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:
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.
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:
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.
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:
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.
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):
Thyroid stimulating hormone (TSH):
Adrenocorticotrophic hormone (ACTC) & corticotrophin
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.
In female: it promotes ovulation and also maintain the corpus luteum and secrete progesterone.
In males: responsible for the secretion of the testosterone.
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 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.
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.
Function of parathyroid gland:
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.
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: 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 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
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:
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 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.
Function of the pineal gland:
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.