The glands which release their secretions directly into the blood (due to absence of ducts) are known as endocrine glands. They are also known as ductless glands. Secretions of these glands are called hormones. The term hormone was proposed by Starling. Secretin was the first invented hormone. The term hormone may be defined as "a regulatory substance produced in an endocrine gland and is transported through body fluids (such as blood) to the target organ/ tissue." Chemically, the hormones are of three types, viz, amine hormones (e.g.: epinephrine, melatonin), peptide and protein hormones (e.g.: oxytocin, insulin) and steroid hormones (e.g.: testosterone, aldosterone). 
      The endocrine glands, and hormone producing tissues in different parts of the body constitute the endocrine system. A branch of biology and medicine dealings with endocrine glands, their secretions, their actions, abnormalities of hyper and hypo secretions is known as Endocrinology. The following are the endocrine glands in the human body.

I. Hypothalamus 
               The floor of diencephalon is the thalamus. Below the thalamus, is the hypothalamus. It connect the neural and endocrine systems. It is closely associated with the pituitary. Hypothalamus contains groups of neurosecretory cells (called nuclei). They secrete neurohormones. They are carried to the neurohypophysis through axons of hypothalamo hypophyseal tract. Neurohormones are of two types, namely, releasing hormones (stimulate secretion of pituitary hormones) and inhibiting hormones (inhibit secretions of pituitary hormones).
e.g.: i. Growth hormone releasing hormone (somatocrinin) stimulates the synthesis and release somatotropin by pituitary.
ii. Growth hormone inhibiting hormone (somatostatin) secreted by hypothalamus inhibits the release of growth hormone from pituitary.

II. Pituitary gland (Hypophysis)
               Pituitary gland lies on the ventral side of diencephalon just below the hypothalamus. It is attached to the hypothalamus through a stalk called infundibulum. Pituitary in lodged and protected in a socket called sella tursica in the sphenethmoid. Pituitary has two main parts, namely, adenohypophysis and neurohypophysis.  Between these two is a small zone called pars intermedia. 
               Pituitary has dual origin. The adenohypophysis develops from roof of buccal cavity in the form of Rathke's pouch. Neuro hypophysis develops from the floor of hypothalamus.
Hormones secreted by adenohypophysis
1. Growth hormone (GH)/ Somatotropin: It mainly stimulates the cells of liver to secrete insulin like growth factors. They cause elongation of bones by stimulating cell division in epiphyseal plates. They promote growth by promoting cell division, differentiation and accelerating protein synthesis and decrease catabolism of proteins.

                                
2. Thyroid Stimulating Hormone (TSH)/ Thyrotropin: It stimulates the synthesis and secretion of hormones from thyroid gland.
3. Adreno Cortico Tropic Hormone (ACTH): It controls the synthesis and secretion of steroid hormones like glucocorticoides by cortex of adrenal gland.

4. Follicle Stimulating Hormone (FSH): It promotes the growth of ovarian follicles in female. In males it regulate the spermatogenesis.
5. Luteinising Hormone (LH): In males it is called interstitial cell stimulating hormone (ICSH) that stimulates the leydig cells of testes to secrete androgens (like testosterone). In females stimulates the process of ovulation. It also maintains corpus luteum. LH also stimulate ovaries to secrete oestrogens and progesterone.
6. Prolactin/ Lactogenic hormone/ Luteotropic Hormone: It causes the enlargement of mammary glands. It also helps in maintaining corpus luteum of the ovary to maintain pregnancy.
Pars intermedia: It secrets melanocyte stimulating Hormone (MSH). Its role is insignificant in man.
Hormones secreted by neurohypophysis: It stores and releases two hormones vasopressin and oxytocin (These are actually secreted by hypothalamus).
i. Vasopressin: It stimulates the reabsorption of water by the distal convoluted tubule and collecting duct of renal tubule. Vasopressin is also called anti diuretic hormone (ADH).

ii. Oxytocin induces the contraction of smooth muscles. In female it cause powerful contractions of uterine muscles during child birth and ejection of milk by milk glands.
 Hypersecretion of growth hormone in children leads to gigantism (over growth of skeleton resulting abnormal height) Hypersecretion of growth hormone in adults results in acromegaly (elongation of jaw bones, hands, feet; thickened nose, lips, eyelids etc., - leading to gorilla posture)
Hyposecretion of growth hormone in childhood leads to dwarfism (retarded growth but sexually and intellectually normal)
Hyposecretion of vasopressin leads to diabetes insipidus. It is characterised by poly urea and loss of water through urine.
 Hypothalamus is the master control centre of endocrine system. It directly control the pituitary gland and in term regulate the functioning of other endocrine glands. 

III. Thyroid gland:

   It is the largest endocrine gland in the body and is a bilobed gland located at the junction of larynx and trachea. The two lobes are connected by an isthmus. Thyroid gland is composed of many follicles among which vascular connective tissue is present. It also contains stromal cells/ para follicular cells.

  Follicular cells synthesise two main hormones, namely, triiodothyronine (T3) and tetraiodothyronine (T4 or thyroxine). These hormones contain Iodine. These two hormones perform same functions, such as, regulation of basal metabolic rate, stimulate erythropoiesis, metabolism of carbohydrates, proteins and fats and maintenance of electrolyte balance besides growth.

   Parafollicular cells secrete calcitonin. It maintains optimum levels of calcium and phosphate in the blood.

Thyroid and parathyroids are endodermal in origin.
Hyperthyroidism causes exopthalmia (protruded eye balls), increased metabolic rate etc.,

Deficiency of iodine in the diet results in hypothyroidism and enlargement of thyroid (simple goitre)
Hypothyroidism during pregnancy leads to cretinism of the developing baby. Cretin suffers from stunted growth, mental retardation, low IQ, abnormal skin, deafness, mutism etc.
Hypothyroidism in adults results in myxedema (low metabolic rate, lethargy, puffiness of skin and face, dry skin etc., are its main symptoms).

IV. Parathyroid glands
    There are two pairs of parathyroid glands present on the back part of thyroid. They secrete Parathormone (Parathyroid hormone - PTH). If augments Ca++ levels in blood. It stimulates demineralisation of bones (bone resorption) by activating osteoclasts releasing Ca in blood. PTH also promotes the absorption of Ca++ in renal tubules. Along with calcitonin PTH play important role in Ca++ ion balance in the blood.
 PTH also activates vitamin - D (Calciferol) into the active form known as calcitriol (hormone). It enhances Ca++ absorption in the digestive tract.

Hyperparathyroidism caused demineralisation of bones leading to deformities and fractures. Also cause stone formation in soft tissues (e.g.: Kidney stones).
Hypoparathyroidism leads to tetany because of low Ca++ in blood.

V. Thymus gland: It is a lobular gland present just below the breast bone, just above the heart. It secretes thymosins. They play important role in differentiation of T - lymphocytes, that are necessary for cell mediated immunity.
Thymus also promotes the formation of antibodies (humoral immunity)
Thymus gland is well developed in young stage but it gradually degenerates in old people in which immune response is weak.
 

VI. Adrenal glands/ Supra renal glands:
In man, a pair of pyramid shaped adrenal glands is present in the anterior parts of kidneys. Each adrenal gland contains outer cortex (mesodermal) and inner medulla (ectodermal).
1. Cortex: Adrenal cortex is formed by three zones namely
a. Outer zona glomerulosa: Secretes mineralocorticoides (e.g.: Aldosterone). Aldosterone regulates water and electrolyte balance in the body. It promotes the absorption of Na+ and elimination of K+.

b. Middle zona fesciculata: Secretes glucocorticoides (e.g.: Calcitriol). They regulate carbohydrate metabolism. They stimulate gluconeogenesis, lipolysis and pterolysis. It causes anti inflammatory reactions. Calcitriol is a stress combat hormone.
c. Inner zona reticularis: Secrete sex hormones that are necessary for development of secondary sexual characters.

2. Medulla: Adrenal medulla secretes adrenaline (epinephrine) and noradrenaline (norepinephrine). They are secreted in response to stress and in emergency. Hence they are called emergency hormones or fight or flight hormones. They enhance alertness, pupil dilation, involuntary erection of hair on skin (piloerection) sweating etc., and also increases rates of heart beat, breathing; stimulates rise in blood glucose etc., 
Hyposecretion of glucocorticoides leads to Addison's disease. It is characterised by loss of weight, muscle weakness, fatigue reduced blood pressure, brozening of skin etc.,
Hypersecretion of glucocorticoids leads to Cushing's syndrome. It is characterised by break down of muscle proteins, redistribution of body fat, moon face, buffalohump etc., and wound healing is poor, obesity etc.,

VII Pancreas
Pancreas is a mixed gland (partly exocrine and partly endocrine) and is located in the posterior part of stomach. Exocrine part is formed by acini and they secrete pancreatic juice. Endocrine part is formed by Islets of langerhans each islet includes four types of cells. They are -
1. Alpha cells: Secrete glucagon. It stimulates glycogenolysis and gluconeogenesis in hepatocytes. Hence blood glucose level rises. Hence it is called hyperglycemic hormone.

2. Beta cells: Secrete insulin. It makes the cell membranes more permeable to absorb glucose from blood. As a result, blood glucose level decreases. Hence, insulin is known as hypoglycaemic hormone. It also accelerates glycogenesis, lipogenesis; enhances uptake of amino acids by cells and inhibits catabolism of proteins.

3. Delta cells: Secrete somatostatin. It inhibits secretion of insulin and glucagon. It slows down the absorption of nutrients in the gut.
4. F- cells: Secrete pancreatic polypeptide. It inhibits secretion of somatostatin, contraction of gall bladder etc.,

Hyposecretion of insulin results in rise in blood glucose level (hyperglycemia). Prolonged hyperglycemia leads to diabetes mellitus. Which is characterised by glucosuria (loss of glucose through urine) and formation of ketone bodies. To control diabetes mellitus, insulin therapy is in practice.
Hypersecretion of insulin leads to fall in blood glucose level (hypoglycaemia). It leads to insulin shock.

VIII Gonads
1. Testes:
In males, there is a pair of testes located in scrotal sacs, out side the abdomen each testes is composed of numerous seminiferous tubules and they produce sperms by spermatogenesis. Among these seminiferous tubules (inter seminiferous tubular spaces), there are Leydig cells (Interstitial cells). They secrete male sex hormones known as androgens such as testosterone. These are necessary for development of secondary sexual characters [maturation and functioning of male sex organs, growth of facial and axillary hair, aggressiveness, low pitch voice (masculine] in male.
2. Ovaries:
In females, there is a pair of ovaries located in abdomen behind the kidneys. Ova are developed from the ovarian follicles. In addition, ovaries secrete oestrogen and progesterone. Oestrogen is responsible for development of secondary sexual characters (development and maturation of female sex organs, mammary glands, high pitch voice (feminine) etc.,) in female progesterone prepare the uterus the development of embryo and maintains pregnancy.

IX Hormones secreted by Kidney and Heart
1. Juxta glomerular cells of kidney secrete erythropoietin. It stimulates erythropoisis.
2. The muscle cells in human heart secrete atrial natriuretic factor (atrial natriuretic peptide). It dilates the blood vessels and hence, blood pressure decreases.
 

X Hormones secreted by gastrointestinal tract
1. Gastrin: It stimulates gastric glands to secrete hydrochloric acid and pepsinogen.
2. Secretin: It stimulates exocrine part of pancreas to stimulate secretion of water and

3. Cholecystokinin: It causes contraction of gall bladder and as a result, bile juice poured into the duodenum.
4. Gastric inhibitory peptide: It inhibits gastric secretions.
                                             Mechanism of Hormone Action
 Hormones are circulated to all parts of the body through blood circulation. But the hormones affects only specific target cells by binding to specific proteins called hormone receptors located in the target cells only. (Receptors of target cell membrane are called membrane bound receptors and those present in the cell are called intracellular receptors). Hormone receptors are specific for each of hormone. Hormone along with its receptor form a hormone receptor complex. It generates biochemical changes in the target cells.

i. Hormone (generally water soluble) with membrane bound receptor do not enter the target cell, but produce second messengers. They regulate cellular metabolism in cascading action amplifying the final effect.
ii. Hormones which interact with intracellular receptors (e.g.: steroid hormones) are fat soluble and diffuse into the cytoplasm of target cell through plasma membrane. They bind to internal receptors, enter the nucleus and regulate gene expression.