Chemical Coordination and Integration in Humans

In the human body, not every cell is directly connected through nerve fibres. So, how do distant organs communicate?

This is where chemical coordination comes into play. Hormones are released into the bloodstream and travel to target cells.

• Compared to neural coordination:
  • Slower in action
  • But long-lasting and widespread effects

Think of it like:

• Nervous system → instant messaging ⚡
• Endocrine system → broadcast message with lasting impact 📡

Endocrine Glands and Hormones

Endocrine glands are called ductless glands because:

• They do not have ducts
• Their secretions (hormones) are released directly into blood

What are Hormones?

• Non-nutrient chemicals, Act as intercellular messengers, Produced in very small quantities. Yet, they have powerful regulatory roles

Modern understanding has expanded this definition:

• Not only traditional glands, but even organs like heart, kidney, liver, gastrointestinal tract also produce hormone-like signalling molecules.

Evolutionary Perspective

• Invertebrates → Simple endocrine system, fewer hormones
• Vertebrates (Humans) → Highly complex, multiple hormones ensuring fine regulation

Human Endocrine System

The endocrine system consists of:

• Dedicated glands: Pituitary, Pineal, Thyroid, Adrenal, Pancreas, Parathyroid, Thymus, Gonads
• Other hormone-secreting organs: Liver, kidney, heart, gastrointestinal tract

This system ensures → Growth, Metabolism, Reproduction, Homeostasis

The Hypothalamus

Located in the forebrain, the hypothalamus acts as a bridge between the nervous and endocrine systems.

It contains neurosecretory cells (nuclei) that produce hormones.

Two Types of Hormones

1. Releasing Hormones
   • Stimulate pituitary secretion
   • Example: Gonadotropin-Releasing Hormone (GnRH)
2. Inhibiting Hormones
   • Suppress pituitary activity
   • Example: Somatostatin (inhibits Growth Hormone)

👉 In simple terms:
The hypothalamus is like a controller, deciding when to start or stop hormone production

The Pituitary Gland: The Master Gland

The pituitary gland:

• Located in sella turcica (a bony cavity of skull)
• Attached to hypothalamus
• Divided into:
  • Adenohypophysis (Anterior Pituitary)
  • Neurohypophysis (Posterior Pituitary)

Hormones of Adenohypophysis (Anterior Pituitary)

Hormone	Function	Key Insight
Growth Hormone (GH)	Body growth	Excess → Gigantism; Deficiency → Dwarfism
Prolactin (PRL)	Mammary gland growth & milk production	Important for lactation
TSH	Stimulates thyroid gland	Controls metabolism indirectly
ACTH	Stimulates adrenal cortex	Releases stress hormones
LH	Ovulation & testosterone production	Reproductive control
FSH	Follicle growth & spermatogenesis	Gamete formation
MSH	Skin pigmentation	Acts on melanocytes

TSH: Thyroid Stimulating Hormone; ACTH: Adrenocorticotropic Hormone; LH: Luteinizing Hormone; FSH: Follicle-Stimulating Hormone; MSH: Melanocyte Stimulating Hormone
👉 Conceptually: Anterior pituitary = “Executive gland” controlling other endocrine glands.

Hormones of Neurohypophysis (Posterior Pituitary)

Hormone	Function	Clinical Relevance
Oxytocin	Uterine contraction & milk ejection	Important in childbirth
Vasopressin (ADH)	Water reabsorption in kidneys	Deficiency → Diabetes Insipidus

👉 ADH ensures → Water conservation, Prevents dehydration
Without it → Excessive urination + water loss.

The Pineal Gland: The Biological Clock

Located at the back of the forebrain, the pineal gland secretes melatonin.

Functions of Melatonin

• Regulates 24-hour (diurnal/circadian rhythm)
• Controls → Sleep–wake cycle, Body temperature, Metabolism, Menstrual cycle, Immunity

👉 In essence:
Pineal gland acts as the body’s internal clock, synchronising biological activities with day–night cycles.

Thyroid Gland: The Metabolic Regulator

The thyroid gland is a butterfly-shaped gland located in the front of the neck. It produces:

• Thyroxine (T4)
• Triiodothyronine (T3)
• Thyrocalcitonin (TCT)

Functions

• Regulates metabolism (Basal Metabolic Rate)
• Supports RBC formation
• Maintains water and electrolyte balance
• TCT regulates blood calcium levels

A key point: Iodine is essential for thyroid hormone synthesis. Its deficiency leads to goitre (enlargement of thyroid).

Thyroid Disorders

• Hyperthyroidism → Excess hormone
  • Symptoms: weight loss, anxiety, rapid heartbeat
• Hypothyroidism → Deficiency
  • Symptoms: fatigue, weight gain, cold intolerance
  • In infants → cretinism (growth + mental retardation)
• Graves’ Disease
  • Enlarged thyroid + bulging eyes (exophthalmos)

👉 Conceptual takeaway:
Thyroid = “metabolic accelerator” of the body

Parathyroid Glands: Calcium Balance Controllers

These are four tiny glands located behind the thyroid.

They secrete Parathyroid Hormone (PTH) → a hypercalcemic hormone.

Functions of PTH

• Increases blood calcium by:
  • Bone resorption
  • Kidney reabsorption of calcium
  • Enhanced intestinal absorption

👉 In short: PTH = “Calcium raiser”

Thymus: The Immunity Trainer

The thymus gland, located in the chest, plays a crucial role in immunity.

• Secretes thymosins
• Helps in T-lymphocyte maturation

Key Insight

• Active in childhood
• Degenerates with age → reduced immunity

👉 Thymus = “training school of immune cells”

Adrenal Glands: Stress Managers of the Body

Located on top of kidneys, adrenal glands have two parts:

1. Adrenal Medulla

• Hormones: Adrenaline & Noradrenaline
• Function: Fight or Flight Response
  • ↑ Heart rate, ↑ Blood glucose, ↑ Alertness

2. Adrenal Cortex

• Glucocorticoids (Cortisol) → metabolism, anti-inflammatory
• Mineralocorticoids (Aldosterone) → electrolyte balance
• Androgens → secondary sexual traits

Disorders

• Addison’s disease → low hormone production
• Cushing’s syndrome → excess cortisol

👉 Adrenal gland = “emergency response system”

Pancreas: The Blood Sugar Regulator

The pancreas is a mixed gland:

• Exocrine (digestive enzymes)
• Endocrine (Islets of Langerhans)

Hormones

• Glucagon (α-cells) → increases blood glucose
• Insulin (β-cells) → decreases blood glucose

Mechanism

• Glucagon → glycogenolysis + gluconeogenesis
• Insulin → glycogenesis + glucose uptake

If imbalance persists → Diabetes Mellitus

👉 Pancreas = “glucose balance regulator”

Testis (Male)

The testis, located in the scrotal sac, performs a dual role:

• Primary sex organ (reproduction)
• Endocrine gland (hormone secretion)

Hormone Produced

• Androgens, mainly testosterone, secreted by Leydig (interstitial) cells

Aspect	Role of Androgen (Testosterone)
Secondary Sexual Characteristics	Promotes facial hair growth, deepening of voice, and increased muscle growth.
Reproductive Function	Regulates development and function of male reproductive organs; stimulates sperm production; regulates libido.
Bone and Muscle Health	Helps maintain bone density and muscle mass.

👉 In essence: Testis = “driver of male identity and reproductive capacity”

Ovary (Female)

The ovaries, located on either side of the uterus, also perform dual functions:

Aspect	Description
Egg Production	Produces one ovum (egg cell) during each menstrual cycle.
Hormone Production	Produces two steroid hormones: estrogen and progesterone.

Hormonal Functions

Hormone	Functions
Estrogen	Promotes growth of female secondary sex organs; development of ovarian follicles; development of secondary sexual characteristics (e.g., high-pitched voice); growth of mammary glands
Progesterone	Supports pregnancy; stimulates formation of milk-producing alveoli; aids in milk secretion

Hormones from Other Organs

Organ/System	Hormone	Function
Heart	Atrial Natriuretic Factor (ANF)	Released during high blood pressure; increases sodium and water excretion; relaxes blood vessels, thereby lowering blood pressure
Kidneys	Erythropoietin (EPO)	Stimulates red blood cell production in response to low oxygen levels
Gastrointestinal Tract	Gastrin	Stimulates secretion of gastric acid and pepsinogen for digestion
	Secretin	Stimulates pancreas to release bicarbonate ions to neutralise stomach acid
	Cholecystokinin (CCK)	Stimulates release of digestive enzymes from pancreas and bile from gallbladder
	Gastric Inhibitory Peptide (GIP)	Slows gastric emptying and stimulates insulin release
Other Tissues	Growth Factors	Regulate cell growth, repair, and regeneration in tissues like skin, bones, and muscles