Grade 12 Zoology Note

Endocrine glands

In multicellular organism various types of glands are found which are grouped as;

1.Exocrine glands:

They are glands of the exocrine system that secrete their essential product by way of a duct to some environment external to itself, either inside the body or on a surface of the body. Exocrine glands are one of two types of glands in the human body, the other being endocrine glands, which secrete their products directly into the bloodstream. Examples of exocrine glands include the sweat glands, salivary glands, mammary glands, and liver.

2.Endocrine glands:

They  are glands of the endocrine system that secrete their products, hormones, directly into the blood rather than through a duct. The major glands of the endocrine system include the pineal gland, pituitary gland, pancreas, ovaries, testes, thyroid gland, parathyroid gland, hypothalamus and adrenal glands. The hypothalamus and pituitary gland are neuroendocrine organs. Local chemical messengers, not generally considered part of the endocrine system, include autocrines, which act on the cells that secrete them, and paracrines, which act on a different cell type nearby.


A hormone a class of signaling molecules produced by glands in multicellular organisms that are transported by the circulatory system to target distant organs to regulate physiology and behaviour. Hormones have diverse chemical structures that include eicosanoids, steroids, amino acid derivatives, peptides, and proteins. The glands that secrete hormones comprise the endocrine signaling system. The term hormone is sometimes extended to include chemicals produced by cells that affect the same cell (autocrine or intracrine signalling) or nearby cells (paracrine signalling).


1.Growth, maturation and regeneration.

2.Help in reproductive activities.

3.Metabolism and homeostasis.


Endocrine glands in human:

In human endocrine glands are found in different parts of the body they are;

1. pituitary gland

2.thyroid gland

3.parathyroid gland


 5.adrenal glands


7.Pineal body

8.Sex glands-Testis and overy

Pituitary gland ( Hypothalamic )


It is also called master gland because its hormones control the functions of other gland.

It is located at the base of the brain and enclosed in a depression of bone of skull.

Structure of the pituitary is small about the size of the large pea. It’s weight is about is reddish gray in color and roughly oval in shape.

Pituitary gland is consists of three parts;

a) Anterior lobe

b) Intermediate lobe

c) Posterior lobe

Anterior lobe and intermediate lobe collectively also called adeonohypophysis and posterior lobe also called neurohypophysis.


Hormones of adenohypophysis and their functions:

1.Follicle stimulating hormone (FSH):

 Stimulate the ovarian follicle to produce eggs in female and spermatogenesis in males.

2.Luteinizing hormone (LH): 

Stimulate ovulation from progesterone and estrogen in female. In male it activate to secret testosterone hormone.

3. Prolactin:

Stimulates to produce milk in pregnant female from mammary gland.

4.Adrenocorticotropin hormone (ATCH): 

Stimulate adrenal cortex produce cortisomes hormone. Hyposecretion-Overgrowth of adrenal cortex.

5.Thyroid stimulating hormone (TSH): 

Hyposecretion, falls on the blood level and atrophy of thyroxin hormone from thyroid gland. Hypersecretion-Production of excessive thyroxin from thyroid gland.

6.Growth stimulating hormone (GSH) or somatotropin: 

Control the growth of body.


Dwarfism(childhood) infertility (in adult)

b.Hypersecretion -

      Gigantism : extra ordinary growth and height(about 2.15m tall)

      Acromegaly: Abnormal growth of hand, feet and face look ugly.


7.Melano stimulating hormone (by intermediate lobe) :

Stimulate pigmentation of skin.


Hormones of neurohypophysis(posterior lobe) and their functions:

1.Oxytocin (pitocin): 

It is also called birth hormone and milk ejection hormone. It affects the contraction uterine wall to give birth of baby and ejection of milk in mammary gland in female.

In male it stimulates the flow of spermatozoa.


2.Vasopressin or antidituretic hormone (ADH):

Balance amount of the water in the body from distal convoluted tubule and collecting duct of nephrone.

Hyposecretion – Production of large volume of urine (diurersis) and diabetes insipidus.

Hypersecretion – Small urine volume (antidiuresis).



Thyroid gland :


It is located on either side of trachea behind the thyroid cartilage. It has two right and left lobes.

TS of Thyroid

Position of thyroid gland

It is the largest endocrine gland. Size is 5x3cm and weight is about 40gm. It as numbers of follicle made up of cubodial cells which secret hormones. Thyroid gland is larger in the female than male.


It produce thyroxin and thyrocalcitonine.



It control the rate of metabolic activities. It is essential for growth.

Hyposecretion –retard growth in child (cretinism)

low metabolism (myxoedema) swelling

Enlargement of thyroid gland (Goiter)


         Hypersecretion-increase of metabolic rate-bulging of eye balls, irregular heartbeat, nervousness, loss of   weight etc (graves   diseases) and exopthalmic goiter.



It regulates metabolism of calcium and phosphorus. It is stimulate by increase in conc. of ca++ in plasma.



Parathyroid glands: 

These are embedded in the dorsal surface of thyroid gland. Two are located in each thyroid lobe.There are about 4mm in long and 50mg of weight the gland is made up of polygonal cells.



It secrets Parathormone which is control by calcium level in blood. Law calcium causes secretion of parathyroid hormones. Parathormone maintain the metabolism of phosphate and calcium.


         Hyposecretion: twitching of muscles and spasm of the body (quick jerky response of muscle) and painful contraction of muscles)(Tetany)

Hypersecretion: Extensive decalcification and may lead to bony deformities and                                  factures.


Adrenal or suprarenal glands:


It is located at the top of the kidney. Their structures are small yellowish and highly vasculated. It’s size is 5x3x1cm and weight is 5gm.


Histological it made up of outer cortex and inner medulla. Cortex has three layers but medulla has no layers.

Functions of Adrenal Cortex:

Adrenal Cortex produces steroid hormones called cortisomes. These are three groups:


           It balances Na+, K+ and water. Hyposecretion: Nervous disorder, convulsion and death (conn’s disease). Hypersecretion: Neuromuscular disorder and hypertension.

            2.Glucocoticoids: Synthesis of glucose from amino acids and fatty acids(gluconeogenesis) and metabolism of carbohydrates.


            Hyposecretion: Low blood pressure, high level of urea and elevation of body temperature.


More deposition of glucose in liver (Hyperglycaemia). Obesity,

weakness of muscles and hypertension.

           3.Sex steroid: Controls the function of sex organs and development of secondary sex characters. Hyposecretion: Reduction in development of sex organs and secondary sex characteristics. Hypersecretion: Male characteristics are seen in female (beards, mustache, voice etc) ovary degenerate and menstruation stop.


Adult physical characteristics develop before teenage.


Functions of medulla:

 It produces adrenaline and nor adrenaline.


It is also called emergency hormone and greatly increases in cold emotions and heat. It affects the muscle of heart to contract fast and rise in blood pressure. It is used to stimulate the heart(cardiovascular effect).


It increases metabolic rate and energy in body. It dilates bronchioles. So it uses to treat asthma and allergy. It affects interior portion of pituitary gland to produce hormones.



It increases diastolic and systolic pressure. It is a powerful exciter. 





It is located in the curvature of duodenum; it is roughly leaf like structure. It is a heterocrine gland. Endrocrine cells are discovered by Langerhans. Therefore these rae known as islets of langerhans. There are three types of cell in the pancreatic islets.

      1.α-cells: Secret glucagons.

      2 .β-cells: Secret insulin

      3. δ-cells: Secret somatostatin.

Glucagons and insulin show antagonistic effects.                                                 

Glucagons increases glucose level in the blood.

Insulin decreases glucose level in the blood.


1.Function of insulin: 

It is synthesized by β-cells as proinsulin. It changes glucose in the glycogen to store in muscle and liver. Insulin promote protein metabolism.   It is essential for growth.


Glucose level increases from 300 to 200mg/100ml(normal is 95mg/100ml)Amount of protein also increases in tissue.


Glucose level decreases from to as low as 20 t0 30mg/100ml and greatly decrease in protein and tissue.

2.Functions of glucagons: 

It is synthesized by α-cells. It consists of 29 amino acids. It is produced during lack of glucose in the blood ( hypopycaemia) and change glycogen into glucose into glucose as required just opposite in hypo and hyper secretion with insulin.

3.Somtostatin functions: 

The hormone is secreted by δ-cells. Effect of this hormone is to inhabit the secretion of both insulin and glucagons.




This gland is located in upper chest behind the breast bone.It secret thymosin which converts WBC to T-cells. T-cells play an important role to develop immunity power.


Pineal gland:

This is also called pineal body. It extends from third ventricle of the brain to from a small cone shaped.The cell of pineal gland (pinealocytes) synthesis the hormone melatonin.

In influences the gonadial functions and other endocrine gland.


Sex glands:

There are testis in male and ovaries in females.

1.Testis: Testis cells (cells of ledydig) secret testosterone and dihydrotestosterone on the influence of luteinzing hormone.

Testosterone helps in the development of male accessory gland and secondary sex characters.

2.Ovaries: Each ovary has two endocrine parts:

a.Graafian follicles: secret oestrogen on the influence of FSH. It helps in the development of secondary six character of female.

b.Corpus luteum: Secret progesterone which responsible for receive fertilized eggs and implantation. It also responsible for relaxation of pelvic ligament towards the end of pregnancy.

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