red blood cells have to squash themselves up to pass through. Veins return blood to the heart, and because the pressure is lower, they have thinner walls than arteries.

*Each day the heart beats about 100,000 times - that's more than 36 million times a year! A woman's heart beats faster than a man's. The heart can pump 7 litres (14 pints) of blood around your body in just one minute; this heart rate can be increased by exercise.

10. Why is our blood red?
The blood which flows through our body contains many different materials and cells. Each part of the blood has its own special job to do. The liquid part of our blood is called the plasma and makes up a little more than half the blood. It is light yellow in colour and thicker than water as many substances are dissolved in it. These substances are proteins, antibodies that fight disease, fibrinogen that helps the blood to clot, carbohaydrates, fats and salts, in addition to the blood cells themselves.

The red cells (also called red blood corpuscles) give the blood its colour. There are so many of them in the blood that it makes it all look red. There are about 35 trillion of these tiny, round, flat discs moving around in your body all at once. As the young red cells grows and takes an adult form in the marrow, it loses its nuclecus and builds up more and more haemoglobin.

Haemoglobin is the red pigment, or colour. Red cells live only for about four months and then are broken up, mostly in the spleen. New red cells always being formed to replace the cells that are worn out and destroyed.

11. Why are arteries different from veins?
There is no transport system in any city that can compare in efficiency with the circulatory system of our body. If you can imagine two systems of pipes, one large and one small, both meeting at a central pumping station, you will have an idea of the circulatory system. The smaller pipes go from the heart to the lungs and back, while the larger ones go from the heart to the various other parts of the body. Arteries are vessels in which the blood is carried away from the heart. In veins, the blood is coming back to the heart. In general terms, arteries are carrying pure blood to various parts of the body, and the veins are bringing back blood loaded with waste products.

Arteries lie deep in the tissues, except at the wrist, over the instep, at the temple and along the side of the neck. At any of these places the pulse can be felt, which gives the doctor an idea of the condition of the arteries. The blood in arteries is red in colour and moves through in spurts.

Veins lie closer to the surface of the skin, and the blood in them is much darker in colour and flows more evenly. Veins have valves at intervals along their course.

*Blood contains red and white blood cells that float inside a liquid called plasma. It also contains thousands of different substances needed by the body. Blood carries all thing round the body and also removes waste poducts.

12. How can muscles work in pair?
Muscles actually work in pairs. A muscle can only pull in one direction so it needs another muscle to pull in the opposite direction in order to return a bone to its original position. When you lift your forearm, the biceps muscles shortens to lift the bone. When you straighten your arm, the triceps muscle pulls it back again and the biceps relaxes. The same action takes place in your legs when you move your fingers and toes. Muscles working in pair are know as antagonistic muscles.

13. How do muscles respond to exercise?
Muscles are made up of long, thin cells called muscles fibres. But muscles differ in what they do and how they do it. When a muscle contracts, it produces an acid know as lactic acid. This acid is like a poison, with the effect of making you feel tired, by making the muscles feel tired. If the lactic acid is removed from a tired muscle, it stops feeling tired and you can go right to work again!

But, of course, lactic acid is not removed normally when you exercise and various toxins are produced when muscles are active. They are carried by the blood through the body and cause tiredness throughout the entire body, especially in the brain. So feeling tired after exercise is really the result of a kind of internal poisoning.

However, the body needs this feeling of tiredness so that it will want to rest. During rest, waste products are removed, the cells recuperate, nerve cells of the brain recharge their battery and the joints replace their supplies of lubricant they have used up. So while exersice is good for the body and muscles - rest is as important.

*The knee is a typical load-bearing joint. The end of the bones are cushioned by a pad of cartilage to protect them from impact. Wear and tear is minimized by a lubricant called synovial fluid.

14. Why do we have a skeleton?
A skeleton is made up of a network of bones. Bones provide a framework that holds the whole body together. Without a skeleton we would not be supported and would simply flop about like a rag doll. This would mean that we would not be able to move about.

The skeleton also gives protection to delicate organs in our bodies such as the brains, heart and lungs. It acts as a support to all the soft parts ofthe body. The skeleton also provides a system of levers taht the muscles can work on, enabling us to carry out all out movements.

*At birth a baby has 300 bones, but 94 join together in eary childhood. Your hand and wrist alone contain 27 bones.