Chemical coordination and integration

 

   

   

Points To Remember

Endocrine glands : These are ductless glands which secrete hormones directly into the blood stream.

Hormones : Non-nutrient chemicals synthesised in trace amount by Endocrine glands that act as intracellular messengers and are specific in their action which are transported by blood from site of production to site of action.

Hypothalamus :

   It is basal part of diencephalon.

 Has neurosecretory cells called nuclei which produce hormones to regulate the synthesis and secretion of pituitary gland hormones.

   Two types of hormones released are :

Releasing hormones : Simulate secretion of pituitary hormones, e.g., Gonadotrophin releasing hormone stimulates pituitary gland to synthesise gonadotrophins.

Inhibiting hormones : Inhibit secretions of pituitary hormones, e.g.,

Somatostatin inhibits secretion of growth hormone.

Pituitary Gland :

   Located in bony cavity called as sella tursica.

   Attached to hypothalamus by a stalk.

 Divided anatomically into : Adenohypophysis (Anterior lobe) and Neurohypophysis (Posterior lobe).

 Hormones released from hypothalamic neurons reach anterior pituitary through portal system and through neurons in Posterior pitutary.

   Posterior pituitary is under neural control of hypothalamus.

1. Pituitary Gland

Pituitary Gland

Adenohypophysis (Anterior lobe)    Neurohypophysis (Posterior lobe)

(It stores hormones sureated by hypothalamus

Pars distalis    Pars intermedia Secretes        MSH

GH    PRL    TSH    ACTH    LH    FSH

Adenohypophysis :

Pars nervosa

Oxytocin    Vasopressin

(ADH)

 Growth hormone (GH) : Oversecretion leads to gigantism and low secretion

causes dwarfism and Proper reaction leads to proper growth of body.

 Prolactin (PRL) : Growth of mammary gland and formation of milk in them.

 Thyroid stimulating hormone (TSH) : Stimulates synthesis and secretion of thyroid hormones from thyroid gland.

 Adrenocorticotrophic hormone (ACTH) : Stimulates synthesis and secretion of steroid hormones called glucocorticoids from adrenal cortex.

 Luteinizing hormone (LH) : Synthesis and secretion of hormones called androgens in males, and helps in ovulation and maintenance of corpus luteum in females.

 Follicle stimulating hormone (FSH) : Regulate spermatogenesis in males, and growth and development of ovarian follicles in females.

 Oxytocin helps in contraction of uterus during child birth and milk ejection from mammary gland in females.

 Vasopressin : Acts on kidney and stimulates reabsorption of water and electrolytes by distal tubules to reduce water loss through urine. It is also called as Anti Diuretic Hormone (ADH).

 Acrommegaly : It is a condition when the pituitary gland makes too much growth hormone. It is due to a tumour in pituitary gland. Person suffering from acromegaly (acro means tip and megaly means enlargement) may gradually develop a long face with protruding lower law, enlarged nose and wider spacing between teeth and enlarged hands and feet.

2. Pineal Gland :

   Located on dorsal side of forebrain.

 Secretes Melatonin to regulation 24-hours rhythm, sleep-wake cycle, menstrual cycle, pigmentation etc.

3. Thyroid Gland :

 Has two lobes on either side of trachea interconnected by isthmus (connective tissue).

   Composed of follicles and stromal tissues.

   Follicular cells synthesis thyroxine (T4) and tri-iodothyronine (T3).

   Iodine is necessary for normal functioning in of thyroid.

 Goitre (Hypothyroidism) : Enlargement of thyroid gland; Hypothyroidism may lead to mental retardation and stunted growth (cretinism) Deaf-mutism in the baby if it occurs during pregnancy.

 Hyperthyroidism : Occurs due to cancer or due to development of nodules in thyroid glands. Effects body physiology as abnormal high levels of thyroid hormones is synthesised. Basic metabolic rate increase.

 Exophthalmic goitre : It is a form of hyperthyroidism, characterised by enlargement of thyroid gland, protrusion of eye balls and increased BMR

   Thyroid hormone controls protein, carbohydrate metabolism.

 Also secretes a protein hormone called Thyrocalcitonin (TCT) which regulates blood calcium level.

4. Parathyroid Gland :

 Present on back side of thyroid gland. Each lobe of thyroid gland has its one pair.

 Secrete peptide hormone called parathyroid hormone (PTH) which increases calcium levels in blood so called hypercalcemic hormone.

 PTH stimulates bone resorption, and reabsorption of calcium from blood and reabsorption of calcium by renal tubules, thus increasing blood Ca++ level.

5. Thymus Gland

   Located on dorsal side of heart and aorta.

 Secrete peptide hormones called Thymosins which play role in differentiation of T-lymphocytes (help in cell mediated immunity.)

   Thymosins also produce antibodies and provide humoral immunity.

 Immunity of old people usually becomes weak as thymus gets degenerated with age.

6. Adrenal Gland

   Located at anterior part of each kidney.

   Has centrally located adrenal medulla and at  periphery in adrenal cortex.

 Adrenal medulla secretes adrenaline (epinephrine) and nor adrenaline (norepinephrine), commonly called as catecholamines or emergency hormones or hormones of fight or flight.

 These hormones increase heart beat, rate of respiration, breakdown of glycogen thus increase blood glucose level, breakdown of lipids and protein, alertness, raising of hairs, sweating etc.

    Adrenal Cortex-(3 layers) :        Zona reticularis (inner layer)

Zona fasciculata (middle layer) Zona glomerulosa (outer layer)

   Adrenal cortex secretes :

1. Androgenic steroids :

   Secreted in small amounts.

   Play role in growth of axial pubic and facial hair during puberty.

2. Glucocorticoids :

   Involved in carbohydrate metabolism.

   Stimulates gluconeogenesis, lipolysis and proteolysis.

 e.g., Cortisol which is also involved in cardio-vascular and kidney functions.

   It also suppresses immune response and stimulates RBC production.

3. Mineralocorticoids :

   Regulate balance of water and electrolytes in body.

  e.g., Aldosterone which also helps in reabsorption of NA+ and water excretion of K+ and phosphates ions from renal tubules.

 When adrenal cartex is damaged, it does not produce enough cortisols (which regulate body’s reaction to stressful situations) and aldosterone.

 It result in Addison’s disease. Symptons of addison’s disease are weak muscles, extreme fatigue, increased skin pigmentation, weight loss, sores in mouth and depression.

Two major causes :

1. Primary adrenal insufficiency where our immunity system mistakes adrenal

for an antigen and tries to damage it.

2. Secondary adrenal insufficiency-when pituitary gland can’t produce ACTH

7. Pancreas : It is called composite/dual gland. As it acts as Exocrine and endocrine gland i.e. has both exocrine and endocrine function.

  Contains about 1-2 million islets of Langerhans which has glucagon secreting

α-cells and insulin secreting β-cell.

 Glucagon : Peptide hormone, stimulates glycogenolysis by acting on liver cells. Also, stimulates gluconeogenesis. Hence called hyperglycemic hormone.

 Insulin : Peptide hormone, acts on hepatocytes and adipocytes to enhance cellular glucose uptake, stimulates conversion of glucose to glycogen (glycogenesis), so decrease blood glucose level called hypoglycemic hormone.

 Deficiency of insulin causes diabetes mellitus in which loss of glucose occurs through urine. Excessive hunger and thirst (polydipsia) are other symptoms of Diabetes.

   Insulin and glucagon are antagonistic hormones i.e. play apposite role.

Glycogenolysis : Breaking of glycogen into glucose.

Gluconeogenesis : Formation of glucose from substances other than glycogen.

Glycogenesis : Conversion of glucose into glycogen.

8. Testis :

 A pair of testis composed of seminiferous tubules and interstilial cells is present in the scrotal sac of males.

 Leydig cells (interstitial cells) produce androgens (mainly testosterone) which regulate development and maturation of male accessory sex organs, formation of secondary sex characters and play stimulatory role in spermatogenesis. Male sexual behaviour (libido) is influenced by androgens.

Ovary : A pair of ovaries which produce one ovum in each menstrual cycle are present in abdomen in females.

   Ovary composed of ovarian follicles and stromal tissue.

 Estrogen synthesised by growing ovarian follicles helps in stimulation of growth of female secondary sex organs, female behaviour, mammary gland development and female secondary sex characters.

 Ruptured follicle form corpus luteum which secretes progesterone. Progesterone supports pregnancy and stimulates alveoli formation and milk secretion in mammory glands.

Hormones secreted by tissues which are not endocrine glands :

1. Heart : Atrial wall secrets Atrial Natriuretic factor (ANF) which decreases blood pressure by dilation of the blood vessels.

2. Kidney : Juxtaglomerular cells secretes erythropoietin which stimulates erythropoiesis (RBC formation).

3. Gastro-intestinal tract : it secrets four peptide hormones.

 Gastrin : Acts on gastric glands and stimulates secretion of hydrochloric acid and pepsinogen.

  Secretin : Acts on pancreas and stimulates secretion of water and bicarbonation.

 Cholecystokinin (CCK) : Act on pancreas and gall bladder to stimulate secretion of pancreatic juice and bile juice respectively.

Gastric inhibitory peptide (GIP) : Inhibits gastric secretion and motility.

Mechanism of hormone action : By hormone receptors of two kinds, i.e.,

1. Located on membrane of target cell

   These are membrane bound receptors.

   From hormone receptor complex.

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Leads to biochemical changes in tissue.

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Release of second messengers like (cyclic AMP, IP3, Ca2+ etc.) which regulate cellular metabolism.

2. Located inside the target cell

   These are intra cellular receptors.

 Hormones (steroid hormones iodothyronines etc.) interact with them and cause physiological and developmental effects of regulating gene expression.