A group of endocrine glands which produces various hormones is called an endocrine system. The endocrine system is also called hormonal system. We will now discuss the endocrine system in humans in detail.
In addition to nervous system, the endocrine system also helps in coordinating the activities of our body. The endocrine system in our body consists of a number of glands (or tissues) which make, store, and release chemicals called hormones. There are a large number of endocrine glands in the human body. The endocrine glands present in the human body are: Pineal gland ; Hypothalamus gland; Pituitary gland Thyroid gland; Parathyroid glands; Thymus; Pancreas; Adrenal glands; Testes (only in males) am Ovaries (only in females). The positions of all these endocrine glands in the human body.
The endocrine glands are located in different parts of the body. The endocrine glands are located in the head, neck and trunk of our body. Different endocrine glands make different types of hormones which act on different organs of our body.
The working of endocrine glands is controlled by our nervous system. The hormones produced by endocrine glands act as messengers between the nervous system and the organs of our body. We will now take the example of adrenal glands to show how the endocrine system (or hormonal system) coordinates our body activities.
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There are two adrenal glands in our body, one on top of each kidney (see Figure 45). The adrenal glands make adrenaline hormone. The adrenaline hormone prepares our body to function at maximum efficiency during emergency situations like danger, anger, excitement, etc.
This happens as follows: When we are faced with a dangerous situation (like being chased by a ferocious dog), then our nervous system stimulates the adrenal glands to secrete more adrenaline hormone into our blood.
This adrenaline hormone increases our ‘heart beats’, ‘breathing rate’, ‘blood flow into muscles’ and causes liver ‘to put more stored glucose into our blood’. All these actions of adrenaline hormone produce a lot of energy in our body very, very quickly. And this energy helps us to run away very fast from the dog to save ourselves. In this way, the adrenaline hormone prepares our body to run away very fast from a frightening object. Similarly, it is the adrenaline hormone which prepares our body to fight an enemy (say, a burglar in our house) by providing us a lot of energy in a very short time. A lot of adrenaline hormone is also secreted by adrenal glands when we are ‘angry’ or ‘excited’. The rapid output of energy thus caused helps us to cope with these extreme emotional situations.
The complete coordination in the human body is achieved by the nervous system and endocrine system working together. The main centres in the body for the coordination of the two systems of control (nervous system and the endocrine system) are the hypothalamus and pituitary gland. The hypothalamus plays an important role in collecting information from other regions of the brain and from blood vessels passing through it. This information is passed on to pituitary gland which by its own secretions, directly or indirectly, regulates the activities of all other endocrine glands.
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The hormones are involved in the regulation of several functions in the human body like growth, metabolic activities and reproduction. We will now give the names of the endocrine glands, the hormones released by these glands, and the functions of these hormones in the human body. Please note that pineal gland which is present in the brain has no known function. Pineal gland is supposed to be a vestigial organ (Vestigial organs are those organs which no longer function). Let us discuss the other endocrine glands now.
1. Hypothalamus :
Hypothalamus gland is present in the brain. Hypothalamus produces ‘releasing hormones’ and ‘inhibitory hormones’. The function of hypothalamus is to regulate the secretions of hormones from pituitary gland. That is, hypothalamus controls the pituitary hormones.
2. Pituitary Gland:
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Pituitary gland is present just below the brain. The pituitary gland secretes a number of hormones. One of the hormones secreted by pituitary gland is growth hormone (or human growth hormone). The growth hormone controls the growth of the human body. For example, growth hormone controls the development of bones and muscles. A person having a deficiency of growth hormone in childhood remains very short and becomes a dwarf. On the other hand, a person having too much growth hormone becomes very tall (or a giant).
Thyroid gland is attached to the wind pipe in our body. Thyroid gland makes a hormone called thyroxin (which contains iodine). The function of thyroxine hormone is to control the rate of metabolism of carbohydrates, fats and proteins in the body. Iodine is necessary for the making of thyroxine hormone by thyroid gland, therefore, a deficiency of iodine in the diet can cause a deficiency of thyroxine hormone in the body.
The deficiency of iodine in the diet of a person produces less thyroxine hormone and causes; disease known as goitre. The main symptom of goitre is that the neck of the person appears to be swollen due to the enlargement of thyroid gland located in the neck). People are advised to use iodised salt for cooking food so as to prevent goitre disease.
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This can be explained as follows: Iodine is required by the thyroid gland to make thyroxine hormone. Iodised salt contains appropriate amount of iodine compounds (such as potassium iodide). Iodised salt can provide all the iodine needed by thyroid gland to make sufficient thyroxine for our body. Since there will be no deficiency of thyroxine in the body, goitre cannot develop.
4. Parathyroid Glands :
There are four small parathyroid glands which are embedded in the thyroid gland. Parathyroid glands secrete a hormone called parathormone. The function of parathormone hormone is to regulate calcium and phosphate levels in the blood.
5. Thymus Gland:
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Thymus gland lies in the lower part of the neck and upper part of chest. Thymus gland secretes thymus hormone which plays a role in the development of the immune system of the body. Thymus gland is large in young children but shrinks after puberty (or sexual maturity).
6. Pancreas:
The pancreas is just below the stomach in the body. Pancreas secretes the hormone called insulin. The function of insulin hormone is to lower the blood sugar level (or blood glucose level). Deficiency of insulin hormone in the body causes a disease known as diabetes. Diabetes disease is characterised by large quantities of sugar in the blood (and even urine). The insulin hormone controls the metabolism of sugar.
If, due to some reason, pancreas does not produce and secrete sufficient amount of insulin into blood, then the sugar level in the blood rises. The high sugar level in the blood can cause many harmful effects to the body of a person. The person having high sugar level in blood (or diabetes) is called a diabetic. Diabetic
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There are two adrenal glands which are located on the top of two kidneys. The adrenal glands secrete adrenaline hormone. The function of adrenaline hormone is to regulate heart rate, breathing rate, blood pressure and carbohydrate metabolism. Adrenaline hormone is secreted in small amounts all e time but in large amounts when a person is frightened or excited. When adrenaline is secreted in large amounts it prepares our body for action. It speeds up heart beat and breathing, raises blood pressure and allows more glucose (carbohydrate) to go into the blood to give us a lot of energy quickly to fight or flight (run away). Adrenal glands are often called ‘glands of emergency’.
Testes are the glands which are present only in males (men). Testes make male sex hormones called testosterone. The function of testosterone hormone is to control the development of male sex organs and male features such as deeper voice, moustache, beard, and more body hair (than females). All these changes caused by testosterone are associated with male puberty which the boys attain at an age of 13 to 14 years. The testes also make the male gametes called sperms.
9. Ovaries:
Ovaries are the glands which are present only in females (women). Ovaries make two female sex hormones called oestrogen and progesterone. The function of oestrogen hormone is to control the development of female sex organs, and female features such as feminine voice, soft skin and mammary glands (breasts).
All these changes caused by oestrogen are associated with female puberty which the girls attain at an age of 10 to 12 years. The function of progesterone hormone is to control the uterus changes menstrual cycle. It also helps in the maintenance of pregnancy. The ovaries also make the female gametes called ova (or eggs). Feedback Mechanism
The excess or deficiency of hormones has a harmful effect on our body. For example, the deficiency of insulin hormone results in a disease called diabetes whereas excess of insulin in the body can lead to coma. So, it is necessary that the hormones are secreted by the glands in our body in precise quantities which are required for the normal functioning of the body.
This means that there should be some ‘mechanism to regulate the production and release of hormones in the body. The timing and amount of hormones released by various glands are controlled by the ‘feedback mechanism’ which is in-built in our body.
For example, if the sugar level in the blood rises too much, they are detected by the cells of pancreas which respond by producing and secreting more insulin into blood. And as the blood sugar falls to a certain level, the secretion of insulin is reduced automatically.
Before we end this discussion we would like to give a comparison of the nervous system and endocrine system (or hormonal system) for the control and coordination in humans (and other higher animals).
Nervous system:
1. Made of neurons (nerve cells)
2. Messages transmitted in the form of electrical impulses
3. Messages transmitted along nerve fibres
4. Messages travel very quickly
5. Effect of message usually lasts for a very short while
Endocrine system (Hormonal system):
1. Made of secretary cells (or glands)
2. Messages transmitted in the form of chemicals called hormones
3. Messages transmitted through blood stream
4. Messages travel more slowly
5. Effect of message usually lasts longer