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Endocrine System


  • The structural of the nervous system is a neuron. It has a nucleus and cytoplasm, forming the cell body. The cell body has elongated extension of cytoplasm. Those extension which are specialized for transmitting messages to two neuron are called ‘dendrites’ and the extension that transmits message from the neuron is called an ‘axon’. A reflex action, or simply a reflex is a fundamental function of the nervous system.
  • The chain of neurons that participants in a reflex action is called a ‘reflex arc’. It consist of at least two neuron —(1) Sensory or afferent neuron which conveys the information about the stimulus to the central nervous system. (2) Motor or afferent neuron – which conveys the message from the central neuron system to the organ (effector organ) which has to respond to the stimulus.
  • The function between two neurons is called a ‘ganglion’.


The endocrine system is a collection of glands that secrete chemical messages we call hormones. These signals are passed through the blood to arrive at a target organ, which has cells possessing the appropriate receptor. Exocrine glands (not part of the endocrine system) secrete products that are passed outside the body. Sweat glands, salivary glands, and digestive glands are examples of exocrine glands. Hormones are grouped into three classes based on their structure: 

a. steroids
b. peptides
c. amines 


Steroids are lipids derived from cholesterol. Testosterone is the male sex hormone. Estradiol, similar in structure to testosterone, is responsible for many female sex characteristics. Steroid hormones are secreted by the gonads, adrenal cortex, and placenta. 



Peptides are short chains of amino acids; most hormones are peptides. They are secreted by the pituitary, parathyroid, heart, stomach, liver, and kidneys. Amines are derived from the amino acid tyrosine and are secreted from the thyroid and the adrenal medulla. Solubility of the various hormone classes varies. 

Synthesis, Storage, and Secretion 

  • Steroid hormones are derived from cholesterol by a biochemical reaction series. Defects along this series often lead to hormonal imbalances with serious consequences. Once synthesized, steroid hormones pass into the bloodstream; they are not stored by cells, and the rate of synthesis controls them.
  • Peptide hormones are synthesized as precursor molecules and processed by the endoplasmic reticulum and Golgi where they are stored in secretory granules. When needed, the granules are dumped into the bloodstream. Different hormones can often be made from the same precursor molecule by cleaving it with a different enzyme.
  • Amine hormones (notably epinephrine) are stored as granules in the cytoplasm until needed.

Evolution of Endocrine Systems 

Most animals with well-developed nervous and circulatory systems have an endocrine system. Most of the similarities among the endocrine systems of crustaceans, arthropods, and vertebrates are examples of convergent evolution. The vertebrate endocrine system consists of glands (pituitary, thyroid, adrenal), and diffuse cell groups secreted in epithelial tissues. More than fifty different hormones are secreted. Endocrine glands arise during development for all three embryologic tissue layers (endoderm, mesoderm, ectoderm). The type of endocrine product is determined by which tissue layer a gland originated in. Glands of ectodermal and endodermal origin produce peptide and amine hormones; mesodermal-origin glands secrete hormones based on lipids. 

i. Endocrine Systems and Feedback Cycles 

The endocrine system uses cycles and negative feedback to regulate physiological functions. Negative feedback regulates the secretion of almost every hormone. Cycles of secretion maintain physiological and homeostatic control. These cycles can range from hours to months in duration.

NCERT Summary: Gist of Biology - 3 Notes | EduRev

ii. Mechanisms of Hormone Action

The endocrine system acts by releasing hormones that in turn trigger actions in specific target cells. Receptors on target cell membranes bind only to one type of hormone. More than fifty human hormones have been identified; all act by binding to receptor molecules. The binding hormone changes the shape of the receptor causing the response to the hormone. There are two mechanisms of hormone action on all target cells. 

ii. Nonsteroid Hormones 

Nonsteroid hormones (water soluble) do not enter the cell but bind to plasma membrane receptors, generating a chemical signal (second messenger) inside the target cell. Five different second messenger chemicals, including cyclic AMP have been identified. Second messengers activate other intracellular chemicals to produce the target cell response.

iv. Steroid Hormones 

The second mechanism involves steroid hormones, which pass through the plasma membrane and act in a two step process. Steroid hormones bind, once inside the cell, to the nuclear membrane receptors, producing an activated hormone-receptor complex. The activated hormone-receptor complex binds to DNA and activates specific genes, increasing production of proteins. 

The Nervous and Endocrine Systems 

The pituitary gland (often called the master gland) is located in a small bony cavity at the base of the brain. A stalk links the pituitary to the hypothalamus, which controls release of pituitary hormones. The pituitary gland has two lobes: the anterior and posterior lobes. The anterior pituitary is glandular. The hypothalamus contains neurons that control releases from the anterior pituitary. Seven hypothalamic hormones are released into a portal system connecting the hypothalamus and pituitary, and cause targets in the pituitary to release eight hormones. 

I. Anteriol Pituitoly 

Growth hormone (GH) is a peptide anterior pituitary hormone essential for growth. GH releasing hormone stimulates release of GH. GH inhibiting hormone suppresses the release of GH. The hypothalamus maintains homeostatic levels of GH. Cells under the action of GH increase in size (hypertrophy) and number (hyperplasia). GH also causes increase in bone length and thickness by deposition of cartilage at the ends of bones. During adolescence, sex hormones cause replacement of cartilage by bone, halting further bone growth even though GH is still present. Too little or two much GH can cause dwarfism or gigantism, respectively. Hypothalamus receptors monitor blood levels of thyroid hormones. Low blood levels of Thyroid-stimulating hormone (TSH) cause the release of TSH-releasing hormone from the hypothalamus, which in turn causes the release of TSH from the anterior pituitary. TSH travels to the thyroid where it promotes production of thyroid hormones, which in turn regulate metabolic rates and body temperatures. Gonadotropins and prolactin are also secreted by the anterior pituitary. Gonadotropins (which include follicle-stimulating hormone, FSH, and luteinizing hormone, LH) affect the gonads by stimulating gamete formation and production of sex hormones. Prolactin is secreted near the end of pregnancy and prepares the breasts for milk production.

II. The Posterior Pituitary 

The posterior pituitary stores and releases hormones into the blood. Antidiuretic hormone (ADH) and oxytocin are produced in the hypothalamus and transported by axons to the posterior pituitary where they are dumped into the blood. ADH controls water balance in the body and blood pressure. Oxytocin is a small peptide hormone that stimulates uterine contractions during childbirth. 

Biological Cycles 

Biological cycles ranging from minutes to years occur throughout the animal kingdom. Cycles involve hibernation, mating behavior, body temperature and many other physiological processes. 

Rhythms or cycles that show cyclic changes on a daily (or even a few hours) basis are known as circadian rhythms. Many hormones, such as ACTH-cortisol, TSH, and GH show circadian rhythms. 

Thyroid secretion is usually higher in winter than in summer. Childbirth is hormonally controlled, and is highest between 2 and 7 AM. Internal cycles of hormone production are controlled by the hypothalamus, specifically the suprachiasmic nucleus (SCN). According to one model, the SCN is signaled by messages from the light-detecting retina of the eyes. The SCN signals the pineal gland in the brain to signal the hypothalamus, etc. 

Endocrines: The Postal System of Communication and Co-Ordination

  • Hormones are chemical substances manufactured by organs called endocrine glands or ductless glands. Ductless glands are also sometimes called ‘exocrine glands’. 


  • Thyroid is situated in the neck in front of the wind pipe. It manufactures two hormones: triodothyro (T3) and tetraiodothysonine (T4), are called tyrosine. Both these hormones contain iodine.
  • Hypothyroidism (hypo, ‘under’)– diminished thyroid activity. Hypothyroi-dism in childhood gives rise to a conditions called cretinism. 

Goiter – is called enlargement of the thyroid gland. It manifests itself as a swelling in the neck. A goiter may be associated with increased, normal or descreased activity of the thyroid gland. 

  • Government of India launched the Universal salt iodisation programme in 1986. 

Pancreas — the endocrine department of the pancreas is scattered throughout its substance in the form of tiny islands. The islands have been named as ‘islets of Langerhans’. The islets have two major type of cells called A and B. The A cells secrete the hormone ‘glucagon’ while the B cells secrete insulin. Insulin has discovered by Frederick Grand banting and Charles Herbert Best. Reduction on the quantity of effective insulin gives rise to diabetes mellitus (diabetes, siphon, mellitus of honey) commonly called simply diabetes. Food and glucose to be blood, while exercise and insulin remove it.
The six endocrine glands of the body are.
a. Thyroid
b. Pancreas
c. Adrenals
d. Gonads
e. Parathyroids
f. Pituitary 

Processing of Food

  • The process of digestion and absortion of food takes place in the alimentary canal.
  • The alimentary canal is a 9-metre long tube extending from the hips to the anus- 
  • Saliva is secreted by a set of three pairs of glands situated near the month. It helps in the process a digestion. It contain an enzyme called ‘anylax’ which breaks down the starch in food into maltose. 
  • The food in the stomach is homogenised by the action of the acidic juice. The juice also contains an enzyme called pepsin, which splits proteins into smallest units called peptides. 

Small Intestine:– The food, after being digested in the stomach is transferred; but by bit from the stomach into the small intestine. The first portion of the small intestine which the food enters is called the ‘duodenum’. Juices from pancreas and gall bladder are discharged into the duodenum.

  • Pancreatic juice contains enzymes for digestion of carbohydrates, protein as well as fat. 

Bite is an essential supplement to the recreations enzyme for digestion of fats.

  • Discharge of Pancreatic and binary recentions into the intestine is under the control of two hormones: ‘secretin’ and cholecystokinin’.

Large Intestine (colon)– The absorption of water is an important function

NCERT Summary: Gist of Biology - 3 Notes | EduRev

Kidneys, The Fascinating Filters 

  • Kidneys are often described as bean shaped.
  • Each kidney is make up a about a million narrow tube-like structures called ‘nephrons’. The urine formed by a kidney in a sum total of a the urine formed by its neprpons.
  • A Nephron consists of a receptacle (Bowman’s capsule) enclosing a bunch of capillaries (glomerular) like a closed fist. The glomerulus and Bowman’s capsule filter the blood. 

Filtration: The glomerular capillaries are fed blood by a blood vessel called ‘afferent arteriole’ and drained by a marrower blood vessel called the ‘efferent arteriole.

  • By secreting acids, the kidneys help in maintaining the acidity of the body fluids constant.
  • Urine excreted is the result of these basic processes: filtration, reabrorption and secretion.
  • Excessive eating (polyphagia), excessive drinking (polydipsia) and too much of urine (polyusia) are three cardinal symptoms of diabetes. The ‘hypothesis’ produces a chemical substance called ‘antidivretic hormone (ADH). This substance travels in the bloodstream to the kidneys and increases the reabsorption of water so that the blood gets a little thinner.
  • The Adrenal gland maintains the regulating salt in the body and is located in an organ lying just over the kidney. As soon as the salt (sodium) concentration become just a little less than normal, it release into the blood stream a substance called ‘aldosterone’.
  • Renal transplantation or dialysis (artificial kidney) are the supportive measure when the damage to kidney reaches a certain point. 

The Lymphatic System 

  • The lymphatic system is composed of lymph vessels, lymph nodes, and organs. The functions of this system include the absorbtion of excess fluid and its return to the blood stream, absorption of fat (in the villi of the small intestine) and the immune system function.
  • Lymph vessels are closely associated with the circulatory system vessels. Larger lymph vessels are similar to veins. Lymph capillaries are scatted throughout the body. Contraction of skeletal muscle causes movement of the lymph fluid through valves.

NCERT Summary: Gist of Biology - 3 Notes | EduRev

  • Lymph organs include the bone marrow, lymph nodes, spleen, and thymus.
  • Bone marrow contains tissue that produces lymphocytes. B-lymphocytes (B-cells) mature in the bone marrow.
  • T-lymphocytes (T-cells) mature in the thymus gland.
  • Other blood cells such as monocytes and leukocytes are produced in the bone marrow.
  • Lymph nodes are areas of concentrated lymphocytes and macrophages along the lymphatic veins.
  • The spleen is similar to the lymph node except that it is larger and filled with blood.
  • The spleen serves as a reservoir for blood, and filters or purifies the blood and lymph fluid that flows through it.
  • If the spleen is damaged or removed, the individual is more susceptible to infections. 
  • The thymus secretes a hormone, thymosin, that causes pre-T-cells to mature (in the thymus) into T-cells. 


  • Immunity is the body’s capability to repel foreign substances and cells.
  • The nonspecific responses are the first line of defense.
  • Highly specific responses are the second line of defense and are tailored to an individual threat.
  • The immune response includes both specific and nonspecific components. Nonspecific responses block the entry and spread of disease-causing agents.
  •  Antibody-mediated and cell-mediated responses are two types of specific response.
  • The immune system is associated with defense against disease-causing agents, problems in transplants and blood transfusions, and diseases resulting from over-reaction (autoimmune, allergies) and under-reaction (AIDS). 


Barriers to entry are the skin and mucous membranes.
1. The skin is a passive barrier to infectious agents such as bacteria and viruses. The organisms living on the skin surface are unable to penetrate the layers of dead skin at the surface. Tears and saliva secrete enzymes that breakdown bacterial cell walls. Skin glands secrete chemicals that retard the growth of bacteria.
2. Mucus membranes lining the respiratory, digestive, urinary, and reproductive tracts secrete mucus that forms another barrier. Physical barriers are the first line of defense.
3. When microorganisms penetrate skin or epithelium lining respiratory, digestive, or urinary tracts, inflammation results. Damaged cells release chemical signals such as histamine that increase capillary blood flow into the affected area (causing the areas to become heated and reddened). The heat makes the environment unfavorable for microbes, promotes healing, raises mobility of white blood cells, and increases the metabolic rate of nearby cells. Capillaries pass fluid into intelstinal areas, causing the infected/injured area to swell.
4. Clotting factors trigger formation of many small blood clots. Finally, monocytes (a type of white blood cell) clean up dead microbes, cells, and debris.
5. If this is not enough to stop the invaders, the complement system and immune response act.
6. Protective proteins that are produced in the liver include the complement system of proteins. The complement system proteins bind to a bacterium and open pores in its membrane through which fluids and salt move, swelling and bursting the cell. The complement system directly kills microbes, supplements inflammatory response, and works with the immune response. It complements the actions of the immune system. Complement proteins are made in the liver and become active in a sequence (C1 activates C2, etc.). The final five proteins form a membrane attack complex (MAC) that embeds itself into the plasma membrane of the attacker.
7. Salts enter the invader, facilitating water to cross the membrane, swelling and bursting the microbe. Complement also functions in the immune response by tagging the outer surface of invaders for attack by phagocytes.
8. Interferon is a species-specific chemical produced by cells that are viral attack. It alerts nearby cells to prepare for a virus. The cells that have been contacted by interferon resist all viral attacks. 


  • The immune system also generates specific responses to specific invaders.
  • The immune system is more effective than the nonspecific methods, and has a memory component that improves response time when an invader of the same type (or species) is again encountered.
  • Immunity results from the production of antibodies specific to a given antigen (antibody-generators, located on the surface of an invader).
  • Antibodies bind to the antigens on invaders and kill or inactivate them in several ways.
  • Most antibodies are themselves proteins or are a mix of protein and polysaccharides. Antigens can be any molecule that causes antibody production.
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