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Endocrine system

Endocrine system: The endocrine system is responsible for secreting hormones (chemical messengers) and regulating the body activities into the blood. These hormones carried throughout the whole body.

Neuro-endocrine system: The nervous system and the endocrine system are sends and receive the sensory information and coordinate the body responses, so these two systems together make neuro-endocrine system.

Exocrine and endocrine glands:

Exocrine glands:

The exocrine glands secrete their products into the body ducts, which carried by the body cavities, the lumen part of that organ, or the outer surface of the body. For example: sebaceous gland, sudoriferous gland, mucous gland and digestive glands.

Endocrine glands:

The endocrine glands secrete hormones into the extracellular space around the secretory cells. These secretions diffused into the blood capillaries and are carried throughout the body by circulatory system.

The endocrine glands include:

  1. The pituitary gland
  2. The thyroid gland
  3. The parathyroid gland
  4. The adrenal gland
  5. The pineal gland
  6. The pancreatic gland

There are the organs that secreted hormone by their cells but these are not endocrine organs these organs are: pancreas, ovaries, testes, kidneys, stomach, hypothalamus, thymus, liver, small intestine, skin, heart & placenta.

Endocrine System


Hormone: The hormones are the chemical substance that behave like molecules in the body. After the secretion the hormone travel to other part of the body where they need to act.

The hormones may have very powerful effects, even when very low in concentration. There are approx. 50 different types or hormones are secreted in human body.

Target cells: These are the specific cells which are affected by hormones.

Receptors: The receptors are the proteins or glycoproteins in the cell membrane, which binds to the ligands and create response in the immune system.

Functions of hormone:

  1. The hormones are responsible for regulating the chemical composition and volume of the internal environment.
  2. The hormones help to regulate metabolism and energy balance.
  3. The hormone helps to regulate the contraction of smooth and cardiac muscle fibers and secretions by various glands.
  4. The hormones are also responsible to maintain the homeostasis (process to prevent and stop bleeding).
  5. The hormone regulates the certain activities in the immune system.
  6. Play a role in basic processes of reproduction, including gamete formation, fertilization, nourishment to the embryo and fetus, delivery, nourishment to the newly born.
  7. Contribute to the sequential integration of growth and development.

The pituitary gland & the hypothalamus:

The pituitary gland is present in the hypothalamus of the brain. The pituitary gland is also called as the master gland. The pituitary gland and the hypothalamus together regulate all aspects of growth, development, hemostasis & metabolism. The pituitary gland is divided into two parts called as anterior pituitary and posterior pituitary. The size of the pituitary gland is about the size of a pea, it weighs around 500mg

Anterior pituitary: The anterior pituitary (adenohypophysis) is the upward growth of the glandular epithelium from the pharynx.

Posterior pituitary: The posterior pituitary (neurohypophysis) is a downward growth of the nervous tissue from the brain.

There is present the wide network of the nerve fibers between hypothalamus and the posterior pituitary gland.

Diagram of the anatomy of the pituitary gland | OHSU

Blood supply: The blood supply to the pituitary gland is from the branches of internal carotid artery.

The hypothalamus controls the release of hormones from both anterior pituitary gland and posterior pituitary gland but is different ways.

The anterior pituitary: The anterior pituitary secretes various hormones that controls the wide range of the body activities. By producing hormones, the hypothalamus regulates the anterior pituitary gland which stimulates and inhibit the anterior pituitary gland hormones.

The anterior pituitary gland has five type of cells that secrete following hormones:

Anterior pituitary hormones and their functions:

  • Growth hormone GH: It stimulates the general body growth
  • Thyroid stimulating hormone TSH: It control the secretions and other activities of the thyroid gland
  • Adrenocorticotrophic hormone ACTH: It stimulates the adrenal cortex to secrete glucocorticoids
  • melanocyte-stimulating hormone (MSH): It affects skin pigmentation.
  • Prolactin PRL: Initiate the milk production in mammary glands.
  • Luteinizing hormone LH and Follicle-stimulating hormone FSH: These two hormones together stimulates the secretion of the estrogen and progesterone, maturation of oocytes in the ovaries and also the secretion of the sperm production is testis.

Growth hormone (GH):

  • This hormone is synthesized by anterior pituitary gland secreted in a huge amount.
  • It stimulates the growth and division of the cells especially in bones and skeletal muscles.
  • The secretion of GH maintains the mass of bone and skeletal muscles. GH is secreted during the childhood and puberty and responsible for body growth.
  • GH regulates the metabolism in some organs example: liver, intestine and pancreas.
  • The process of releasing growth hormone release hormone (GHRH) and release suppressing it by growth hormone release inhibiting hormone (GHRIH) is known as somatostatin. Both the hormones secreted by hypothalamus.
  • The secretion of hormone is more in the night during sleep and it is also stimulated by hypoglycemia (low blood sugar), anxiety and exercise.
  • Release of GH is peak at puberty and low with age.

Thyroid stimulating hormone (TSH):

  • The release of TSH is stimulated by thyrotropin releasing hormone (TRH) from the hypothalamus.
  • It stimulates the activity and growth of thyroid gland by secreting the hormone thyroxine (T4) & tri-iodothyronine (T3).
  • It secreted highest during the night and lowest in evening early.

Adrenocorticotrophic hormone (ACTC) & corticotrophin

  • The corticotrophin releasing hormone (CRH) by hypothalamus promotes the synthesis and secretion adrenocorticotrophic hormone (ACTC) by anterior pituitary gland.
  • This hormone is responsible for increasing the concentration of cholesterol and steroids in adrenal cortex.
  • The highest level of ACTC are secreted at night and lower in midnight.


  • Prolactin is secreted during the pregnancy; it prepares mammary gland for the lactation after the child birth.
  • The prolactin releasing hormone (PRH) from hypothalamus stimulate the blood level in prolactin.
  • It is lowered by the prolactin inhibiting hormone (PIH).


  • There is secretion of two gonadotrophins (sex hormone) just before the puberty. These hormones gradually increase by anterior pituitary gland with response to luteinizing hormone releasing hormone (LHRH) or gonadotrophin releasing hormone (GnRH).
  • The rising level of these hormones promotes the mature functioning to reproductive organs at the puberty. The hormones are responsible in both male and female are:
  • Follicle stimulating hormone (FSH)

In female it regulates the development of the sex organs and also development of the immature follicles from the ovaries. The secretion of estrogen and progesterone takes place during mensural cycle.

In male: it is responsible for the initiation of spermatogenesis.

  • Luteinizing hormone (LH)

In female: it promotes ovulation and also maintain the corpus luteum and secrete progesterone.

In males: responsible for the secretion of the testosterone.

Posterior pituitary:

The posterior pituitary gland work as a unit with the hypothalamus. It doesn’t synthesis the its own hormone, it stores the release the hormone produced by hypothalamus that is oxytocin & ADH.

The posterior hormones and their functions:

Oxytocin (OT): The function of oxytocin is to control the uterine contraction at the time of delivery and also the ejection of milk during breast feeding. It also keeps the bond between new born and the mother.

Anti-diuretic hormone (ADH): The function of ADH is to control the water balance mechanism in the body and causes the retention of body water.

The thyroid gland:

The thyroid gland is located in the neck in front of trachea and larynx, at the level of 5th, 6th and 7th cervical and the 1st thoracic vertebrae. The weight is around the 25g, high vascularity. It is surrounded by fibrous capsule. It looks like similar to the butterfly shape. There are two lobes are present one on the either side of thyroid cartilage and other on the upper cartilaginous rings of trachea. These lobes are joined by a narrow isthmus lying in front of the trachea. The lobes are cone shaped the length is about 5cm and the width about 3cm.

Blood supply: The arterial blood supply to the thyroid gland is by the superior (branch of external carotid artery) and inferior thyroid (branch of subclavian artery) arteries. The venous drainage is by the thyroid veins of internal jugular vein.

Thyroid Gland

Hormones related to the thyroid gland:

Thyroxine and tri-iodothyronine: The iodine is most important for the synthesis of the thyroid hormones, thyroxine (T4) and tri-iodothyronine (T3).

The release of the hormone T3 & T4 is stimulated by thyroid stimulating hormone (TSH) from the anterior pituitary. Secretion of TSH is stimulated by thyrotropin releasing hormone (TRH) by the hypothalamus and secretion of TRH is stimulated by malnutrition, stress, exercise, low plasma glucose level & sleep.

Iodine trapping: the process of taking iodine from the blood by the thyroid gland is called as iodine trapping.

Calcitonin: This hormone is secreted by the C-cells in the thyroid gland. This is responsible of lower raise blood calcium levels by acting on bone cells promoting the calcium and kidney tubule inhibit the reabsorption of calcium. Release of calcitonin hormone is stimulated by the increase calcium levels in blood.

The parathyroid glands:

There are present four small parathyroid glands, having weight around 50g each, two glands are embedded in posterior surface of each lobe of the thyroid gland. These glands are surrounded by fine connective tissue capsule which contains spherical cells that are arranged in columns with sinusoids containing the blood between them.

Hyperparathyroidism | Maitland Chiropractor - The Bolick Clinic of ...

Function of parathyroid gland:

  1. Secretion of parathyroid hormone (PTH)
  2. The major function performed by PTH is to increase the blood calcium levels. This is done by increasing the calcium absorption from small intestine and reabsorption from renal tubules.

The adrenal glands:

These are present in pair located at the renal fascia of kidney. They are about 4cm in length and 3cm in thickness. These are composed of outer adrenal cortex and inner adrenal medulla. There are total 40 hormones produced by adrenal cortex known as corticosteroids.

The complete loss of adrenocortical hormone leads to death within a week due to dehydration and electrolyte imbalance.

Adrenal gland

The adrenal glands are composed of two parts and perform different structure and function. The outer part of adrenal gland is cortex and the inner part is medulla. Adrenal cortex is essential to life.

Hormones of adrenal gland:

Adrenal cortex:

  • Cortisol
  • Corticosterone
  • Cortisone
  • Aldosterone
  • Androgens

Adrenal medulla:

  • Epinephrine
  • Non-epinephrine

Adrenal cortex: It produces three groups of steroid hormones from cholesterol collectively they are known as adrenocorticoids (corticosteroids). The groups are glucocorticoids, mineralocorticoids & sex hormones (androgens). Structurally they are the same but functionally different.

Glucocorticoids: the main glucocorticoid is cortisol (hydrocortisone), there is corticosterone and cortisone are also produced in small amount. There are collectively known as steroids, which are essential for life, they regulate metabolism and responses to stress. This hormone is stimulated by ACTH from anterior pituitary, and having anti-inflammatory actions.

Mineralocorticoids (aldosterone): The main mineralocorticoid is the aldosterone. It functions as the maintaining water and electrolyte balance. It also stimulates the reabsorption of sodium by renal tubule and excretion potassium by in the urine. The aldosterone is also involved in the regulation of blood volume and blood pressure because sodium is reabsorbed by retention of water also. The aldosterone regulates the blood potassium levels. The rising in blood potassium level leads to release of more aldosterone and low blood potassium decrease the releasing of aldosterone.

Renin-angiotensin-aldosterone system: When the renal blood flow is reduced or blood sodium levels reduced, the enzyme renin is secreted by kidney. The function of renin is to convert plasma protein angiotensinogen to angiotensin.

Sex hormones: The sex hormones are secreted by the adrenal cortex, mainly include androgens (male sex hormones).

Adrenal medulla: The adrenal medulla is surrounded by adrenal cortex completely. It is a part of sympathetic nervous system and develops from the nervous tissue in the embryo. When this hormone is stimulated by extensive sympathetic nerve supply the glands release the hormones called adrenaline (epinephrine 80%) and noradrenaline (norepinephrine, 20%).

The pancreas:

The pancreas is both endocrine and exocrine organ. It is the part of the digestive system. There are the three main types of cells present in the pancreas named as

  • (alpha) cells that secrete glucagon
  • (beta) cells that these are the most numerous and secrete hormone insulin
  • (delta) cells that secrete somatostatin

The blood glucose levels are controlled by the reverse actions of insulin and the glucagon.


Insulin: The main function of insulin is to lower the raised blood nutrients levels, not only glucose but also amino acids and fatty acids.

Mechanism of insulin:

  • It acts on the cell membrane and stimulate uptake and use of the glucose by connective tissue cells
  • Increase the conversion of glucose to glycogen (glycogenesis), specially in the liver and muscles of skeletal system
  • By protein synthesis it accelerates the uptake of amino acids cells
  • Promote the synthesis of fatty acids and storage of fat in adipose tissue (lipogenesis)
  • Decreases the glycogenesis (breaking of glycogen into glucose)

Glucagon: This increases blood glucose levels by the conversion of glycogen to glucose and secretion of glucagon is stimulated by low glucose level and physical activities and lower by somatostatin and insulin.

Somatostatin (GHRIH): It is also produced by the hypothalamus; it inhibits the secretion of both insulin and glucagon.

The pineal gland:

The pineal gland is small structure attached to third ventricle. The length is about 10mm long reddish brown in color. It is surrounded by the capsule. This gland turns to atrophy after adolescence and may calcified later.

What is Pineal Gland? •

Function of the pineal gland:

  • It secretes melatonin, which promotes the sleepiness and regulates the biological clock
  • Physiology of pineal gland is undefined

The thymus gland: The thymus gland is a small organ behind the sternum. The thymus plays important role in the immune system and the endocrine system also. It begins atrophy during the puberty.

Hormones produced by thymus gland: the hormone thymosin, this hormone promote the maturation and the proliferation of T cells. The thymic hormones also help in retarding the aging process.

Thyroid Gland


Other hormones:

  • Histamine: This is produced and stored by mast cells in the tissues and basophils in blood. This is released as a part of the inflammatory responses especially when allergic.
  • Serotonin (5-hydroxytryptamine, 5-HT): This is present in the platelet cells, brain and the intestinal wall. It also causes the intestinal secretions and contraction of smooth muscle. This is also work as a neurotransmitter in the CNS and also influence the mood.
  • Prostaglandins (PGs): This is the lipid substance that found in most of the tissues. They have physiological effects in the inflammatory response, fever, uterine contractions during labor, blood clotting etc.