Why do people wear glasses?
At the back of the human eyeball, there is a kind of screen which is called the ‘retina’. The lens of the eye focuses the image of an object we ‘see’ on this retina.
But sometimes due to some reason, the image does not focus correctly on the retina. If the image is focused in front of the retina, the person is said to be ‘nearsighted’. A person who is nearsighted can see things that are not very far from the eye but cannot see things which are at a distance.
When spectacles with lenses of the right size and curve are used by a nearsighted person, the image is correctly focused on the retina instead of in front of it.
On the other hand, when the image of an object is focused behind the retina, we say that the person is farsighted. The closer the object is, the farther behind the retina is the image produced.
To read a book, such people need glasses, but they may be able to see things which are very far away. The lenses of the glasses used for such persons are curved in the opposite direction as the ones used by nearsighted persons.
What makes you feel tired?
When you work, you have to use your muscles. When the muscles move, they produce a substance called ‘sarcolactic acid’. The same substance is also found in your blood.
When too much of this sarcolactic acid is formed because of too much work, all the muscles in your body become tired.
When you are sleeping or resting the muscles are relaxed and the acid is removed from the muscles and blood. New cells are formed, blood is cleaned, and joints are lubricated. You again feel fresh and energetic.
Some other substances in the body also make us feel tired. These are called ‘fatigue toxins’. These are also created in our bodies when we work. Our blood carries these toxins to all parts and not only the muscles, but all other parts, even the brain feel tired.
Why can’t the heart ever rest?
The non-stop working pump is an amazing organ of the human body. Even when you are sleeping it goes on pumping blood to reach every part of your body to keep you alive and healthy.
With every beat of the heart, one hundred cubic centimetres of blood is pumped out. In one day about 10,000 litres of blood is pumped through our blood vessels, which means that in an average life, a heart pumps about 250,000,000 litres of blood! And it never stops to rest!
What we call a heartbeat is one contraction and relaxing of the heart. During the contracting purified blood is pumped out, during the relaxing impure blood from the parts of the body enters.
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Why do we need air?
Nobody has to teach a child how to breathe. We must breathe
to live. In fact, all living plants and animals breathe for keeping alive. All
living organisms require oxygen which is present in the air which we inhale.
The cycle of inhaling and exhaling, or inspiration and
expiration, is called breathing or respiration. Inhaling means taking the
air inside our body through the nose or mouth or both. And Exhaling means
sending out air through the same channels.
The fresh air from outside reaches to inside us, and our body uses some of its oxygen to keep us healthy and alive. The air that the body sends out has about one-fifth of the oxygen exchanged for the same amount of carbon dioxide.
The carbon dioxide that we exhale is not useful to the human body system, but plants make use of this for their living and give out oxygen which is so important for us.
The oxygen that we inhale, reaches our lungs and is then
carried to the tissues of the body with the help of the red cells of the blood.
In the tissues, this oxygen helps the food that we eat change into usable form
and gives us energy that keeps the body cells alive. The waste products
including carbon dioxide and water return to the lungs for exhaling.
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