Skin, the outer protection, has two layers known as: epidermis and dermis. The delicate nerves and tissues lie beneath them. Epidermis has hard and tough cells which get flattened, die and are transformed into a material called Keratin, which is finally shed as tiny scales. In the dermis are sweat glands and ducts. The skin is water-proof and heat proof. It will not dry up in heat or melt in the rain. It protects from harmful radiations; acts as a shield against injury; conserves heat during winters and cools body during summer. It keeps the internal temperature constant.
Muscles make our movements possible through contraction and expansion. There are three types of muscles: Striped muscles are composed of tissues, and are called voluntary for they are under the control of the brain. Each major muscle consists of several bundles, made up of a number of fibres. Smooth muscles consist of long cells tapered at both ends. They are found at the lining of gut and the walls of the arteries. They are called involuntary muscles for they are not in the control of the brain. On the other hand they are responsible for the muscular construction. They work on their own. Cardiac muscles are similar in structure to voluntary muscles but the fibres are thick and short; and form a dense mass.
A cell is a unit in which all life activities take place. The basic structure of every living being is composed of cells. Each cell lives its own life and at the same time makes it possible for the living being as a whole to carry on its life-activities. The useful matter in the cell is the protoplasm, which carries on all the processes necessary for life. It takes in food and oxygen, changes food into living matters, gives out waste, repairs its worn-out parts, and forms and produces itself.
There are plants and animals ranging from one cell to millions of cells. The most complicated cell structure is of human. There are animals of lower order and lesser cells also. Cells are organized in group of cells of a particular kind to perform a particular type of work. They are called tissues as bone tissues or muscle tissues etc. When several tissues are combined to perform a given task then it’s called an organ.
In human body there are five important types of cells: Epithelial cells make up the skin, the glands and line the body vessels. Muscle cells make up all the three kinds of muscles. Nerve cells make brain, spinal cord and nerves. Blood cells are found in the blood and lymph. Connective tissue cells make up the framework tissues of the body. Cells are the building blocks.
In the circulatory system the great wonders are the Arteries and the Veins. They are blood vessels. Blood is sent from the heart through arteries that carry pure blood to various parts of the body; and through veins impure blood comes back to the heart. The right auricle receives blood from the body. The heart or the auricle contracts and blood flows into the right ventricle. The ventricle contacts, it pumps blood into the lungs. The left auricle receives blood from the lungs and passes it to the ventricle. Within seconds the process is over; but it continues incessantly to keep us alive. In that process and in the split seconds the blood is purified and sent to the whole body as well as the impure blood is collected from the whole body. The whole circulatory system works on its own.
The cells live and work separately and in conjunction with others. So do the tissues. In this way all the organs work independently and also as a part of the complete system. The functions and activities of each organ are co-related and synchronized. A slight deviation will cause illness.
Brain is the most important organ of the body. It controls and regulates the functioning of whole body. All other organs are equally important. What can the brain do, if blood is not supplied to it? A momentary lapse of blood supply will paralyse the brain. Man will be alive but the brain will not be functioning.
Brain is located in the skull, well protected because it’s very tender and subtle. It has three main parts: Forebrain, Midbrain and Hindbrain. The anterior part is the forebrain which includes the olfactory lobes (the centre of smells); and cerebral hemisphere (the seat of intelligence). The midbrain includes optic lobes, the centre of vision. The hindbrain is the posterior part which includes cerebellum (co-ordination centre) and the medulla oblongata (centre of involuntary actions) Medulla Oblongata lies behind the spinal cord. All the functions are controlled by the brain. The brain is connected to each part of the body with nerves which form a delicate nervous system. It receives the messages and passes the orders which are immediately sent to the organs for necessary action.
The brain and the spinal cord form the central processing unit of nervous system. The brain stem, which links the brain with the spinal cord, is a part of the hindbrain. Sleep and wakefulness are located in the brain stem.
Liver, weighing between 1.36 to 1.81 kg, is the largest organ in the human body. The proteins, both of vegetarian and animal origin, form ‘raw proteins’. Liver makes them acceptable to the body first by breaking them down. It manufactures certain proteins on its own such as Fibrogen, the blood clotting protein. It turns carbohydrates into two forms: one is “instant energy” in the form of glucose and the other is “stored energy” in the form of glycogen. It produces a hormone which can store excess sugar present in the blood as glycogen. Fats are also turned into different forms by liver as subcutaneous layer and act as insulation and shock absorbers. It produces bile (pitta) which is a thick yellow or greenish fluid and it neutralizes the acidity of partly digested food in the intestine so that enzymes can continue to work there. It is also a storage organ. Several vitamins, iron (removed from blood pigment) are stored in liver. It also neutralizes poisons and wastes.
The urinary system is a part of the cleaning system and consists of two kidneys, ureters, urinary bladder and urethra. Kidneys are situated at the back of the abdominal cavity, one on each side of the vertebral column. They are made up of a large number of ‘caoated tube-shaped filters’ called nephrons. They extract nitrogenous waste material and excess water from blood and form urine which passes to the urinary bladder via ureters. From there it’s expelled through the urethra by contraction of muscles. The removal of such waste material is essential in all living organisms. Normal function of kidney ensures good health.
The other important cleaning organ is the spleen which is an integral part of the lymphatic system. It lies just below the diaphragm, at the top of the left side of the abdomen. It’s one of the main filters of blood. It removes the old and worn out blood cells and also abnormal cells. It plays a major part in getting rid of the harmful bacteria. It also makes antibodies. Enlarged spleen is an indication of some disease in the body.
A gland is an organ which produces a chemical substance that regulates the metabolic activities of the body. There are two kinds of glands: Exocrine glands which have ducts leading to the outside; and Endocrine glands which are ductless. Sweet glands secrete sweet through coiled ducts to the surface of the skin. Digestive glands secrete digestive juices in the stomach and intestine. Salivary glands produce saliva. They are all exocrine glands. Endocrine glands secrete hormones which are chemical messengers released directly into the bloodstream and are used to control metabolic activities such as growth and development. One of the most important endocrine glands is pituitary gland which is situated in the brain. It is called the master gland. Thyroid gland is a special large bi-lobed gland situated behind the larynx (voice-box) in the neck. It secretes throxin hormone that contains iodine. It controls general metabolism of the body and accelerates the production of energy and consumption of oxygen. Excessive secretion increases body temperature, fat is depleted and the body loses weight. Reduced secretion will cause hair-loss and swelling of tongue and vocal cords.
Blood in itself is a wonder. It absorbs, carries and distributes all the elements and energy to each part of the body. Blood is a tissue that has different kinds of cells in pale liquid called plasma. Red cells are red due to the presence of pigments called haemoglobin and carry oxygen. White cells are fighter cells. It fights against germs and other foreign bodies. It helps in maintaining a constant temperature in the body. Blood contains antibodies. These are proteins manufactured by the body that fight against foreign substances called antigens. When an antigen enters the body, it stimulates the immune system to produce antibodies (The immune system is the body’s natural defence system). The antibodies attach, or bind, themselves to the antigen and inactivite it.
Eyes, the organs of vision are very delicate organs. Eyeballs are protected in a socket with eye-lids, eye-lash and eye-brows. The outer layer of the eye-ball has three layers: the outermost layer is tough and elastic. It’s opaque at the back and is called sclera; and it’s transparent at the front and is called cornea. The middle layer supplies blood to the eye. The innermost layer is the retina which is an extremely delicate tissue. At the centre of the eye is a circular opening called the pupil which is a hole in the middle of iris. Iris is just behind the cornea. The pupil appears black because no light is reflected from it. The lens is immediately behind the pupil. The space between cornea and eye lens is filled up with a vision liquid called ‘aqueous humour’. The light coming from an object enters the eye through cornea. When we see an object, the light-waves entering our eyes are focused on retina by the lens.
The lids of our eyes are our built in wind-screen wipers. The lids are made up of folds of skin, and they can be raised and lowered by certain muscles. Our blinking provides automatic lubrication to the eyes. The eyelid applies suction to the opening of the tear gland and takes out some of the fluid. This prevents the eyes from drying out. The tears contain an antiseptic which kills germs. It protects against bacteria. The eye-lids are directly in contact with spinal cord. So, when suddenly an object approaches our eyes, the eye-lids are closed in a reflex action.
