Biology

Endocrine System

The activities such as growth, reproduction. maintenance of glucose concentration in blood, reabsorption of water in kidneys etc. need to be regulated. Endocrine system performs this job. This system uses chemicals to “communicate” with its effectors.

Many glands in our body are exocrine. Such glands have ducts for releasing their secretions e.g. digestive glands, skin glands etc.

What is Hormones?

These chemicals are known as hormones. A hormone is a specific messenger molecule synthesized and secreted by an endocrine gland. These glands are ductless and release their secretions (hormones) directly into bloodstream. Blood carries the hormones to target organs or tissues, upon which they act.

The stepwise process of metamorphosis in many animals is controlled by hormones. Life activities such as cell division in invertebrates are also regulated by hormones. Hormones also control activities like migration in birds, Hormones have been identified even in unicellular organism.

Important Endocrine Glands

There are many Important Endocrine Glands are as followings:

1. Pituitary Gland

It is a pea-shaped gland attached to the hypothalamus of brain. Many hormones (trophic hormones) of pituitary gland influence the secretions of other endocrine glands However some hormones of this gland act directly on various tissues of body. There are two lobes of pituitary gland i.e. anterior lobe and posterior lobe.

a. Anterior Lobe:

It produces many hormones. One of its important hormones is somatotrophin (growth hormone). It promotes the growth of body. If the production of this hormone is diminished during growing age, the rate of growth decreases. This condition is called dwarfism.

If this hormone is excessively produced during growing age, it leads to gigantism (very tall and overweight). If somatotrophin is excessively produced after growing age, internal organs and body extremities alone grow large. This condition is known as acromegaly. Such persons will have large hands, feet and jawbones.

Another important hormone secreted by the anterior lobe of pituitary gland is thyroid-stimulating-hormone (TSH). It stimulates thyroid gland to secrete its hormones. The remaining hormones of anterior lobe influence reproductive organs and also control adrenal glands.

b. Posterior Lobe:

The posterior lobe of pituitary gland stores and secretes two hormones i.e. oxytocin and vasopressin (antidiuretic hormone: ADH). These hormones are produced by hypothalamus (a part of brain).

Vasopressin increases the rate of reabsorption of water from nephrons. When we have low amount of water in body fluids, pituitary gland secretes vasopressin and so more reabsorption of water occurs from nephrons into blood. In this way, body retains water and less amount of urine is produced.

Have you noticed that during summer, the urine output is low? Due to increased sweating, the water level of blood is lowered. As a result, pituitary gland releases more ADH into blood.

What is diabetes insipidus?

On the other hand, when body fluids have more than normal water, there is a decline in the secretion of this hormone. If pituitary gland does not secrete this hormone in the required amount, less water is reabsorbed from nephrons and there is excessive loss o water through urine. This condition is known as diabetes insipidus.

The hormone, oxytocin stimulates the contraction of uterus walls in mothers for child birth. Moreover, this hormone is necessary for the ejection of milk from breast.

2. Thyroid gland

This is the largest endocrine gland in human body. It is  present in neck region, below larynx, and produces a hormone thyroxin. iodine is required for the production of this hormone. If a person lacks iodine in diet, thyroid gland cannot make its hormone. In this condition, thyroid gland enlarges. This disorder is called goitre.

Our government encourage salt refiners to add iodine to salt It also encourages people to choose this iodized salt.

Thyroxin increases the break down of food (oxidation) and release of energy in body. It is also responsible for the growth of body. Hypothyroidism is caused by the under-production of thyroxin. It is characterized by low energy production in body and slowing down of heart-beat. Hyperthyroidism is caused by over-production of thyroxin. Its symptoms are increase in energy production, increased heart-beat, frequent sweating and shivering of hands.

The thyroid gland produces another hormone called calcitonin. It decreases the level of calcium ions in blood and promotes the absorption of calcium from blood into bones.

3. Parathyroid glands

These are four glands situated on the posterior side of thyroid gland. They produce a hormone known as parathormone. It increases the level of calcium ions in blood.

Calcitonin and parathormone complement each other and regulate the level of calcium ions in the blood.

When there is increased production of parathormone, more than normal calcium salts are absorbed from the bones and added to blood. Consequently the bones become brittle. If there is deficiency in the production of parathormone, blood calcium level falls. It leads to tetany, which affects the functioning of muscles.

Tetany is marked by sharp flexion of the wrist and ankle joints, muscle twitching, cramps and convulsions. It is due to decreased blood calcium level which makes the nerves and muscles more excitable.

4. Adrenal glands

Two adrenal glands are situated above kidneys. Each adrenal gland consists of two parts. The outer part is cortex and the inner part is medulla. Adrenal medulla secretes a hormone called epinephrine or adrenaline in response to stress. It prepares our body to overcome emergency situations. Therefore, adrenaline is also termed as ’emergency hormone.

When a person experiences fear, anger or anxiety, the rate and intensity of heartbeat increases, blood pressure increases, blood flow to the limbs increases, blood flow to the alimentary canal and skin is reduced. Such changes prepare the body to face any emergency situation.

The adrenal cortex secretes many hormones called corticosteroids which maintain the balance of salts and water in blood.

5. Pancreas

This organ has two functions. The major part of pancreas is a ducted (exocrine) gland. This portion secretes digestive enzymes, through a duct, into the small intestine. Some portions of pancreas serve as ductless (endocrine) gland. This portion contains groups of endocrine cells referred to as islets of Langerhans. These islets secrete two hormones i.e. insulin and glucagon. Glucagon influences the liver to release glucose in blood and so the blood glucose concentration rises. Insulin influences the liver to take excess glucose from blood and so the blood glucose concentration falls.

If a person’s pancreas does not make normal quantity of insulin, the blood glucose concentration rises and we say that the person has diabetes mellitus.

The blood glucose concentration is maintained at the rate of 80 to 120 mg  per 100 ml of blood.

Persons with diabetes have loss of body weight, weakening of muscles and tiredness. The disease can be controlled by insulin administration. Formerly, insulin extracted from animals was used for this purpose. But now human insulin produced from bacteria through genetic engineering is available.

Blood Glucose Concentration (BGC) Test:

The amount of glucose in blood is measured by this test. It is used to diagnose diabetes. Blood glucose may be measured on a fasting basis (collected after an 8 to 10 hour fast), randomly (anytime) and after a meal. The results of some BGC tests are given here.

6. Gonads

Testes (Singular testis) and ovaries are the male and female reproductive organs i.e. gonads. In addition to producing gametes, gonads also secrete hormones, called sex hormones. Testes secrete hormones e.g. testosterone, which is responsible for the development of male secondary sex characters such as growth of hair on face and coarseness of voice etc.

Ovaries secrete estrogen and progesterone, which are responsible for the development of female secondary characters such as the development of breast etc.

Feedback Mechanisms

Endocrine glands do not secrete their hormones at a constant rate. The rate varies with the needs of the body. Like many other functions in body, the secretion of hormones is also regulated by feedback mechanisms. Feedback mechanism means the regulation of a process by the output of the same process. Feedback mechanisms are of two types

Endocrine glands in human body
                                                             Endocrine glands in human body

i.e. positive and negative feedbacks.

In negative feedback, the output of a process decreases or inhibits the process. This mechanism works to return a condition towards its normal value. For example; when the blood glucose concentration rises, pancreas secretes insulin. It decreases the blood glucose concentration. Decline in the blood glucose concentration to a normal set-point inhibits the secretion of insulin. Similarly, when blood glucose concentration drops below normal, pancreas secretes glucagon. It raises the blood glucose concentration. In this case, rise in the blood glucose concentration to a normal set-point inhibits the secretion of glucagon. In other words, the blood glucose concentration (output) controls the process i.e. the secretion of insulin and glucagon.

In positive feedback, the changes resulting from a process increase the rate of process. For example; suckling action of an infant stimulates the production of a hormone in mother. This hormone works for the production of milk. More suckling leads to more hormone, which in turn leads to more milk production.

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