Endocrine System of Class 11

Hormones (derived from Greek word meaning “I excite or arouse”) are biochemical messengers secreted into blood or extracellular fluid by one cell and effect the functioning of other cells. In other words these are biochemical messengers that regulate the biological processes (Hormones not only stimulate biological processes but can also inhibit biological processes). Meaning of term hormone ie . “I excite or arouse” signifies the role of hormone discovered first of all.

The first hormone discovered was secretin by two English physiologists William M.Bayliss and Ernest H. Starling in 1903 (Secretin comes from duodenal mucous membrane and stimulates secretion of pancreatic juice) They gave modern definition of hormones.

The term hormone was coined by Starling in 1905. Huxley called hormones as chemical messengers. Claude Bernard termed hormones as internal secretions. Selye defined hormones as functional, chemically complex organic compounds. Scharrer stated that hormones generally control or regulate the reproductive activities, growth, maturation, metabolism, homeostasis & regeneration. Hormones are also called internal secretions, biochemical messengers, chemical coordinators. Metabolic traffic man, autocoids, informational molecules.

Both plants and animals produce hormones

Hormones, enzymes and vitamins regulate metabolism. Hormones have many similarities with enzymes. These are

Not obtained from food, but synthesized in body.

Easily soluble and diffusible.

Required and secreted in minute quantities.

Not stored in body (except thyroxine & posterior pituitary hormones i.e. oxytocin & vasopressin) and have high specificity.

Differences between hormones and enzymes, hormones and vitamins.




Hormones may be amino acid derivatives, peptides, proteins or steroidal in nature.


All enzymes are complex proteins


They have low molecular weight and often readily diffuse through cell membrane


They have very high molecular weight and are not diffusible


They are secreted by cells at one site and pass via blood to another site to act


They are secreted by cells and may act in themselves or pass via ducts to act in some cavity in the body


They are often used up in their regulatory action


They remain unaffected in the reaction they catalyze


Hormone-controlled actions are not reversible


Enzyme-controlled reactions are reversible


Hormones may accelerate or inhibit specific physiological processes


Enzyme speed up the biochemical reactions


They are effective in low concentrations. Their excess or deficiency may cause physiological disorders


They also act in low concentrations, but the rate of enzyme-catalyzed reactions steadily increases with an increase in their concentration


They may act slowly or quickly


They act quickly




All hormones are synthesized in the animal body itself


Most vitamins are taken by animals in their food.


Both excess and deficiency of hormones may disrupt life functions, causing hormonal disorders


Excess water-soluble vitamins are excreted. Vitamin deficiency causes malfunctioning called deficiency diseases or avitaminosis


Hormones act by activating existing enzymes, or by changing permeability of cell membrane, or by activating genes to produce fresh enzymes


Vitamins generally act as coenzymes


Hormones are amino acid derivatives, or peptides, or proteins, or steroids


Vitamins are simple organic compounds such as amines, organic acids, esters, etc.


RIA : Radioimmunoassay is a technique to measure hormones, their precursor, their metabolic end products quantitatively in a living body .

Most hormones circulate in blood, coming into contact with essentially all cells. However a given hormone usually effects only a limited number of cells which are called Target cells, or Effector cells, where it regulates a definite physiological effect. A particular cell is the target cell for a hormone if it contains functional receptors for that hormone and cells which do not have such receptors cannot be influenced directly by that hormone. Hormone receptors are found either exposed on the surface of cell or within the cell (cytoplasm /nucleoplasm) depending on the type of hormone (steroid / proteinaceous).

Local hormones : Local hormones are those hormones which do not pass into blood but diffuse to the target as the same is nearby. They are also called paracrines or parahormones. Eg Acetylcholine, Bradykinin, Prostaglandins, Histamine

Prostaglandins (PGs)

These were discovered from human semen in 1930. Name prostaglandin was introduced by Von Euler. Seminal vesicles are chief source of prostaglandins in semen

Immediate precursors of PGs in the body are essential unsaturated fatty acids i.e. linoleic acid and arachidonic acid Commonest prostaglandins are PGA1, PGA2, PGE1, PGE2, PGF PGs perform variety of functions like, mild uterine contractions to suck the sperm upwards when semen is deposited in vagina, blood flow regulation, relaxation of arterial smooth muscles.

Bradykinin : These are polypeptides that cause contraction of smooth muscles and dilation of blood vessels.

Nitric oxide : The gaseous hormone nitric oxide (also an air polluntant is a local hormone) Molecule of the year 1994, it has been used in rescuing end stage patients in respiratory distress. It is a signalling molecule of cardiovascular system (Furchgott and Ignarro, 1986 ) that regulates blood pressure and blood flow besides relaxing smooth muscle cells. Furchgott , Ignarro and Murad were awarded Nobel Prize for their work on nitric oxide in 1998. It is currently used in male potency drug viagra (sildenafil citrate)

Pheromones (semiochemicals) :

The term pheromone literally means “to carry” and “ to excite”Term pheromone was proposed by Karlson and Butenandt (1959)

Pheromones are biochemical substance which are secreted outside the body by an animal and when detected by member of the same species, these bring about physiological or behavioral change in the recipient. Pheromones are species specific.

Earlier these were also called Ectohormones but this is wrong term because hormones are secreted by endocrine glands and act inside the body whereas pheromones are produced by exocrine glands (of skin) and act outside the body .

Pheromones can be perceived through sense of smell i.e. olfaction, through skin and even through taste buds.

These perform variety of functions like sex attraction, sex repulsion, territory marking, warning , alarming etc.

These exist in all the animals (invertebrates as well as vertebrates) including humans.

Distance and darkness in not a barrier for pheromones. But one disadvantage is that once released into the environment their effect cannot be nullified.

Among all the pheromones, insect pheromones have been the most intensely studied e.g. female silk moth Bombyx mori releases Bombykol

pheromone to attract male and Bombykal pheromone to repel male. Queen honey bee releases ketodecnoic acid to attract male and to supress

sexual development of worker bees. Honeybees also release an aggregation pheromone called Geradiol. Civet cat produces Civetone,

muskdeer produces Muskone. Female rhesus monkey releases Copulin pheromone from her vagina to attract male rhesus monkey. Underground fungi truffles produces chemicals which are similar to pheromones released by boars (male pigs) which is why sows (female pigs) can be trained to hunt for truffle.

Humans also produce pheromones. Those girls who study in coed. schools (i.e. in the company of boys) attain sexual maturity at an early age. A pheromone called androsterone produced by male accelerates puberty in females. This pheromone has been isolated from sweat, saliva and urine of male. Another example of human pheromones is Dormitory effect. It is synchronization of menstrual cycles of a group of females living together in a hostel. Menstrual cycle of two females living together in a room can also get sychronized after a period of few months. In routine we may not observe this effect because we take bath daily and use different kinds of aromatic substance like soaps, deodorants, oils, shampoos etc. These substances interfere with action of pheromones.

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