22. CHEMICAL CO-ORDINATION AND INTEGRATION
Endocrine system includes endocrine (ductless) glands and their secretions (hormones).
Hormones are non-nutrient chemicals that act as intercellular messengers and are produced in trace amounts.
Hormones are non-nutrient chemicals that act as intercellular messengers and are produced in trace amounts.
HUMAN ENDOCRINE GLANDS
They include
Neurosecretory cells (nuclei) of hypothalamus secrete the following types of hormones:
It is located in a bony cavity called sella tursica.
It is attached to hypothalamus by a stalk.
It is divided into anterior Adenohypophysis & posterior Neurohypophysis.
It has 2 parts: Pars distalis and Pars intermedia.
Pars distalis (Anterior pituitary): It produces
- Hypothalamus
- Pituitary
- Pineal
- Thyroid
- Parathyroid
- Thymus
- Adrenals
- Pancreas (Islets of Langerhans)
- Gonads (Testis & Ovary)
1.
HYPOTHALAMUS
- Releasing hormones: Stimulate secretion of pituitary hormones. E.g. gonadotropin releasing hormone (GnRH) stimulates pituitary to release gonadotropins (FSH & LH).
- Inhibiting hormones: Inhibit secretion of pituitary hormones. E.g. Somatostatin inhibits release of growth hormone from pituitary.
- Oxytocin & vasopressin: These are transported axonally and stored in pituitary. (See pituitary gland).
2. PITUITARY GLAND
It is attached to hypothalamus by a stalk.
It is divided into anterior Adenohypophysis & posterior Neurohypophysis.
a. Adenohypophysis
It has 2 parts: Pars distalis and Pars intermedia.
Pars distalis (Anterior pituitary): It produces
- Somatotropin (Growth hormone, GH): For body growth.
Its over-secretion causes Gigantism (abnormal growth). Hyposecretion causes Dwarfism (stunted growth).Over-secretion of GH in adults (mainly in middle age) causes Acromegaly (severe disfigurement especially of face). It leads to serious complications and premature death. Early diagnosis of the disease is difficult. It may be undetected for many years.
- Prolactin (PRL): Regulates growth of mammary glands and milk production.
- Thyroid stimulating hormone (TSH): Stimulates thyroid gland to secrete thyroid hormones.
- Adrenocorticotrophic hormone (ACTH): Stimulates adrenal cortex to synthesise & secrete steroid hormones (glucocorticoids).
- Follicle stimulating hormone (FSH): Stimulates gonadal activity.
In males, FSH & androgens regulate sperm formation (spermatogenesis).
In females, FSH stimulates growth and development of ovarian follicles.
- Luteinizing hormone (LH): Stimulates gonadal activity.
In males, it stimulates synthesis and secretion of androgens from testis.In females, it induces ovulation and maintains the corpus luteum.
Pars intermedia:
- In human, it is almost merged with pars distalis.
- It produces Melanocyte stimulating hormone (MSH). It acts on melanocytes to regulate skin pigmentation.
b. Neurohypophysis
It stores Oxytocin & Vasopressin from hypothalamus.
i. Oxytocin:
- Contracts smooth muscles.
- In females, it stimulates contraction of uterus during child birth, and milk ejection from the mammary gland.
ii. Vasopressin or Anti-diuretic hormone (ADH):
- Stimulates reabsorption of water & electrolytes by DCT of kidney and thereby reduces diuresis (loss of water through urine).
- Deficiency of ADH results in diminished ability of the kidney to conserve water. It leads to water loss and dehydration. This is called Diabetes insipidus.
3. PINEAL GLAND
Smallest endocrine gland.
It is located on dorsal side of forebrain.
It secretes melatonin.
Functions of melatonin:
Functions of melatonin:
- Regulates diurnal (24-hour) rhythm of body. E.g. sleep-wake cycle, body temperature etc.
- Influences metabolism, pigmentation & menstrual cycle.
- Influences defense capability.
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