Endocrine Glands and Hormones

Endocrine glands and hormones provide chemical coordination in the human body, working together with the neural system. This page covers what hormones are, the hypothalamus, pituitary, pineal, thyroid, parathyroid, thymus, adrenal, pancreas, testis and ovary, the hormones of the heart, kidney and gut, the related disorders, and how hormones act on target cells. It follows the NCERT Class 11 chapter Chemical Coordination and Integration. NEET often tests endocrine glands and hormones through match lists of hormones with their sources and actions, and through statements on hormone disorders.

1. Neural and Chemical Coordination

• The neural system provides rapid, point-to-point coordination among organs.
• Neural coordination is fast but short-lived.
• Nerve fibres do not innervate (supply nerves to) all the cells of the body.
• Cellular functions, however, need to be regulated continuously.
• This special kind of coordination and integration is provided by hormones.
• ★ Exam imp The neural system and the endocrine system jointly coordinate and regulate the physiological functions of the body.

2. Endocrine Glands and Hormones

• Endocrine glands lack ducts, so they are called ductless glands.
• The secretions of endocrine glands are called hormones.
• Classical definition: a hormone is a chemical produced by an endocrine gland, released into the blood and carried to a distantly located target organ.

• The new definition covers many new molecules, in addition to the hormones of the organised endocrine glands.
• Invertebrates have very simple endocrine systems with few hormones.
• ★ Exam imp In vertebrates, a large number of chemicals act as hormones and provide coordination.

3. Human Endocrine System

• The endocrine system is made of the endocrine glands and the hormone-producing diffused tissues or cells located in different parts of the body.
• ★ Exam imp The organised endocrine bodies are the pituitary, pineal, thyroid, adrenal, pancreas, parathyroid, thymus and gonads (testis in males, ovary in females).
• Some other organs also produce hormones: the gastrointestinal tract, liver, kidney and heart.
• The hypothalamus also produces hormones, so it is studied along with the major endocrine glands.

4. The Hypothalamus

• The hypothalamus is the basal part of the diencephalon of the forebrain.
• It regulates a wide spectrum of body functions.
• It contains several groups of neurosecretory cells called nuclei, which produce hormones.
• ★ Exam imp Hypothalamic hormones regulate the synthesis and secretion of pituitary hormones.

How hypothalamic hormones reach the pituitary

1. The hormones are produced in the hypothalamic neurons.
2. They pass down through the axons of these neurons.
3. They are released from the nerve endings.
4. They reach the pituitary gland through a portal circulatory system.
5. They regulate the functions of the anterior pituitary.

• ★ Exam imp The posterior pituitary is under the direct neural regulation of the hypothalamus, not under the portal route.

5. The Pituitary Gland

5.1 Location and parts

• The pituitary lies in a bony cavity called the sella turcica (also printed as sella tursica).
• It is attached to the hypothalamus by a stalk.
• Anatomically, it is divided into the adenohypophysis and the neurohypophysis.
• The adenohypophysis has two portions: the pars distalis and the pars intermedia.
• The pars distalis is commonly called the anterior pituitary.
• The neurohypophysis (pars nervosa) is also called the posterior pituitary.
• ★ Exam imp In humans, the pars intermedia is almost merged with the pars distalis.

5.2 Pituitary hormones and their actions

• ★ Exam imp LH and FSH stimulate gonadal activity, so they are called gonadotrophins.
• Graafian follicles: fully mature ovarian follicles, which LH makes ovulate.
• Corpus luteum: the structure formed from the remnants of the graafian follicle after ovulation.
• Melanocytes: melanin-containing cells of the skin.
• Diuresis: loss of water through urine. Vasopressin reduces diuresis, so it is also called the anti-diuretic hormone (ADH).

5.3 Disorders of pituitary hormones

• Acromegaly is hard to diagnose in its early stages.
• It often goes undetected for many years, until changes in external features become noticeable.

6. The Pineal Gland

• The pineal gland is located on the dorsal side of the forebrain.
• It secretes a hormone called melatonin.
• ★ Exam imp Melatonin plays a very important role in regulating the 24-hour (diurnal) rhythm of the body.
• For example, it maintains the normal rhythms of the sleep-wake cycle and body temperature.
• Melatonin also influences metabolism, pigmentation, the menstrual cycle and our defence capability.

7. Thyroid and Parathyroid Glands

7.1 Structure of the thyroid gland

• The thyroid has two lobes, located on either side of the trachea.
• The two lobes are joined by a thin flap of connective tissue called the isthmus.
• The gland is composed of follicles and stromal tissues.
• Each thyroid follicle is made of follicular cells enclosing a cavity.

7.2 Thyroid hormones and their functions

• ★ Exam imp The follicular cells synthesise two hormones: tetraiodothyronine or thyroxine (${\mathrm{T}}_4$) and triiodothyronine (${\mathrm{T}}_3$).
• ★ Exam imp Iodine is essential for the normal rate of hormone synthesis in the thyroid.
• ★ Exam imp The thyroid also secretes a protein hormone, thyrocalcitonin (TCT), which regulates blood calcium levels by lowering them.

Functions of thyroid hormones

• Regulate the basal metabolic rate (BMR).
• Support the process of red blood cell formation (erythropoiesis).
• Control the metabolism of carbohydrates, proteins and fats.
• Influence the maintenance of water and electrolyte balance.
• Help in the development and maturation of the central neural system, and influence the menstrual cycle.

7.3 Disorders of the thyroid

7.4 Parathyroid glands

• ★ Exam imp In humans, four parathyroid glands lie on the back side of the thyroid, one pair in each of the two lobes.
• They secrete a peptide hormone called parathyroid hormone (PTH).
• The secretion of PTH is regulated by the circulating levels of calcium ions.

Actions of PTH

• Acts on bones and stimulates bone resorption (dissolution or demineralisation).
• Stimulates reabsorption of $\mathrm{C}{\mathrm{a}}^{2+}$ by the renal tubules.
• Increases $\mathrm{C}{\mathrm{a}}^{2+}$ absorption from the digested food.
• ★ Exam imp Net result: blood $\mathrm{C}{\mathrm{a}}^{2+}$ rises, so PTH is a hypercalcemic hormone.
• Along with TCT, PTH plays a significant role in calcium balance in the body.

8. The Thymus

• The thymus is a lobular structure located between the lungs, behind the sternum, on the ventral side of the aorta.
• It plays a major role in the development of the immune system.
• It secretes peptide hormones called thymosins.
• ★ Exam imp Thymosins play a major role in the differentiation of T-lymphocytes, which provide cell-mediated immunity.
• Thymosins also promote the production of antibodies, which provide humoral immunity.
• The thymus degenerates in old individuals, so thymosin production falls.
• As a result, the immune responses of old persons become weak.

9. The Adrenal Gland

9.1 Structure

• The body has one pair of adrenal glands, one above each kidney.
• Each gland has two types of tissue: the central adrenal medulla and the outer adrenal cortex.
• The adrenal cortex has three layers:

9.2 Adrenal medulla

• The adrenal medulla secretes adrenaline (epinephrine) and noradrenaline (norepinephrine).
• These two are commonly called catecholamines.
• They are secreted rapidly in response to stress of any kind and during emergency situations.
• ★ Exam imp Hence they are called emergency hormones or hormones of fight or flight.

Effects of catecholamines

• Increase alertness, pupillary dilation, piloerection (raising of hairs) and sweating.
• Increase the heart beat, the strength of heart contraction and the rate of respiration.
• ★ Exam imp Stimulate the breakdown of glycogen, which raises the concentration of glucose in blood.
• Stimulate the breakdown of lipids and proteins.

9.3 Adrenal cortex

• The adrenal cortex secretes many hormones, commonly called corticoids.

Actions of glucocorticoids (mainly cortisol)

• Stimulate gluconeogenesis, lipolysis and proteolysis.
• Inhibit the cellular uptake and utilisation of amino acids.
• Cortisol helps maintain the cardio-vascular system (including blood pressure) and kidney functions (including the glomerular filtration rate).
• ★ Exam imp Cortisol produces anti-inflammatory reactions and suppresses the immune response.
• Cortisol stimulates RBC production.

Actions of aldosterone

• Acts mainly at the renal tubules.
• ★ Exam imp Stimulates reabsorption of $\mathrm{N}{\mathrm{a}}^{+}$ and water and excretion of ${\mathrm{K}}^{+}$ and phosphate ions.
• Thus maintains electrolytes, body fluid volume, osmotic pressure and blood pressure.

• ★ Exam imp Addison's disease: underproduction of adrenal cortex hormones alters carbohydrate metabolism and causes acute weakness and fatigue.

10. The Pancreas

• The pancreas is a composite gland: it acts as both an exocrine and an endocrine gland.
• The endocrine pancreas consists of the Islets of Langerhans.
• ★ Exam imp A normal human pancreas has about 1 to 2 million islets, forming only 1 to 2 per cent of the pancreatic tissue.
• The two main cell types of an islet are $\alpha$-cells, which secrete glucagon, and $\beta$-cells, which secrete insulin.

• Glucagon plays an important role in maintaining normal blood glucose levels.
• Insulin plays a major role in the regulation of glucose homeostasis.
• Under insulin, glucose moves rapidly from blood into hepatocytes and adipocytes, so blood glucose falls.
• Glucose homeostasis in blood is maintained jointly by insulin and glucagon.

10.1 Diabetes mellitus

• ★ Exam imp Prolonged hyperglycemia leads to a complex disorder called diabetes mellitus.
• It is associated with loss of glucose through urine.
• Harmful compounds called ketone bodies are formed.
• It results from insulin deficiency and/or insulin resistance.
• Diabetic patients are successfully treated with insulin therapy.

11. Testis and Ovary

11.1 Testis

• A pair of testes is present in the scrotal sac, outside the abdomen, of males.
• The testis has dual functions: it is a primary sex organ and an endocrine gland.
• It is composed of seminiferous tubules and stromal or interstitial tissue.
• ★ Exam imp The Leydig cells (interstitial cells), present in the intertubular spaces, produce androgens, mainly testosterone.

Actions of androgens

• Regulate the development, maturation and functions of the male accessory sex organs: epididymis, vas deferens, seminal vesicles, prostate gland, urethra.
• Stimulate muscular growth, growth of facial and axillary hair, aggressiveness and a low pitch of voice.
• Play a major stimulatory role in spermatogenesis (formation of spermatozoa).
• Act on the central neural system and influence male sexual behaviour (libido).
• Produce anabolic (synthetic) effects on protein and carbohydrate metabolism, and stimulate erythropoiesis.

11.2 Ovary

• Females have a pair of ovaries located in the abdomen.
• The ovary is the primary female sex organ; it produces one ovum during each menstrual cycle.
• It also produces two groups of steroid hormones: estrogen and progesterone.
• The ovary is composed of ovarian follicles and stromal tissues.
• ★ Exam imp Estrogen is synthesised and secreted mainly by the growing ovarian follicles.
• After ovulation, the ruptured follicle becomes the corpus luteum, which secretes mainly progesterone.

12. Hormones of Heart, Kidney and Gastrointestinal Tract

• Some tissues that are not endocrine glands also secrete hormones.

How ANF lowers blood pressure

1. Blood pressure increases.
2. The atrial wall of the heart secretes ANF.
3. ANF causes dilation of the blood vessels.
4. Blood pressure is reduced.

• ★ Exam imp The GI tract secretes four major peptide hormones: gastrin, secretin, CCK and GIP.
• Together they regulate the secretion of digestive juices and help in digestion.

13. Mechanism of Hormone Action

• Hormones act on target tissues by binding to specific proteins called hormone receptors.
• ★ Exam imp Hormone receptors are located in the target tissues only.
• Membrane-bound receptors: receptors present on the cell membrane of target cells.
• Intracellular receptors: receptors present inside the target cell, mostly nuclear receptors (in the nucleus).
• Binding of a hormone to its receptor forms a hormone-receptor complex.
• ★ Exam imp Each receptor is specific to one hormone only, so receptors are specific.
• The complex brings about biochemical changes in the target tissue.
• Thus hormones regulate target tissue metabolism and hence physiological functions.

13.1 Chemical nature of hormones

13.2 Hormones with membrane-bound receptors

1. A hormone such as FSH binds its membrane-bound receptor on the target cell (an ovarian cell).
2. The hormone normally does not enter the target cell.
3. The hormone-receptor complex generates second messengers, such as cyclic AMP, $\mathrm{I}{\mathrm{P}}_3$ and $\mathrm{C}{\mathrm{a}}^{2+}$.
4. Second messengers regulate cellular metabolism, giving biochemical responses.
5. These lead to physiological responses, such as ovarian growth.

13.3 Hormones with intracellular receptors

1. A hormone such as estrogen (a steroid) enters the target cell (a uterine cell).
2. It binds its intracellular receptor, mostly in the nucleus, forming a hormone-receptor complex.
3. The complex interacts with the genome.
4. This regulates gene expression or chromosome function: mRNA forms, and then proteins.
5. Cumulative biochemical actions give physiological and developmental effects, such as tissue growth and differentiation.

14. Exam Essentials

All hormones at a glance

Hormones and their disorders

Pairs to Match

Numbers to Remember

• Organised endocrine bodies named: 8. Types of hypothalamic hormones: 2 (releasing, inhibiting).
• Pituitary: 2 anatomical divisions and 3 parts; hormones 6 (pars distalis) + 1 (pars intermedia) + 2 (pars nervosa).
• Melatonin: regulates the 24-hour (diurnal) rhythm.
• Thyroid: 2 lobes; 2 iodothyronines (${\mathrm{T}}_4$ and ${\mathrm{T}}_3$) plus TCT.
• Parathyroid glands: 4, one pair on each thyroid lobe.
• Adrenal glands: 1 pair; 2 tissues; 3 layers in the cortex; 2 medullary hormones.
• Islets of Langerhans: about 1 to 2 million, only 1 to 2 per cent of pancreatic tissue; 2 main cell types.
• Ovary: 1 ovum per menstrual cycle; 2 groups of steroid hormones.
• GI tract: 4 major peptide hormones. Chemical groups of hormones: 4.

15. Quick Revision

• Neural coordination is fast, point-to-point and short-lived; hormones provide continuous chemical coordination.
• Hormones: non-nutrient chemicals, intercellular messengers, produced in trace amounts; endocrine glands are ductless.
• Organised endocrine bodies: pituitary, pineal, thyroid, parathyroid, thymus, adrenal, pancreas, gonads.
• Hypothalamus: releasing (GnRH) and inhibiting (somatostatin) hormones; portal circulation to the anterior pituitary.
• Anterior pituitary: GH, PRL, TSH, ACTH, LH, FSH; pars intermedia: MSH; posterior pituitary stores oxytocin and ADH.
• GH excess: gigantism (or acromegaly in adults); GH deficiency: pituitary dwarfism; ADH defect: diabetes insipidus.
• Pineal: melatonin, the 24-hour rhythm of sleep-wake cycle and body temperature.
• Thyroid: ${\mathrm{T}}_4$, ${\mathrm{T}}_3$ (need iodine) and TCT; hypothyroidism gives goitre and cretinism; Graves' disease is hyperthyroidism.
• Parathyroid: four glands, PTH, hypercalcemic; acts on bone, renal tubules and gut.
• Thymus: thymosins for T-lymphocytes and antibodies; degenerates in old age.
• Adrenal medulla: catecholamines (fight or flight). Cortex: cortisol, aldosterone, androgens; Addison's disease.
• Pancreas: glucagon ($\alpha$) raises blood glucose; insulin ($\beta$) lowers it; prolonged hyperglycemia gives diabetes mellitus.
• Testis: androgens from Leydig cells. Ovary: estrogen from growing follicles, progesterone from the corpus luteum.
• ANF lowers BP; erythropoietin from JG cells forms RBCs; gastrin, secretin, CCK and GIP control digestive secretions.
• Membrane-bound receptors use second messengers (cAMP, $\mathrm{I}{\mathrm{P}}_3$, $\mathrm{C}{\mathrm{a}}^{2+}$); intracellular receptors act on the genome.

16. Solved Examples

17. Practice Questions

Common Mistakes to Avoid