Category Zoology

When do we produce saliva?

The major function of saliva is to help in the process of digestion. It keeps the mouth moist and comfortable when we eat and helps to moisten dry food, allowing it to be chewed and swallowed more easily. The mucus in saliva coats the bolus (or chewed food) and acts as a lubricant to help us to swallow.

The enzyme ptyalin which is found in saliva begins the first stage of digestion. It begins to break down starchy food into simpler sugars. Saliva also allows us to taste our food and drink. Each day we all usually produce about 1.7 litres (3 pints) of saliva.

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The four main tastes are sweet, salt, bitter and sour, and you can taste them with different parts of your tongue. You can check where the four tastes are by dabbing it with a little salt, sugar, coffee grounds (bitter) and lemon juice (sour).

 

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When do we learn to talk?

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.

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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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When do red and white blood cells die?

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.

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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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When are our blood groups determined?

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.

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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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When does the spleen produce red blood cells?

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.

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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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When does blood flow from veins to arteries?

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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