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

      These chemicals are called hormones. Some hormones may be a specific messenger molecule synthesized by a gland. These glands are ductless and release their secretions directly into the bloodstream in the vessel. Blood carries the hormones to focus on organs or tissues, upon which they act.


1.Important Endocrine Glands:


1. Pituitary Gland:

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


 a. Anterior Lobe: It produces many hormones. One of its necessary hormones is somatotrophin (growth hormone). It promotes the expansion of the body. If the assembly of this hormone is diminished during growing age, the speed of growth decreases. This condition is termed dwarfism. If this hormone is excessively produced during growing age, it results in gigantism (very tall and overweight). If somatotrophin is excessively produced after growing age, internal organs and body extremities grow large. This condition is understood as acromegaly. In which persons will have large hands, feet, and jawbones.

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

b. Posterior Lobe: The posterior lobe of hypophysis stores and secretes two hormones, i.e. oxytocin and vasopressin (antidiuretic hormone: ADH). The Hypothalamus (a part of the brain) produces these hormones.

     Vasopressin increases the speed of reabsorption of water from nephrons. after we have a coffee amount of water in body fluids, endocrine secretes vasopressin, then more reabsorption of water occurs from nephrons into the blood. During this way, the body retains water and fewer amount of urine is produced. On the opposite hand, when body fluids have quite normal water, there's a decline in the secretion of this hormone. If the ductless gland doesn't secrete this hormone within the required amount, less water is reabsorbed from nephrons and there's excessive loss of water through urine. This condition is understood as diabetes. The hormone oxytocin stimulates the decline of uterus walls in mothers for childbirth. Moreover, this hormone is critical for the ejection of milk from the breast.

2. Thyroid gland:

   This is often the most important secreted in the material body. It's present in the neck region, below the larynx, and produces the hormone thyroxin. Iodine is required for the assembly of this hormone. If someone lacks iodine in the diet, the ductless gland cannot make its hormone. During this condition, the ductless gland enlarges. This disorder is named goiter.


     Thyroxin increases the breakdown of food (oxidation) and the release of energy in the body. It's also chargeable to expand the body. The under-production of thyroxin causes hypothyroidism. It's characterized by low energy production in the body and slowing down of heart-beat. Over-production of thyroxin causes hyperthyroidism. Its symptoms are an increase in energy production, increased heartbeat, frequent sweating, and trembling of hands. The endocrine produces another hormone called calcitonin. It decreases the number of calcium ions in the blood and promotes the absorption of calcium from the blood into bones.


3. Parathyroid glands

    These are four glands situated on the posterior side of the endocrine. They produce a hormone called parathormone. It increases the number of calcium ions in the blood. When there's increased production of parathormone, quite normal calcium salts are absorbed from the bones and added to the blood. Consequently, the bones become brittle. If there's a deficiency in the production of parathormone, blood calcium level falls. It ends up in tetany, which affects the functioning of muscles.


4. Adrenal glands:


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

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


5. Pancreas:


     This organ has two functions. The main part of the pancreas may be a ducted (exocrine) gland. This portion secretes digestive enzymes, through a duct, into the tiny intestine. Some portions of the pancreas function ductless (endocrine) gland.

    This portion contains groups of endocrine cells stated as islets of Langerhans. These islets secrete two hormones, i.e. insulin and glucagon. Glucagon influences the liver to release glucose in the blood and then the blood sugar concentration rises. Insulin influences the liver to require excess glucose from the blood, then the blood sugar concentration falls.

     If a person’s pancreas doesn't make a normal quantity of insulin, the glucose concentration rises, and that we say that the person has diabetes. Persons with diabetes have a loss of weight, weakening of muscles, and tiredness. Insulin administration controls the disease. Formerly, insulin takes out from animals was used for this purpose. But now human insulin produced from bacteria through gene-splicing is on the market.


 6. Gonads:

     Testes (Singular: testis) and ovaries are the male and feminine reproductive organs, i.e. gonads. additionally, to producing gametes, gonads also secrete hormones called sex hormones. Testes secrete hormones e.g. testosterone, which is accountable for the event of male secondary sex characters like growth of hair on face and coarseness of voice, etc.

    Ovaries secrete estrogen and progesterone, which are accountable for the event of female secondary characters like the event of breast, etc. 


Feedback Mechanisms

     Endocrine glands don't secrete their hormones at a relentless rate. The speed varies with the wants of the body. Like many other functions in the body, the secretion of hormones is additionally regulated by feedback mechanisms. Feedback mechanism means the regulation of a process by the output of the identical process. Feedback mechanisms are of two types, i.e. positive and negative feedback.

      In feedback, the output of a process decreases or inhibits the method. This mechanism works to bring a condition towards its normal value. For example, when the glucose concentration goes up, the pancreas secretes insulin. It decreases the glucose concentration. A decline within the glucose concentration to a standard set-point inhibits the secretion of insulin. Similarly, when glucose levels drop below normal, the pancreas secretes glucagon. It raises the blood sugar concentration. During this case, a rise within the blood sugar concentration to a traditional setpoint inhibits the secretion of glucagon. In other words, the glucose concentration (output) controls the method, i.e. the secretion of insulin and glucagon.

     In feedback, the changes resulting from a process increase the speed of the process. For example, the suckling action of an infant stimulates the assembly of a hormone in the mother. This hormone works for the assembly of milk. More suckling ends up in more hormones, which successively results in more milk production.