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Endocrine System- An Introduction

Endocrine System uses chemical signals for cell to cell communication. It coordinates the function of cells.The response to an endocrine signal occurs within minutes to hours.

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Hormones – cell to cell communication molecules

  • Use chemical signals for cell to cell communication
  • Coordinate the function of cells
  • Response to an endocrine signal occur within minutes to hours

Endocrine Glands

  • Ductless glands
  • Clumps or cords of secretory cells surrounded by network of small blood vessels
  • Hormones are poured directly into blood

The signal chemicals are
• Made in endocrine cells
• Transported via blood
• Receptors are  on target cells

Neurotransmitters are chemicals secreted by neurons that diffuse across a small gap to the target cell. Neurons use electrical signals as well.

Hormones secreted by neurons are known as neurohormones. Thus they have a neural origin. e.g.

• Anti Diuretic Hormone (ADH)

Exocrine Glands- Pour secretion through duct system

Dual Glands- Endocrine/ Exocrine function – both present e.g., Pancreas

Paracrine Horomones

• Site of synthesis & site of action close together
• Act on adjacent cells

Autocrine Horomnes

Site of synthesis & effector (target) – same cell

Recurrent / Periodic Glands

• Appear & disappear according to physiological demand
• eg., Corpus Luteum & Placenta

Classification of Hormones

1. Proteins & Polypeptides
2. Steroids
3. Amino Acid – Tyrosine derivative

Proteins & Polypeptides

< 100 amino acids – peptide
> 100 amino acids – protein

• Ant. & Post. Pituitary hormones
Insulin / Glucagon
• Growth hormone
• Parathyroid hormone etc.


Have Cyclopentanoperhydrophenanthrene nucleus


• Cortisol
• Aldosterone
• Testosterone
• Estrogen etc.

Amino Acid – Tyrosine Derivatives

• Thyroid Hormones
• Epinephrine / Norepinephrine


– Globulin
– Albumin
only 2 – 3% is in FREE FORM (biologically active)

• Not usually stored – except

1. THYROXINE –by combining with TYHYROGLOBULIN
2. INSULIN – can be stored in β – cells of pancreas
in complex form with Zinc
3. ADH – by combining with
– Neurophysin – I
– Neurophysin – II

Inactivation / Clearance from Plasma

i. Metabolic destruction by tissues
ii. Binding with tissues
iii. Excretion  by LIVER into BILE
– By conjugation with glucoronide etc.
iv. Excretion by kidneys into urine
v. Skin

Modes of Hormonal Regulation

(A) Feed back system
(B) Neural Control
(C) Chronotropic Control

(A) Feed back system (Mainly)
– Negative Feed back (almost always)
– Positive Feed back
i. Hormone – Hormone level
ii. Substrate – Hormone level
iii.Mineral – Hormone level

(B) Neural Control
i. Adrenergic
ii. Cholinergic
v. GABAergic

(C) Chronotropic Control
i. Diurnal Rhythm
ii. Sleep – Wake Rhythm
iii.Menstrual Rhythm
iv.Seasonal Rhythm
v. Developmental Rhythm
vi.Circhoral Rhythm
vii.Ultradian Rythm

Feedback System

i. Hormone – Hormone Negative Feed Back

ii. Substrate – Hormone Negative Feed Back

iii. Mineral – Hormone Negative Feed Back

iv. Parturition – Oxytocin Positive Feed Back

v. Suckling – Oxytocin Positive Feed Back

Negative Feedback controls
Long & Short loop reflexes

(C) Chronotropic Control

1. Diurnal Rhythm

– ↑ Prolactin secretion during night

2. Sleep Wake Rhythm

– Infants / Children – ↑ GH during sleep
– ↑ ACTH & ↑ Cortisol during sleep

3. Menstrual Rhythm

– ♀ reproductive life (15 – 45 yrs)
– Ovarian steroids – Estrogen & Progesterone
4. Seasonal Rhythm

– Esp in lower animals (Frogs – Hibernate)
5. Developmental Rhythm

– eg., Growth Rythm
6. Circhorral  Rhythm

– change on hourly basis eg., insulin after food intake
7. Ultradian Rhythm

– moment to moment change eg., epinephrine

Want a clearer concept, also see

Endocrine Functions of Pancreas

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