Chemical Coordination and Integration

Endocrine Glands and Hormones

• The neural system provides fast coordination between organs, but its effects are short-lived.
• Also, nerve fibres do not reach all body cells and many body functions need continuous regulation.
• Therefore, the neural system works together with the endocrine system to regulate and coordinate body functions through hormones.

Endocrine Glands and Hormones

• Ductless Glands: Endocrine glands lack ducts and release their secretions directly into the blood.
• Hormones: These glands secrete chemicals called hormones, which are carried by blood to target organs located far from the site of secretion.
• Definition of Hormones:
  • “Hormones are non-nutrient chemicals produced in trace amounts that act as intercellular messengers, and their site of action is usually different from their site of synthesis.”

Examples:

• Invertebrates: possess very simple endocrine systems with only a few hormones.
• Vertebrates (including humans): have a well-developed endocrine system with many chemicals acting as hormones for coordination.

Additional Gland Types (for clarity):

• “Exocrine glands release their secretions through ducts (e.g., sweat and salivary glands).”
• “Heterocrine or mixed glands have both endocrine and exocrine functions (e.g., pancreas).”

Properties of Hormones

• Produced in very small quantities.
• Have low molecular weight.
• They do not provide energy or building material.
• They act at very low concentrations.
• They may act slowly or rapidly.
• They are non-antigenic.
• They usually do not show cumulative effects.
• Hormonal actions are generally irreversible.
• Excess or deficiency of hormones causes disorders.
• Some hormones are produced as inactive prohormones (e.g., proinsulin).

Human Endocrine System

• Components: The endocrine system includes endocrine glands and hormone-producing tissues or cells distributed throughout the body.
• Major Endocrine Glands:
  • Hypothalamus
  • Pituitary
  • Pineal
  • Thyroid
  • Parathyroid
  • Adrenal
  • Pancreas
  • Thymus
  • Gonads: Testis in males and ovary in females

Other Hormone-Producing Organs:

• Gastrointestinal Tract
• Liver
• Kidney
• Heart

“The study of endocrine glands and hormones is called endocrinology.”

1. Hypothalamus

• Location: In the basal part of the diencephalon in the forebrain.
• Function: Regulates a wide range of body functions and acts as a link between the neural and endocrine systems.
• “The hypothalamus is considered the supreme coordinating centre of the endocrine system.”
• Hormone Production: Hormones are produced by groups of neurosecretory cells called nuclei.
• Types of Hypothalamic Hormones:
  1. Releasing Hormones: Stimulate the secretion of specific pituitary hormones.
     • Example: Gonadotrophin releasing hormone (GnRH) – stimulates the synthesis and release of gonadotrophins from the pituitary.
  2. Inhibiting Hormones: Inhibit the secretion of specific pituitary hormones.
     • Example: Somatostatin – inhibits the release of growth hormone from the pituitary.
• Imortant hypothalamic hormones: synthesized by hypothalamus but stored in Pituitary.
  1. Oxytocin
  2. Vasopresssin
• Pathway of Hypothalamic Hormones:
  • Hypothalamic hormones are synthesized in hypothalamic neurons, pass through axons, and are released from nerve endings.
  • They reach the pituitary gland through a portal circulatory system.
• Regulation of Pituitary by Hypothalamus:
  • Anterior Pituitary: is regulated by hypothalamic releasing and inhibiting hormones.
  • Posterior Pituitary: is under direct neural control of the hypothalamus.

2. Pituitary Gland

• Location:
  • The pituitary gland is a small, pea-shaped gland located in a bony cavity called sella turcica.
  • It is attached to the hypothalamus by a stalk (infundibulum).
• “Pituitary gland is often called the master endocrine gland because it regulates and controls the activity of other endocrine glands.”
• Structural Division:
  • The pituitary gland is divided into two main parts:

Structural Division:

• The pituitary gland is divided into two main parts:
  1. Adenohypophysis (anterior pituitary)
  2. Neurohypophysis (posterior pituitary)

Adenohypophysis (Anterior Pituitary):

• The adenohypophysis has two regions:
  1. pars distalis
  2. pars intermedia.

1. Pars Distalis (Anterior Pituitary Proper)

• Hormones Produced :
  1. Growth Hormone (GH):
     • Function: Promotes body growth by stimulating protein synthesis.
     • Hypersecretion Causes:
       • In children – Gigantism
       • In adults – Acromegaly (severe disfigurement, mainly of face and extremities)
     • Hyposecretion: Pituitary dwarfism
  2. Prolactin (PRL):
     • Function: Stimulates growth of mammary glands and milk production during lactation.
  3. Thyroid Stimulating Hormone (TSH):
     • Function: Stimulates the thyroid gland to produce thyroid hormones.
  4. Adrenocorticotrophic Hormone (ACTH):
     • Function: Stimulates the adrenal cortex to secrete glucocorticoids.
  5. Gonadotrophins:
     1. Luteinizing Hormone (LH):
        • Males: Stimulates Leydig cells of testis to produce androgens (testosterone).
        • Females: Induces ovulation and maintains corpus luteum.
     2. Follicle Stimulating Hormone (FSH):
        • Males: Regulates spermatogenesis.
        • Females: Stimulates growth and maturation of ovarian follicles.

LH and FSH together are called gonadotrophins because they stimulate gonadal activity.

2. Pars Intermedia

• Hormones Secreted :
  1. Melanocyte Stimulating Hormone (MSH):
     • Function:
       • Regulates skin pigmentation by influencing melanin distribution.
       • In humans, the pars intermedia is very small and almost merged with pars distalis.
     • Hypersecretion:
       • May cause hyperpigmentation.

“In humans, pars intermedia is almost merged with pars distalis, and the role of MSH in pigmentation is considered doubtful.”

Neurohypophysis (Posterior Pituitary)

• The posterior pituitary does not synthesize hormones.
• It stores and releases hormones produced by the hypothalamus and transported through axons.

1. Oxytocin:
   • Function:
     • Stimulates contraction of uterine muscles during childbirth.
     • Causes milk ejection from mammary glands during breastfeeding.
     • Acts mainly on smooth muscles.
2. Vasopressin (Antidiuretic Hormone, ADH):
   • Function:
     • Promotes reabsorption of water and electrolytes in kidney tubules.
     • Reduces urine output (anti-diuretic effect) and helps maintain water balance.
     • Also causes contraction of blood vessels, increasing blood pressure.
   • Hyposecretion:
     • Leads to diabetes insipidus, characterised by excessive urination and dehydration.

Important Disorder – Degeneration of the anterior pituitary may lead to Simmonds disease, marked by dry, wrinkled facial skin and premature ageing.

Pituitary Part	Hormones	Functions
Adenohypophysis (Anterior Pituitary)	Growth Hormone (GH)	Promotes growth
	Prolactin (PRL)	Stimulates mammary gland growth and milk production
	Thyroid Stimulating Hormone (TSH)	Stimulates thyroid hormone production
	Adrenocorticotrophic Hormone (ACTH)	Stimulates glucocorticoid production in adrenal cortex
	Luteinizing Hormone (LH)	Gonadal activity, ovulation, androgen secretion
	Follicle Stimulating Hormone (FSH)	Regulates spermatogenesis in males and ovarian follicle growth in females
Pars Intermedia	Melanocyte Stimulating Hormone (MSH)	Regulates skin pigmentation
Neurohypophysis (Posterior Pituitary)	Oxytocin	Uterine contraction and milk ejection
	Vasopressin (ADH)	Water reabsorption and reduced urine output

3. Pineal Gland

• Location: Located on the dorsal side of the forebrain.
  • “The pineal gland is also called epiphysis cerebri due to its position.”
• Hormone Secreted: Melatonin.
• Functions:
  • Regulates the 24-hour (diurnal) rhythm of the body.
  • Maintains normal sleep–wake cycle.
  • Helps regulate body temperature.
  • Influences metabolism, pigmentation, menstrual cycle, and defence capability.

Additional Points

• Melatonin acts antagonistically to melanocyte stimulating hormone.
• It has an inhibitory effect on sexual maturation and sexual activity by opposing gonadotrophins.
• Serotonin, a neurotransmitter, is also present in the pineal gland.

4. Thyroid Gland

• Location:
  • The thyroid gland is one of the largest endocrine glands.
  • It is located in the neck, just below the larynx, with two lobes present on either side of the trachea.
  • The two lobes are connected by a thin flap of connective tissue called the isthmus.
• Structure:
  • The thyroid gland is made up of thyroid follicles and stromal tissue.
  • Each follicle is lined by follicular cells and filled with a homogenous material called colloid.
  • Follicular cells synthesize thyroid hormones.
  • The stromal tissue contains blood capillaries and specialised parafollicular or C cells.
• Hormones Secreted:
  1. Tetraiodothyronine (T4) or Thyroxine.
  2. Triiodothyronine (T3).
  3. Thyrocalcitonin (TCT)
• Thyroxine and triiodothyronine are iodinated derivatives of the amino acid tyrosine and are produced by follicular cells.
• Iodine is essential for their synthesis.
• T3 is more active and potent than T4.
• A large amount of T4 is converted into T3 in the liver, kidneys, and some other tissues.
• Thyrocalcitonin is a protein hormone secreted by C cells.

Functions of Thyroid Hormones (T3 and T4)

• Regulate basal metabolic rate (BMR) of the body.
• Regulate metabolism of carbohydrates, proteins, and fats.
• Support red blood cell formation.
• Regulate growth of body tissues and development of mental faculties.
• Stimulate tissue differentiation and development.
• Help maintain water and electrolyte balance.

Function of Thyrocalcitonin (TCT)

• Lowers blood calcium level.
• Suppresses release of calcium ions from bones.
• Acts antagonistically to parathyroid hormone to maintain calcium balance.

Disorders of Thyroid Gland

• Hypothyroidism:
  • Caused due to deficiency of thyroid hormones, commonly due to iodine deficiency.
  • In infants:
    • Cretinism – characterised by stunted physical growth, undeveloped sex organs, and mental retardation.
  • In adults:
    • Myxoedema – characterised by puffy appearance, fat accumulation under skin, reduced alertness, and slow heart rate.
    • Simple goitre – enlargement of thyroid gland due to iodine deficiency. It may lead to cretinism or myxoedema.
    • Hypothyroidism may also lead to irregular menstrual cycles in adults.

• Hyperthyroidism:
  • Caused due to excessive secretion of thyroid hormones.
  • Graves’ disease or exophthalmic goitre is a common form, characterised by:
    • Enlarged thyroid gland
    • Protruding eyes (exophthalmos)
    • Increased metabolic rate
    • Weight loss
    • Nervousness and rapid heartbeat

5. Parathyroid Gland

• Location:
  • In humans, four parathyroid glands are present on the posterior surface of the thyroid gland, one pair in each thyroid lobe.
• Structure:
  • Parathyroid glands contain two types of cells:
    1. Chief cells – small cells that secrete parathyroid hormone
    2. Oxyphil cells – larger cells with unclear function
• Hormone Secreted:
  • Parathyroid Hormone (PTH) or Parathormone:
    • It is a peptide hormone, and its secretion is regulated by the level of calcium ions in the blood.
• Functions of Parathyroid Hormone (PTH):
  • Increases blood calcium level.
  • Stimulates bone resorption, releasing calcium from bones into blood.
  • Enhances reabsorption of calcium by kidney tubules.
  • Increases absorption of calcium from digested food.
  • Works along with thyrocalcitonin to maintain calcium balance in the body.

Disorders of Parathyroid Gland

• Hyperparathyroidism
  • Caused by persistent excess secretion of PTH.
  • Leads to high blood calcium levels.
  • May result in osteoporosis, increased bone fractures, kidney stones, and osteitis fibrosa cystica.
• Hypoparathyroidism
  • Caused by deficiency of PTH.
  • Leads to low calcium levels in blood and tissues due to excessive loss of calcium in urine.
  • Increases excitability of nerves and muscles.
  • Causes muscle cramps, convulsions, and sustained muscle contractions called tetany, especially affecting muscles of face, larynx, hands, and feet.

6. Thymus

• Location:
  • located between the lungs, behind the sternum, and on the ventral side of the aorta.
• Nature:
  • It is a lymphoid organ. At birth, it is well developed, but it gradually shrinks (atrophies) with age.
• Hormones:
  • The thymus secretes peptide hormones called thymosins.
• Functions of Thymosins:
  • Stimulate the development and differentiation of T-lymphocytes.
  • Help in establishing cell-mediated immunity.
  • Promote production of antibodies, thus supporting humoral immunity.
  • Increase resistance to infections.
  • Help in the development of the immune system during early life.
  • Hasten attainment of sexual maturity.
• Age-related Change:
  • With advancing age, the thymus gradually degenerates.
  • This leads to reduced secretion of thymosins.
  • As a result, immune responses become weaker in old individuals.
• Additional Structural Feature:
  • The thymus contains Hassall’s corpuscles, which are spherical or oval bodies and have a phagocytic role.

7. Adrenal Gland

• Location:
  • Adrenal glands are paired structures, with one gland located above each kidney.
• General Structure:
  • Each adrenal gland has two distinct regions:
    1. Adrenal cortex – outer part
    2. Adrenal medulla – inner part

The adrenal glands are often called emergency glands because of their role during stress.

Adrenal Medulla

Nature and Control

• The adrenal medulla develops from neuroectoderm.
• It contains chromaffin cells, which are modified postganglionic sympathetic neurons.
• It functions together with the sympathetic nervous system as a single unit called the sympathetico-adrenal system.

Hormones

1. Adrenaline (Epinephrine)
2. Noradrenaline (Norepinephrine)

• These hormones are collectively called catecholamines and are derived from the amino acid tyrosine.
• They are also known as emergency hormones or fight-or-flight hormones.

Adrenal Cortex

Nature

• The adrenal cortex develops from mesoderm.
• It secretes a group of hormones collectively called corticoids.

Layers of Adrenal Cortex

• Zona glomerulosa – outer layer
• Zona fasciculata – middle and widest layer
• Zona reticularis – inner layer

Hormones of Adrenal Cortex

1. Mineralocorticoids (e.g., Aldosterone)
   • Regulate water and electrolyte balance.
   • Promote reabsorption of sodium and water from kidney tubules.
   • Increase excretion of potassium and phosphate ions in urine.
   • Help maintain blood volume, osmotic pressure, and blood pressure.
   • Aldosterone secretion is stimulated by low sodium levels in blood.
2. Glucocorticoids (e.g., Cortisol)
   • Regulate carbohydrate metabolism.
   • Stimulate gluconeogenesis, lipolysis, and proteolysis.
   • Inhibit uptake and utilisation of amino acids by cells.
   • Maintain cardiovascular and kidney functions.
   • Produce anti-inflammatory effects and suppress immune responses.
   • Stimulate red blood cell production.
   • Cortisol is secreted in higher amounts during stress and is therefore called the stress hormone.
3. Androgenic Steroids (Sex Corticoids)
   • Include male and female sex hormones.
   • Androgens promote development of male secondary sexual characters.
   • They also contribute to growth of body hair during puberty.

Disorders of Adrenal Gland

• Addison’s Disease
  • Caused by deficiency of adrenal cortex hormones, mainly glucocorticoids and mineralocorticoids.
  • Leads to weakness, fatigue, and altered carbohydrate metabolism.
  • Associated with low blood sugar, low sodium levels, and dehydration.
• Cushing’s Syndrome
  • Caused by excess secretion of cortisol.
  • Leads to high blood sugar, increased blood pressure, and electrolyte imbalance.
• Conn’s Syndrome
  • Caused by excessive secretion of aldosterone.
  • Leads to high sodium levels, low potassium levels, increased blood volume, and high blood pressure.
• Adrenal Virilism
  • Caused by excess secretion of androgenic steroids.
  • Leads to appearance of male secondary sexual characters in females.
• Gynaecomastia
  • Enlargement of mammary glands in males.
  • Caused by excessive estrogen secretion or reduced testosterone levels.

8. Pancreas

• Type:
  • The pancreas is a composite gland because it functions as both an exocrine and an endocrine gland.
• Endocrine Part:
  • The endocrine portion of the pancreas consists of Islets of Langerhans.
  • There are about 1–2 million islets in a normal human pancreas, forming only about 1–2% of the pancreatic tissue.
• Cells of Islets of Langerhans:
  1. Alpha (α) cells
     • Secrete glucagon.
  2. Beta (β) cells
     • Secrete insulin.
• Additional cells (NCERT important):
  • Delta (D) cells – secrete somatostatin.
  • PP or F cells – secrete pancreatic polypeptide.
• Hormones and Functions:
  1. Glucagon:
     • It is a peptide hormone.
     • It increases blood glucose level (hyperglycemic hormone).
     • Acts mainly on liver cells.
     • Stimulates breakdown of glycogen into glucose (glycogenolysis).
     • Stimulates formation of glucose from non-carbohydrate sources (gluconeogenesis).
     • Reduces cellular uptake and utilisation of glucose.
  2. Insulin:
     • It is a peptide hormone.
     • It decreases blood glucose level (hypoglycemic hormone).
     • Acts mainly on liver cells and adipose tissue.
     • Enhances cellular uptake and utilisation of glucose.
     • Stimulates conversion of glucose into glycogen (glycogenesis).

Insulin and glucagon together maintain glucose homeostasis in the blood

• Disorder:
  • Diabetes Mellitus:
    • Caused due to prolonged high blood glucose level (hyperglycemia).
    • Characterised by loss of glucose through urine and formation of harmful ketone bodies.
    • Managed effectively by insulin therapy.

9. Testis

• Location:
  • In males, a pair of testes is present in the scrotal sac, outside the abdominal cavity.
• Functions:
  • Acts as the primary male sex organ.
  • Functions as an endocrine gland.
• Structure:
  • Testis is composed of seminiferous tubules and interstitial (stromal) tissue.
• Hormone Secreting Cells:
  • Leydig cells (interstitial cells) present in the spaces between seminiferous tubules.
• Hormones:
  • Leydig cells secrete androgens, mainly testosterone.
• Functions of Androgens:
  • Regulate development, maturation, and function of male accessory sex organs such as epididymis, vas deferens, seminal vesicles, prostate gland, and urethra.
  • Stimulate spermatogenesis.
  • Promote muscular growth.
  • Cause growth of facial and axillary hair.
  • Produce male secondary sexual characters like deep voice and aggressiveness.
  • Influence male sexual behaviour (libido).
  • Produce anabolic effects on protein and carbohydrate metabolism.

10. Ovary

• Location:
  • In females, a pair of ovaries is located in the abdominal cavity.
• Functions:
  • Acts as the primary female sex organ.
  • Produces one ovum during each menstrual cycle.
  • Functions as an endocrine gland.
• Structure:
  • The ovary is composed of ovarian follicles and stromal tissue.
• Hormones:
  1. Estrogen:
     • Synthesised mainly by growing ovarian follicles.
     • Stimulates growth and activity of female reproductive organs.
     • Promotes development of ovarian follicles.
     • Causes appearance of female secondary sexual characters such as high-pitched voice.
     • Stimulates development of mammary glands.
     • Regulates female sexual behaviour.
  2. Progesterone:
     • Secreted mainly by the corpus luteum formed after ovulation.
     • Supports pregnancy.
     • Stimulates formation of alveoli in mammary glands.
     • Promotes milk secretion.

Hormones of Heart, Kidney, and Gastrointestinal Tract

Heart

• Atrial Natriuretic Factor (ANF):
  • It is a peptide hormone secreted by the atrial walls of the heart.
  • Decreases blood pressure by causing dilation of blood vessels.

Kidney

• Erythropoietin:
  • It is a peptide hormone secreted by juxtaglomerular cells of the kidney.
  • Stimulates formation of red blood cells in bone marrow (erythropoiesis).

Gastrointestinal Tract

1. Gastrin:
   • Stimulates gastric glands to secrete hydrochloric acid and pepsinogen.
2. Secretin:
   • Stimulates exocrine pancreas to secrete water and bicarbonate ions.
3. Cholecystokinin (CCK):
   • Stimulates pancreas to secrete digestive enzymes.
   • Stimulates gall bladder to release bile.
4. Gastric Inhibitory Peptide (GIP):
   • Inhibits gastric secretion and gastric motility.

• Growth Factors:
  • Secreted by various non-endocrine tissues.
  • Essential for normal growth, repair, and regeneration of tissues.

Mechanism of Hormone Action

How Hormones Work

• Hormones bring about their effects by binding to specific proteins called hormone receptors.
• These receptors are present only in target tissues, which is why hormones act only on specific cells.

Receptor Types

1. Membrane-bound receptors:
   • present on the cell membrane of target cells.
2. Intracellular receptors:
   • located inside the cell, usually in the cytoplasm or nucleus.

Hormone–Receptor Complex

• When a hormone binds to its specific receptor, a hormone–receptor complex is formed.
• This complex initiates biochemical changes inside the target tissue.
• These changes regulate cellular metabolism, growth, differentiation, and overall physiological functions.

Chemical Nature of Hormones

Based on chemical structure, hormones can be grouped into the following types:

1. Protein, Peptide, and Polypeptide Hormones
   • Examples: Insulin, glucagon, pituitary hormones, hypothalamic hormones, parathyroid hormone, and vasopressin.
2. Steroid Hormones
   • Examples: Cortisol, testosterone, estradiol, and progesterone.
3. Iodothyronines:
   • Example: Thyroid hormones.
4. Amino-acid Derivatives:
   • Example: Epinephrine and norepinephrine.

Mechanism of Action (Based on Receptor Type)

1. Action Through Membrane-bound Receptors
   • Protein, peptide, and polypeptide hormones, as well as most amino acid derivative hormones, are large and cannot enter the target cell.
   • These hormones act by binding to membrane-bound receptors and function as first messengers.
   • After hormone binding, the receptor activates enzymes on the inner side of the cell membrane.
   • This leads to the formation of second messengers such as cyclic AMP (cAMP), IP3, and calcium ions (Ca⁺⁺).
   • Second messengers amplify the hormone signal inside the cell.
   • They activate protein kinases and other enzymes, which bring about changes in cellular metabolism and physiological responses such as secretion, growth, or muscle contraction.
2. Action Through Intracellular Receptors
   • Steroid hormones and iodothyronines are lipid-soluble and can easily enter the target cell.
   • Inside the cell, these hormones bind to specific intracellular receptor proteins.
   • Each receptor binds to one hormone molecule, forming a hormone–receptor complex.
   • This complex moves into the nucleus and interacts directly with DNA.
   • It activates specific genes and stimulates transcription, leading to the formation of messenger RNA (mRNA).
   • The mRNA moves into the cytoplasm and directs ribosomes to synthesise specific proteins.
   • These proteins may be enzymes, structural proteins, receptor proteins, or secretory proteins.
   • The newly formed proteins produce long-lasting physiological and developmental effects such as growth, differentiation, and sexual maturation.

• Role in Homeostasis:
  • Hormones help maintain homeostasis by regulating the internal chemical environment of the body.
  • Their actions are coordinated through feedback mechanisms, ensuring stable body conditions despite external or internal changes.

Chapter Summary

Chemical Coordination by Hormones

• Chemical coordination in the body is carried out by hormones.
• Hormones are special chemical messengers that coordinate, integrate, and regulate various physiological functions.
• They act in very small amounts and produce specific effects on target tissues.

Functions of Hormones

• Hormones regulate metabolism, growth, development, reproduction, and maintenance of internal balance.
• They may act on endocrine glands or directly on specific target cells and tissues.

Components of the Endocrine System

• The human endocrine system includes the following major glands:
  • Hypothalamus
  • Pituitary
  • Pineal
  • Thyroid
  • Parathyroid
  • Thymus
  • Adrenal
  • Pancreas
  • Gonads – testis in males and ovary in females
• In addition, organs such as the gastrointestinal tract, kidney, and heart also secrete hormones.

Pituitary Gland

• The pituitary gland is divided into three parts: pars distalis, pars intermedia, and pars nervosa.
• Pars distalis secretes six trophic hormones that regulate growth and control other endocrine glands.
• Pars intermedia secretes one hormone.
• Pars nervosa stores and releases two hormones synthesised by the hypothalamus.
• Pituitary hormones play a central role in regulating growth, development, reproduction, and activity of peripheral endocrine glands.

Pineal Gland

• The pineal gland secretes melatonin.
• Melatonin regulates 24-hour (diurnal) rhythms such as the sleep–wake cycle and body temperature, and influences reproductive and metabolic functions.

Thyroid Gland

• Thyroid hormones regulate basal metabolic rate.
• They are essential for normal growth, neural system development, and erythropoiesis.
• They regulate metabolism of carbohydrates, proteins, and fats and influence menstrual cycle in females.
• Thyrocalcitonin lowers blood calcium level and helps maintain calcium balance.

Parathyroid Glands

• Parathyroid glands secrete parathyroid hormone (PTH).
• PTH regulates blood calcium levels and maintains calcium homeostasis by acting on bones, kidneys, and intestine.

Thymus Gland

• The thymus gland secretes thymosins.
• Thymosins help in differentiation and maturation of T-lymphocytes for cell-mediated immunity.
• They also promote antibody production for humoral immunity.
• The thymus shrinks with age, leading to reduced immune efficiency.

Adrenal Gland

• The adrenal gland consists of an outer adrenal cortex and an inner adrenal medulla.
  
• Adrenal medulla secretes epinephrine and norepinephrine, which:
  • Increase alertness, pupil dilation, sweating, and piloerection
  • Increase heart rate and respiration rate
  • Stimulate glycogenolysis, lipolysis, and proteolysis

• Adrenal cortex secretes:
  1. Glucocorticoids, which regulate carbohydrate, protein, and fat metabolism, stimulate gluconeogenesis, lipolysis, proteolysis, erythropoiesis, support cardiovascular and kidney functions, maintain blood pressure and glomerular filtration rate, and suppress inflammation
  2. Mineralocorticoids, which regulate water and electrolyte balance and maintain blood volume and blood pressure

Pancreas

• The endocrine part of the pancreas secretes insulin and glucagon.
• Glucagon increases blood glucose level by stimulating glycogenolysis and gluconeogenesis, causing hyperglycemia.
• Insulin lowers blood glucose level by stimulating glucose uptake, utilisation, and glycogenesis, causing hypoglycemia.
• Deficiency or resistance to insulin leads to diabetes mellitus.

Testis

• Testis secretes androgens.
• Androgens regulate development and function of male reproductive organs, male secondary sexual characteristics, spermatogenesis, male sexual behaviour, anabolic pathways, and erythropoiesis.

Ovary

• Ovary secretes estrogen and progesterone.
• Estrogen stimulates growth and maintenance of female reproductive organs and secondary sexual characteristics.
• Progesterone supports pregnancy, promotes development of mammary glands, and supports lactation.

Hormones of Heart and Kidney

• The heart secretes atrial natriuretic factor, which lowers blood pressure.
• The kidney secretes erythropoietin, which stimulates red blood cell production (erythropoiesis).

Hormones of Gastrointestinal Tract

• The gastrointestinal tract secretes gastrin, secretin, cholecystokinin, and gastric inhibitory peptide.
• These hormones regulate secretion of digestive juices, enzyme activity, bile release, and overall digestive processes.