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PITUITARY GLAND notes of Endocrine System

PITUITARY GLAND notes of Endocrine System

-The pituitary gland (hypophysis) and the hypothalamus act as a unit, regulating the activity of most of the other endocrine glands. That’s why it is called a master gland. 
-The pituitary gland lies in the hypophyseal fossa (sella turcica) of the sphenoid bone below the hypothalamus, to which it is attached by a stalk
-It is the size of a pea, weighs about 500 mg and consists of two distinct parts that originate from different types of cells.
-The anterior pituitary (adenohypophysis) is an upgrowth of glandular epithelium from the pharynx and the posterior pituitary (neurohypophysis) is a down growth of nervous tissue from the brain.

The anterior pituitary. This is supplied indirectly with arterial blood that has already passed through a capillary bed in the hypothalamus. This network of blood vessels form part of the pituitary portal system, which transports blood from the hypothalamus to the anterior pituitary

The posterior pituitary. This is formed from nervous tissue and consists of nerve cells surrounded by supporting cells called pituicytes. Posterior pituitary hormones are synthesised in the nerve cell bodies, transported along the axons and then stored in vesicles within the axon terminals

Anterior pituitary:
-Some of the hormones secreted by the anterior lobe (adenohypophysis) stimulate or inhibit secretion by other endocrine glands (target glands).
-The whole system is controlled by a negative feedback mechanism. That is when there is a low level of a hormone in the blood supplying the hypothalamus it produces the appropriate releasing hormone which stimulates the release of a trophic hormone by the anterior pituitary. This, in turn, stimulates the target gland to produce and release its hormone. As a result, the blood level of that hormone rises and inhibits the secretion of releasing factor by the hypothalamus. The hormones secreted by the anterior pituitary are as following: 

 1. Growth hormone (GH)This is the most abundant hormone synthesized by the anterior pituitary.
Its release is stimulated by growth Hormone Releasing Hormone (GHRH) and suppressed by growth
hormone release inhibiting hormone (GHRIH) both of which are secreted by the hypothalamus.

-It stimulates growth and division of most body cells but especially those in the bones and skeletal muscles.
-It also regulates aspects of metabolism in many organs, e.g. liver, intestines and pancreas
-It stimulates protein synthesis; promotes the breakdown of fats; and increases blood glucose levels

Hypersecretion of GH causes gigantism and acromegaly and hyposecretion causes dwarfism.

2. Thyroid Stimulating Hormone (TSH)

This hormone is synthesised by the anterior pituitary and its release is stimulated by TRH from the

Function: It stimulates the growth and activity of the thyroid gland, which secretes the hormones thyroxine (T4) and triiodothyronine (T3).
Secretion is regulated by a negative feedback mechanism. When the blood level of thyroid hormones is high, secretion of TSH is reduced, and vice versa.
Its release is lowest in the morning and highest during the night.

3. Adrenocorticotrophic hormone (corticotrophin, ACTH)
Corticotrophin releasing hormone (CRH) from the hypothalamus promotes the synthesis and release of ACTH by the anterior pituitary.

Function: This increases the concentration of cholesterol and steroids within the adrenal cortex and the output of steroid hormones, especially cortisol.
-It maintains circadian rhythm.
Secretion is also regulated by a negative feedback mechanism, being suppressed when the blood level of ACTH rises.

4. Prolactin
-This hormone stimulates lactation (milk production) and has a direct effect on the breasts immediately after parturition (childbirth).
-It stimulates the duct system in mammary glands.
The blood level of prolactin is stimulated by Prolactin Releasing Hormone (PRH) released from the hypothalamus and it is lowered by prolactin inhibiting hormone (PIH, dopamine) and by an increased the blood level of prolactin.

5. Gonadotrophins
After puberty two gonadotrophins (sex hormones) are secreted by the anterior pituitary in response to
luteinizing Hormone Releasing Hormone (LHRH), also known as Gonadotrophin Releasing Hormone
In both males and females these are:
• Follicle Stimulating Hormone (FSH)
• luteinising hormone (LH).

-In both sexes. FSH stimulates the production of gametes (ova or spermatozoa).
-In females. LH and FSH are involved in secretion of the hormones oestrogen and progesterone during the menstrual cycle
-In males. LH also called Interstitial Cell Stimulating Hormone (ICSH) stimulates the interstitial cells of the testes to secrete the hormone testosterone

Posterior pituitary
Oxytocin and antidiuretic hormone (ADH or vasopressin) are the hormones synthesized in the
hypothalamus and then released from the axon terminals within the posterior pituitary gland

1. Oxytocin
Oxytocin stimulates two target tissues during and after parturition (childbirth): uterine smooth muscle
and the muscle cells of the lactating breast.
-it helps in the ejection of milk from lactating breast.
-It helps in contraction uterine wall during parturition and helps in expulsion of baby’s head out of the uterus.


2. Antidiuretic hormone (ADH) or vasopressin
The main effect of antidiuretic hormone is to reduce urine output (diuresis is the production of a large the volume of urine).
ADH increases the permeability to water of the distal convoluted and collecting tubules of the
nephrons of the kidneys
As a result, the reabsorption of water from the glomerular filtrate is increased.

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