chemical coordination

The endocrine system includes:

• Endocrine glands (well-organised, ductless)

• Diffuse hormone-producing tissues/cells (present in various organs)

💡 These glands secrete hormones directly into the bloodstream to regulate body functions.

Gland	Location	Key Hormones Produced

Gland	Location	Key Hormones Produced
Pituitary	Base of brain (below hypothalamus)	GH, TSH, FSH, LH, ACTH, Prolactin, ADH, Oxytocin
Pineal	Center of brain (epithalamus)	Melatonin (biological clock)
Thyroid	Neck (around trachea)	Thyroxine (T₄), Triiodothyronine (T₃), Calcitonin
Parathyroid	Behind thyroid (4 lobes)	Parathyroid hormone (PTH)
Adrenal	Top of each kidney (2 glands)	Adrenaline, Noradrenaline, Cortisol, Aldosterone
Pancreas	Below stomach	Insulin, Glucagon, Somatostatin
Thymus	Chest, behind sternum (temporary)	Thymosin (immunity in children)
Gonads	Testes (males), Ovaries (females)	Testosterone, Estrogen, Progesterone

⚙ Functions

• Controls a wide range of body functions like:

  • Hunger, thirst

  • Sleep, body temperature

  • Emotional behaviour

  • Endocrine activities

🧪 Neurosecretory Cells

• Hypothalamus contains groups of neurosecretory cells called nuclei.

• These cells secrete hormones – NOT into blood directly, but into the pituitary portal circulation.

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🔄 Types of Hormones from Hypothalamus

Hormone Type	Function	Example
Releasing Hormones (RH)	Stimulate anterior pituitary to release its hormones	GnRH – stimulates FSH & LH release
Inhibiting Hormones (IH)	Inhibit anterior pituitary hormone secretion	Somatostatin – inhibits GH release

🔁 How Hormones Work

• Hormones are made in hypothalamic neurons

• Travel down axons to nerve endings

• Released into pituitary portal circulation

• Act on anterior pituitary to regulate its hormone secretion

📌 Anterior Pituitary = Controlled via chemical hormones (RH/IH)
📌 Posterior Pituitary = Controlled via direct neural signals from hypothalamus

🧠 Key Examples

1. 🧬 Gonadotropin-releasing hormone (GnRH)

   • Stimulates release of FSH & LH from anterior pituitary.

2. 🛑 Somatostatin

   • Inhibits growth hormone (GH) release.

NEET Highlights

• Hypothalamus controls both anterior and posterior pituitary, but in different ways:

  • Anterior Pituitary = RH/IH via blood

  • Posterior Pituitary = Direct neural stimulation

• This system ensures precise hormonal control over the body.

c) Somatostatin ✅

🧪 NEET Practice MCQ

Q: Which hormone from the hypothalamus inhibits GH secretion?
a) GnRH
b) CRH
c) Somatostatin
d) TRH

🧠 Pituitary Gland – The Master Gland 📍 Location & Structure

• Located in a bony cavity called sella turcica of the skull.

• Attached to the hypothalamus by a stalk.

• Divided into:

  • Adenohypophysis (anterior part)

  • Neurohypophysis (posterior part)

🔷 Adenohypophysis (Anterior Pituitary) Subdivisions:

1. Pars distalis → Major hormone-producing region.

2. Pars intermedia → Secretes MSH, almost merged with pars distalis in humans.

✨ Hormones from Pars Distalis

Hormone	Function
GH (Growth Hormone)	Growth of body. Excess → Gigantism / Acromegaly; Less → Dwarfism
PRL (Prolactin)	Milk production & growth of mammary glands
TSH (Thyroid Stimulating Hormone)	Stimulates thyroid to release T₃ and T₄
ACTH (Adrenocorticotropic Hormone)	Stimulates adrenal cortex to release glucocorticoids
LH (Luteinizing Hormone)	Males: Stimulates androgen (testosterone) secretion from testes Females: Triggers ovulation, forms corpus luteum
FSH (Follicle Stimulating Hormone)	Males: Stimulates spermatogenesis Females: Growth of ovarian follicles

✨ Hormone from Pars Intermedia

• MSH (Melanocyte Stimulating Hormone)
  → Regulates skin pigmentation by acting on melanocytes.

🔷 Neurohypophysis (Posterior Pituitary)

• Also called Pars Nervosa

• Stores & releases hormones made by hypothalamus:

Hormone	Function

Hormone	Function
Oxytocin	Contracts uterus during childbirth & ejects milk from breasts
Vasopressin / ADH (Antidiuretic Hormone)	Acts on kidneys → Reabsorbs water → Prevents water loss via urine

🔥 Deficiency of ADH → Diabetes Insipidus
💧 Symptoms: Excessive urination, dehydration

Quick NEET Pointers

🧪Gigantism → GH excess in childhood

• Acromegaly → GH excess in adulthood

• Diabetes Insipidus ≠ Diabetes Mellitus (No sugar in urine here)

🧠 NEET Memory Tricks:

• FLAT PEG = Hormones of Anterior Pituitary:

  • FSH

  • LH

  • ACTH

  • TSH

  • Prolactin

  • Endorphins (not mentioned here)

  • GH

🌙 Pineal Gland – The Body's Biological Clock

📍 Location:

• Situated on the dorsal side of the forebrain.

✨ Hormone Secreted:

• Melatonin

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🧠 Functions of Melatonin:

Function	Description
⏰ Circadian Rhythm	Regulates the 24-hour (diurnal) rhythm – sleep-wake cycle
🌡 Body Temperature	Helps maintain the body’s temperature cycle
♀ Menstrual Cycle	Influences timing & rhythm of the menstrual cycle
🛡 Immunity	Enhances defense capability
🎨 Pigmentation	Affects skin pigmentation
🔄 Metabolism	Influences metabolic processes

🧪 NEET Quick Points:

• Darkness increases melatonin secretion → Induces sleep

• Light suppresses melatonin → Promotes wakefulness

• Regulates biological clock in humans

• Secreted more during night

💡 NEET Tip:

• Think "Melatonin = Moon hormone" 🌓

• It helps you "melo-down" for sleep 🛏💤

🦋 Thyroid Gland – The Metabolism Master

📍 Location & Structure:

• Two lobes on either side of trachea

• Connected by a thin flap of connective tissue → Isthmus

• Made of follicles (with follicular cells) & stromal tissue

✨ Hormones Secreted:

Hormone	Type	Function
T₃ (Triiodothyronine)	Iodine-containing	More active form
T₄ (Thyroxine / Tetraiodothyronine)	Iodine-containing	Major hormone secreted
TCT (Thyrocalcitonin)	Protein hormone	Regulates blood calcium levels ↓

Regulates metabolism and energy production. T₃ is more active, while T₄ is the primary secreted hormone.

🧂 Iodine is essential for T₃ and T₄ production!

🧬 Functions of Thyroid Hormones (T₃ & T₄):

• 🚀 Increase BMR (Basal Metabolic Rate)

• 🔴 Stimulate RBC formation

• 🍞 Control metabolism of carbs, fats, proteins

• 💧 Help maintain water & electrolyte balance

• 🧠 Crucial for growth & nervous system development

🚨 Disorders:

Condition	Cause	Features
Hypothyroidism	Iodine deficiency	Goitre, weight gain, tiredness
Cretinism (in infants)	During pregnancy	Stunted growth, mental retardation
Hyperthyroidism	Cancer/nodules	Exophthalmic goitre, weight loss, high BMR
Graves’ Disease	Autoimmune hyperthyroidism	Eye bulging, anxiety, heat intolerance

💡 NEET Tip:

• Low Iodine → Goitre

• T₃ is 3-5x more active than T₄

• TCT = Calcium Tone = Lowers Ca²⁺ in blood

🧬 Parathyroid Gland – The Calcium Balancer

📍 Location:

• 4 small glands on the posterior surface of thyroid gland

• 1 pair per lobe of the thyroid

✨ Hormone Secreted:

• Parathyroid Hormone (PTH)
  🔹 Type: Peptide hormone
  🔹 Secreted in response to low blood Ca²⁺ levels

🔁 Functions of PTH (Hypercalcemic Hormone):

Target	Action	Result
Bones	Stimulates bone resorption (breakdown)	Releases Ca²⁺ into blood
Kidneys	Promotes Ca²⁺ reabsorption in tubules	Less Ca²⁺ lost in urine
Intestine	Enhances Ca²⁺ absorption from digested food	↑ Blood Ca²⁺ levels

👆 Hypercalcemic hormone = Raises blood calcium

Works With:

⚖ Works With:

• Thyrocalcitonin (TCT) from thyroid:

  • Opposite action (lowers blood Ca²⁺)

  • Together, they maintain calcium homeostasis

💡 NEET Tip:

• PTH = "Pulls The calcium High"

• Deficiency → Hypocalcemia

• Excess → De-mineralization of bones (fragile bones)

Thymus – The Immunity Giver

📍 Location:

• Between the lungs, behind the sternum, in front of the aorta

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🧠 Structure:

• Lobular gland

• More active in children & adolescents

• Begins to degenerate with age

✨ Hormone Secreted:

• Thymosins (Peptide hormones)

💪 Functions of Thymosins:

Role	Action
🧬 Cell-mediated Immunity	Helps in differentiation of T-lymphocytes
🧫 Humoral Immunity	Promotes antibody production by B-cells

⏳ Age-Related Changes:

• Thymus degenerates in old age

• ↓ Thymosin → Weaker immune response in elderly

NEET Focus Point:

• T-cells mature in Thymus (T for Thymus!)

• Degeneration leads to immunosenescence

🌟 Adrenal Gland: The Emergency Worker

• Location: One pair of adrenal glands located on top of each kidney.

• Structure:

  • Adrenal Medulla (inner region)

  • Adrenal Cortex (outer region)

🧠 Adrenal Medulla: Hormones of "Fight or Flight"

• Main Hormones:

  • Adrenaline (Epinephrine)

  • Noradrenaline (Norepinephrine)

• Functions:

  • 🏃‍♂ Rapid response to stress and emergency (fight or flight)

  • ✅ Increase heart rate, strengthen heart contractions, increase respiration rate

  • 👀 Dilate pupils, raise hairs (piloerection), increase sweating

  • 💥 Breakdown of glycogen → Raises blood glucose levels

  • 🏋‍♂️ Stimulates breakdown of lipids and proteins

Adrenal Cortex

🌍: The Three Layers and Their Functions

The adrenal cortex has 3 layers:

1. Zona Glomerulosa (Outer)

2. Zona Fasciculata (Middle)

3. Zona Reticularis (Inner)

Main Hormones:

• Glucocorticoids (Cortisol):

  • Regulate carbohydrate metabolism

  • ✅ Stimulate gluconeogenesis, lipolysis, and proteolysis

  • 🔥 Anti-inflammatory, immune-suppressive

  • 🩸 Stimulate RBC production

  • 🫀 Maintain cardiovascular and kidney function

• Mineralocorticoids (Aldosterone):

• • ⚖ Regulate water and electrolyte balance

  • ✅ Stimulate Na+ and water reabsorption in kidneys

  • ✅ Excrete K+ and phosphate ions

  • Maintain blood pressure and osmotic pressure

• Androgenic Steroids:

• • 🎯 Influence the growth of axial hair, pubic hair, and facial hair during puberty

🚨 Disorders:

• Addison's Disease: Caused by underproduction of adrenal cortex hormones → Fatigue, weakness

• Hypersecretion of Adrenaline → Increased stress response (e.g., during emergencies)

Pancreas: Dual Role Gland

🩺

➡ It functions as both:

• Exocrine Gland: Secretes digestive enzymes.

• Endocrine Gland: Secretes hormones from Islets of Langerhans.

🧬 Islets of Langerhans – Tiny but Mighty!

🧪 Makes up only 1–2% of pancreas but controls sugar levels like a boss 😎
Has two main cell types:

Cell Type	Hormone Secreted	Function
α-cells	Glucagon	Raises blood sugar (hyperglycemia)
β-cells	Insulin	Lowers blood sugar (hypoglycemia)

💉 Insulin (Secreted by β-cells)

• Target Cells: Hepatocytes (liver) + Adipocytes (fat cells)

• ✅ Increases glucose uptake by cells

• ✅ Promotes glycogenesis (glucose → glycogen)

• ✅ Reduces blood sugar → Prevents hyperglycemia

💥 Deficiency leads to Diabetes Mellitus:

• Glucose in urine

• Formation of harmful ketone bodies

• Treated with insulin therapy

🍬 Glucagon (Secreted by α-cells)

• Target Organ: Liver

• ✅ Stimulates glycogenolysis (glycogen → glucose)

• ✅ Stimulates gluconeogenesis (non-carbs → glucose)

• ⬆ Increases blood sugar = Hyperglycemic hormone

⚖ Balance Between Insulin and Glucagon

• Insulin 🧊 cools down sugar levels

• Glucagon 🔥 fires them up
  ➡ Together, they maintain glucose homeostasis in blood

🌟 Testis: The Male Reproductive and Endocrine Organ

• Location: A pair of testes located in the scrotal sac (outside the abdomen).

• Functions:

  • Primary sex organ: Responsible for sperm production.

  • Endocrine gland: Produces hormones, especially androgens like testosterone.

🧬 Structure:

• Seminiferous Tubules: Site of spermatogenesis (sperm formation).

• Leydig Cells (Interstitial Cells): Located between the seminiferous tubules, they produce androgens (mainly testosterone).

🧪 Role of Androgens (Testosterone):

• Male Sexual Development:

  • Regulate growth and function of male accessory sex organs: epididymis, vas deferens, seminal vesicles, prostate gland, and urethra.

• Secondary Sexual Characteristics:

• • Stimulate muscle growth, facial & axillary hair growth, and a low-pitched voice.

• Spermatogenesis:

• • Stimulate sperm production in the seminiferous tubules.

• Sexual Behavior:

• Influence libido (sexual drive).

• Metabolism:

  • Have anabolic effects: Increase protein and carbohydrate metabolism.