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All multicellular organisms produce hormones. Plant hormones are known as phytohormones. Blood is the medium of transport of animal hormones. The term hormones is derived from Greek word homao which means to excite. It was first used by William M. Bayliss and Ernest H. Starling, both of the London University College, in the year 1904. They showed a chemical substance - secretin, secreted in the intestine can stimulate the action of a pancreatic secretion. These substances were known as 'chemical messengers'. 

The tissues that produce the hormones are known as the effectors while those tissues are that are influenced by the effectors are called as targets. Hormones are of low molecular weight and they diffuse readily. The effects brought about by the hormones are not permanent as the get readily oxidized. 


Hormones Definition

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Hormones definition - Hormones are organic chemical substances released by a cell, or organ or a gland or any body part of plants and animals that functions in the regulation of physiological activities and to maintain homeostasis. The chemical discharged from one part affects the cells in other part of the organism. Hormones are released in very minute quantities. Minute quantity of hormones carry out functions evoking responses from the target organs or tissues. The target organs or tissues are adapted to the minute quantities of the hormones. Hormones acts as chemical messengers that transport signal from one cell to the other. 

Hormones are transm
itted to their target organs in the blood stream after they are discharged from the glands secreting them. Cells express a specific receptor molecule to the hormone molecules to which they respond. Endocrine secretions is the mode of discharge directly into the bloodstream. 

Characteristics of Hormones

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General characteristics of hormones are as follows:
  • Hormones are secreted by endocrine cells. 
  • Hormones are chemical messengers.
  • The are chemical signals that circulate in the body fluids. 
  • The hormones regulate the behavior of the target cells. 
  • Hormones, unlike enzymes do not catalyze any reaction. 
  • They are secreted only when needed, they are not stored prior to requirement. 
  • Hormones may be proteinaceous or non-proteinaceous in nature (amino-acids or steroids). 
  • The secretion of hormones is regulated by the nervous system through the feed back effect. 
  • Hormones usually cause long term effects like change in behavior, growth, etc.
  • The hormones function is to stimulate or inhibit the target organs. 

Classification of Hormones

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Traditional Classification

Hormones are classified traditionally into three types:

Classical hormones - These hormones are secreted from the endocrine cells into the interstitial fluid. These hormones diffuse into the bloodstream and are distributed to all body parts by the circulatory system. 

Neurohormones - These hormones are synthesized by the neuroendocrine cells and are secreted at the nerve terminals. They are transported around the body through the blood vessels, into which they were diffused. 

Local hormones - These hormones are secreted into the interstitial fluid and they act locally in two ways. Some hormones act on the neighboring cells and are known as paracrine hormones and some hormones act on the cells from which they were secreted, they are autocrine hormones. 

Structural Classification

Hormones are structurally classified into four groups steroids, peptides, amino acids and fatty acids.

Steroid hormones are derived from cholesterol and are soluble in lipids. The steroid hormones include the sex hormones and the hormones produced by the adrenal gland. The sex hormone include androgens, estrogens and progesterone. The adrenal hormones are mineralcorticosteroids and glucocorticosteroids.Steroids hormones are important as they take part in important functions including water balance, sexual development and stress response. 

Amino acid derivative hormones - These hormones are derived from amino acids like tyrosine and tyroptophan. Two types of tyrosine derived hormones, they are thyroid hormone and catecholamines. 

Thyroid hormone is the most important as it regulates the develpment of organs and metabolism. 
Catecholamines - Norepoinephrine and epinephrine are catecholamines. They are stress hormones and ar neurotransmiiters., 
Tryptophan amino acid is the precursor of hormones like serotonin and melatonin. Serotonin regulates the movement of the intestines and is also associated with mood and low levels of this hormones often result in depression. 

Peptide hormones - These hormones are derived from peptides. Prohormones are the precursors of for peptide hormones. The prohormones are synthesized by the endoplasmic reticulum. Proper structural configuration is necessary for their functioning. The peptide hormones are stored in the cell vesicle until there is stimuli signals for their release into the blood stream. Examples of peptide hormones are TSH (thyroid stimulating hormone),insulin, prolactin, vassopressin. 

Fatty acids derived hormones - Hormones derived from the fatty acids are called eicosanoids, they are derived from arachidonic acid. These hormones are produced by every cell in the body. They have important roles in the body including inflammation, blood pressure and blood clotting.  

On the Basis of Mode of Action

Based on the mode of action hormones are classified into quick acting hormones and short acting hormones. 

Quick acting hormones - These hormones initiate immediate response from their target cells. These hormones have outer plasma membrane receptors on the target cell, they are large sized. Example: Protein and amine hormones.

Short acting hormones - These hormones initiate a delayed response. These hormones are small in size and they bind to the protein receptors present in the cytosol. Example: steroid hormones of reproductive organs and sdrenal cortex. 

Functions of Hormones

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Effects of hormones in mammals: 
  • They stimulate or inhibit growth.
  • Hormones control the wake-up cycle and the circadian rhythms.
  • Are responsible for mood swings.
  • Induces or suppresses apoptosis. 
  • Activates or inhibits the immune system.
  • Regulates metabolism.
  • Prepares body for mating, fleeing, flighting and other activity.
  • Also prepares body for new mode of life like puberty, parenting and menopause. 
  • It controls the activity of the reproductive cycle. 
  • Controls hunger. 

Functions of steroid hormones 
The male sex hormone - testosterone is produced by the testis and is responsible for male characteristics like deep voice, facial hair during puberty.

Estradiol is the female sex hormone and is responsible for the development of the secondary sexual characteristics in females. It is also participates in control of menstrual cycle. 

Progesterone is responsible for preparing the uterus for implantation of the fertilized egg. It also plays a important role as birth control agents. 

Hormones of Adrenal Gland
Mineral corticoids are made from different cells of the adrenal cortex. This hormone is concerned with the water-salt balance in the body. It also regulates the NaCl content of blood and causes excretion of potassium in urine. 

Glucocorticoids are made by the adrenal cortex, these hormones modify certain metabolic reactions and has and anti-inflammatory effect. 

Function of Peptide hormones
Peptide hormones like insulin take part in carbohydrate metabolism. It increases penetrations of cell membranes to facilitate entry of glucose. Hence decreasing the concentration of glucose int eh blood. Insulin is often referred as hypoglycemic factor. 
Deficiency of insulin causes diabetes mellitus. 

Functions of Amino Acid hormones
Thyroid hormones thyroxin and tri-iodothyroxine, they affect the general metabolism of the body. The thyroid hormones are known as the pace setter of the endocrine system.

List of Hormones

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List of vertebrate hormones and the secretory organs is as follows:

 S. No.  Secretory Organ  Hormone 
 1.  Ovary 


 2.  Adrenal Cortex


 3. Corpus Leuteum  Progesterone

 4.  Islets of Langerhans


α- cells


 5.Pars distalis

Pars inermedia

Pars nervosa

 6. Gastrointestinal Tract  Secretin
 7. Parathyroid  Parathormone
 8. Thyroid   T3,

Hormone Regulation

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Controlling how much hormones are secreted and released from the cells is hormones regulation. This regulatory activity is done in two mechanisms - the negative feedback mechanism and positive feedback mechanism and also by counter regulatory hormones. 

Negative Feedback Mechanism - Mostly hormones regulation is done by negative feedback mechanism. In this mechanism hormone causes an effect, the cells that make hormones detect this effect and the production of hormones is ceased. 

Example of negative feedback mechanism is with the hormone insulin. Insulin hormone is produced by ß-cells of pancreas. The release of insulin by the pancreas is the response to the consumption of glucose. Rise in the glucose levels in blood, is detected by the pancreas and secretes insulin into the blood. Insulin increase the uptake of glucose in the target cells. Some of the glucose is used by the cells and the other is converted and stored in the form of glycogen. The uptake of glucose by the cells decreases glucose levels, this decrease is detected by the pancreas and as a response to the decrease in glucose levels,it stops secreting insulin into the bloodstream. Decrease in insulin levels in the blood decreases glucose uptake by the cells. This negative feedback mechanism helps to maintain normal blood  glucose levels and prevents extremities. 

Positive Feedback Mechanism - A few hormones are regulated through positive feedback mechanism. In the positive feedback mechanism the effect of hormones, make the gland secrete more hormones. This is the opposite of negative feedback mechanism. 

Example of positive feedback mechanism is the hormone that causes childbirth. The hormones is oxytocin which is made by the pituitary gland. The onset of labor stretches the muscles in the cervix, the nerves here sends signals to the pituitary. This signal makes the pituitary release more oxytocin. The oxytocin hormone causes the muscles of the uterus to contract which causes more stretching in the cervix. This stretching causes even more secretion of oxytocin. The levels of oxytocin keeps rising until the contractions leads to childbirth. 

Counter regulatory hormones - Activities in the body is sometimes controlled by two or more hormones.

Example of counter regulatory hormones - Glucose levels in blood is very important to an organism. This is just not controlled by one hormone, other hormones also make  glucose levels to increase or decrease. If the levels of glucose is too low, the body release hormones that function opposite to the activity of insulin. These hormones do not let the cells uptake of glucose from the blood. They make the cells put back glucose into the blood. These hormones that work opposite to other hormones are called counter regulatory hormones. Counter regulatory hormones for insulin are glucagon and epinephrine. 
More topics in Hormones
Plant Hormones Animal Hormones
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