My Science School

As air flows out of the lungs, we can use it to make the sounds of speech and other noises. At the top of the windpipe, in the sides of the voice box or larynx, are two stiff, shelf-like folds – called the vocal cords. Criss-crossed muscles in the voice box can pull them together so that air passes through a narrow slit between them and makes them vibrate, creating sounds. As the vocal cords are pulled tighter, they make higher-pitched sounds. As the vocal cords loosen, they make lower-pitched sounds. Of course, when we actually learn to talk, our speech depends on the development of the brain and its ability to copy the sounds that we hear.

Fact File: Although many people think of speech as our main way of communicating, we do not have to use spoken words. People who can’t speak learn a language called signing, in which hands and fingers are used to signal letters and words.

 

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Both white and red blood cells are formed in the bone marrow. Each red blood cell measures about 7.5 microns (thousandths of a milimetre) in diameter and is shaped a little like a doughnut. Red blood cells contain haemoglobin, which gives them their red pigment. There are 5 to 6 million red cells per cubic mm of blood. The red cell only survives about 120 days and the damaged and old cells are removed by the spleen and liver.

A white blood cell is not really white but almost transparent. It can change shape, push out folds and finger-like projections and crawling like an amoeba in a pond. These cells survive less than a week.

Fact File: An average adult body has about 5 litres of blood. At any one time, about 1,250 ml are in the arteries, 3,500 ml in the veins and about 250 ml in the capillaries. The cells in blood flow through a capillary for only half a second before they move into the next type of vessels, small veins.

 

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Our individual blood group is determined by our parents at the time of our conception. Blood groups are determined by the presence of antigens (a substance capable of stimulating an immune response) on the surfaces of the red cells. Although the red blood cells in different people look the same they are, in fact, dissimilar. They can be divided up into four main groups: A, B, AB and O.

Blood can be transplanted from one person to another by what we call a blood transfusion. It is very important that the blood given matches the person’s group, because if the wrong types of blood are mixed together the result can be serious blood clots.

Fact File: Blood begins to clot as soon as it is exposed to the air, plugging the wound. White blood cells gather around the wound to kill invading microbes, and new skin cells grow into the healing wound beneath the scab.

 

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The spleen is one of the main filters of the blood. Not only do the reticular cells remove the old and worn-out blood cells, but they will also remove any abnormal cells. This applies in particular to red blood cells, but white cells and platelets are also filtered selectively by the spleen when it is necessary.

The spleen will also remove abnormal particles that are floating in the bloodstream. It therefore plays a very important part in ridding the body of harmful bacteria.

In some circumstances the spleen has a major role in the manufacture of new red blood cells. This does not happen in the normal adult, but in people who have a bone marrow disease. The spleen and liver are major sites of red blood cell production. Another function of the spleen is to manufacture a great deal of the blood of a foetus while it is in the uterus during its period of gestation.

Fact File: The spleen is situated in the top left-hand corner of the abdomen, just below the diaphragm. It is in a relatively exposed position, which is why it is frequently damaged in accidents and has to be removed.

 

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Your body has an amazingly complex and delicate system of blood vessels, carrying blood to every nook and cranny, and then returning to the heart. These vessels are called arteries and veins and they are both tubes made up of four different layers. The arteries carry the blood away from the heart and the veins return it.

The vein frequently anastomose (or join together) with each other so that the blood flow can alter direction. This is caused if there is any constriction or pressure from movement of muscles or ligaments.

Fact File: Just over half of blood is plasma, a pale yellow, sweet-smelling, sticky fluid. It contains hundreds of dissolved substances, from sugars for energy, to hormones, to wastes like carbon dioxide.

 

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Your heart is a muscular pump that never stops beating. It has its own timing device that produces tiny electrical signals. These signals cause the heart muscle to contract rhythmically. The pump on the right side of the heart receives blood that has been pumped around the body. This blood is dark red and has used up most of its oxygen. The right pump sends it on a short circuit through the lungs that surround the heart. The blood comes back bright red and rich in oxygen, to the heart’s left side, ready for its journey around the body. When the heart stops beating, body tissues no longer receive fresh blood carrying oxygen and nutrients. So life ends.

However, in a hospital, the cardiopulmonary machine can take over the job of heart and lungs. This means doctors can resuscitate people or carry out operations on the heart, such as replacing diseased valves.

Fact File: When the body is very active, the heart can pump 20 gallons of blood each minute. That would fill a bathtub within two minutes.

 

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Your heart is a powerful muscle which pumps blood around your body. It is only the size of your fist and weights less than half a kilogram. Each and every day it pumps about 18,000 litres of blood around your body, and yet you are not normally aware that it is even beating. Run quickly upstairs, though, and you will soon feel it thumping away inside your rib cage.

A heart attack can occur when either or both sides of the heart are unable to pump sufficient blood to meet the needs of our body. Other prominent causes of a heart attack are abnormally high blood pressure (hypertension), coronary atherosclerosis (the presence of fatty deposits in the lining of the coronary arteries), and rheumatic heart disease.

A person with left-sided heart failure experiences shortness of breath after exertion, difficulty in breathing while lying down, spasms of breathlessness at night, and abnormally high pressure in the pulmonary veins. A person with right-sided failure experiences abnormally high pressure in the systemic veins, enlargement of the liver, and accumulation of fluid in the legs. A person with failure of both ventricles has an enlarged heart that beats in gallop rhythm – that is, in groups of three sounds rather than two.

Fact File: An electrocardiogram, or ECG, measures the electrical signals that the heart produces as it beats. These signals change when a person is suffering from certain medical conditions that affect the heart.

 

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Haemophilia  is an inherited deficiency whereby the substance necessary for blood clotting is missing. The transmission of this condition is sex linked, being present mostly in males but carried solely by females. Sons of a haemophilic male are normal, but daughters, although outwardly normal, may transmit this deficiency to half their sons.

The existence of haemophilia in certain royal families of Europe is well known. Working from family trees it seems probable that Queen Victoria naturally produced the gene for haemophilia.

Fact File: When we look at other human bodies, we usually concentrate on the face. Our features are largely inherited, under control of the gens, which is why we resemble our parents.

 

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In the 1800s a monk named Gregor Mendel experimented with characteristics in pea plants by cross-fertilizing plants with different traits. He kept a careful track of the traits displayed by the pea plants produced by cross-fertilization, discovering that the characteristics from the parent plants were inherited by the progeny (off-spring) plants in specific patterns.

Mendel also discovered during his experiments that certain genes seemed more dominant than others. For example, if a pea with a white flower is cross-fertilized with a pea with a pink flower, the resulting flowers will all be pink.

This is clear in human beings. A parent with brown-brown genes produces only children with brown eyes, while a parent with brown-blue genes could produce children with eyes other than brown.

Fact file: Chromosomes are tiny threads that are present in all cells apart from red blood cells. They contain all the information for an entire person to develop. There are46 chromosomes in each cell. They come in 22 pairs, plus another special pair that determine the person’s sex.

 

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As well as being packed with information, the DNA of chromosomes also has the ability to reproduce itself. Without this, the cells could not pass on information from one generation to the other. The process of cell division in which the cell duplicates itself is called mitosis, which works as follows:

1 the chromosomes become shorter and the nuclear envelope breaks,

2 the chromosomes are released, which duplicate and attach themselves to a cytoplasmic network,

3 they are then drawn apart

4-7 to form two new cells with reformed nuclear envelopes.

Mitosis is absolutely essential to life because it provides new cells for growth and for replacement of worn-out cells. Mitosis may take minutes or hours, depending upon the kind of cells and species of organisms. It is influenced by time of day, temperature, and chemicals. Strictly speaking the term mitosis is used to describe the duplication and distribution of chromosomes, the structures that carry our genetic information.

Fact File: DNA strands look like a twisted ladder. Sections of DNA are called genes. All the instructions for growing a new human being are coded into the DNA molecule.

 

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A baby’s genetic make-up is decided right from the time when the egg is fertilized. A very important part of cell division involves the nucleus. Each nucleus contains two sets of genes. One came originally from the person’s father and one from the mother. So before a cell divides, both sets of genes are copied. This is termed DNA replication. Each new offspring cell then receives a full double set of gens, one from the father and one from the mother.

Every human body gets half its genes from each parent, therefore it inherits some of the features of both the mother and the father. This is why all kinds of characteristics, for example height, run in families.

However, every human body has its own unique physical circumstances, such as the food we eat, our environment and the illnesses that we catch.

We also each have our own behavioral and mental development. So we inherit a mix of physical features from our parents, but also we are all individual and unique.

Fact File: A long thread or axon extends from the body of a neurone, and it is along this that the nerve impulses are carried.

 

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Cells are able to divide very quickly indeed to replace those that are old or have died. Nerve cells are the only ones that cannot be replaced. However, even nerve cells can sometimes grow new connections if the message paths become damaged.

Dead and dying cells are removed by white blood cells in the bloodstream, which actually eat them. The liver is also able to break down red blood cells, which are only able to survive for a very short time. The cell’s control area, the nucleus, contains all the information and instructions to keep the cells alive and functioning. The information the cell needs is in the form of immensely long coils of chemicals. These structures are known as DNA, which make up the genes.

Fact File: The largest cell in the human body is the egg cell or ovum, which may be fertilized by a sperm cell and grow into a baby.

 

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All living things, plant and animal alike, are made from cells. An individual body cell is too small to be seen by the naked eye. Cells have a fixed lifespan and are replaced automatically as they die off. The more active the cell, the shorter the time it will live.

Some white blood cells live for a very short time, and some types that consume dead cells and bacteria survive for only about 30 hours.

White cells that fight disease live for two to four years. Cells lining the intestine live for about five days before being replaced.

Below is a list of the lifespan of certain cells:

• Skin cells live for 19 days


• Sperm live for 2 months


• Eyelashes live for 3 to 4 months


• Red blood cells live for 4 months


• Liver cells live for 8 months


• Scalp hairs live for 2 to 4 years


• Bone cells live for 15 to 25 years
  
  
  
  
  
  
  

  Fact File: There are three types of nerve cell each with a different function. Motor neurons control the way your muscles work. Sensory neurons carry messages from your sense organs. Connector neurons pass messages between different parts of the nervous system.

  
  
  

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A joint is the meeting point between bones and usually controls the amount of movement. Some joints have to be strong, while others need to be very mobile. As it is not possible for joints to be both strong and mobile we require many different kinds of joints:

Ellipsoid Joint: Allows circular and bending movement but no rotation

Saddle Joint: Allows movement in two directions, but without rotation.

Ball and socket joint: A joint freely moving in all directions.

Hinge Joint: Allows extension and flexion.

Condylar Joint: This is similar to a hinge joint, but with slight rotation to allow the joint to ‘lock’ into an extended position.

Pivot Joint: Allows rotation but no other movement.

Plane Joint: A flat surface allows the bones to slide on each other, but they are restricted by ligaments to a small range.

Fact File: The knee joint is the largest and most complex joint in the body. As it reaches full extension it rotates slightly and ‘locks’ into a rigid limb from hip to ankle.

 

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The skeleton is the framework of our bodies which keeps the organs, blood vessels and nerves in place and also acts as protection. The connective tissue is needed to act as a support and to bind them all together. It also supplies the ligaments and tendons for the joints and muscles, tethers the larger organs to keep them in place, and provides softness for protection and rigidity in the form of cartilage.

There are many forms of connective tissue, but they are all developed from the same jelly-like substance which is made up of salts, water, protein and carbohydrate. Inside this jelly are elastic threads to give elasticity; collagen to give strength; reticulin to give support; white cells and macrophages to fight infection; fat cells for storage; and finally plasma cells to produce antibodies.

Fact File The shape and appearance of a cell depends on what job it does. Cells consist of jelly-like cytoplasm, surrounded by a membrane. Nutrients pass through this membrane and substances produced by the cell leave.

 

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