Class 11 Biology NCERT Notes- Chapter 19: Chemical Coordination and Integration

Detailed Study Notes – Chapter 19: Chemical Coordination and Integration (Class 11 Biology, Notes, PDFs, Quizzes, MCQs)

1. Introduction to Body Coordination Systems

The physiological functions within the body are coordinated and regulated by two primary systems: the neural system and the endocrine system.

• Neural System: Provides rapid, point-to-point coordination. Its effects are fast but short-lived. Nerve fibres do not innervate all cells in the body, necessitating a complementary system for continuous regulation.
• Endocrine System: Works in conjunction with the neural system. It provides chemical coordination through hormones. This system is essential for continuous regulation of cellular functions throughout the body.

2. Endocrine Glands and Hormones

Endocrine Glands:

• These are ductless glands.
• Their secretions, called hormones, are released directly into the bloodstream.

Hormones:

• Classical Definition: A chemical produced by endocrine glands, released into the blood, and transported to a distantly located target organ.
• Current Scientific Definition: Hormones are non-nutrient chemicals that act as intercellular messengers and are produced in trace amounts. This broader definition includes molecules secreted by organized endocrine glands as well as other tissues.

Endocrine systems in invertebrates are typically simple with few hormones, whereas vertebrates have a large number of chemicals that function as hormones to provide complex coordination.

3. The Human Endocrine System

The human endocrine system consists of organized endocrine glands and hormone-producing diffused tissues/cells.

Organized Endocrine Glands:

• Hypothalamus
• Pituitary Gland
• Pineal Gland
• Thyroid Gland
• Parathyroid Gland
• Thymus
• Adrenal Gland
• Pancreas
• Gonads (Testis in males, Ovary in females)

Other Hormone-Producing Organs/Tissues:

• Gastrointestinal tract
• Liver
• Kidney
• Heart

3.1. The Hypothalamus

• Location: Basal part of the diencephalon (forebrain).
• Structure: Contains groups of neurosecretory cells called nuclei.
• Function: Regulates a wide spectrum of body functions, primarily by controlling the pituitary gland. It produces two types of hormones:
  • Releasing Hormones: Stimulate the secretion of pituitary hormones.
    • Example: Gonadotropin-releasing hormone (GnRH) stimulates the pituitary to synthesize and release gonadotrophins (LH and FSH).
  • Inhibiting Hormones: Inhibit the secretion of pituitary hormones.
    • Example: Somatostatin inhibits the release of growth hormone (GH) from the pituitary.
• Connection to Pituitary:
  • Anterior Pituitary: Hormones from hypothalamic neurons travel through axons, are released at nerve endings, and reach the anterior pituitary via a portal circulatory system.
  • Posterior Pituitary: Is under direct neural regulation of the hypothalamus. Hormones are synthesized in the hypothalamus and transported axonally to the posterior pituitary for storage and release.

3.2. The Pituitary Gland

• Location: Located in a bony cavity called the sella tursica, attached to the hypothalamus by a stalk.
• Anatomical Divisions:
  • Adenohypophysis: Consists of the Pars Distalis and Pars Intermedia.
  • Neurohypophysis: Also known as the Pars Nervosa or posterior pituitary.

Division	Common Name	Hormones Produced/Released	Functions
Pars Distalis	Anterior Pituitary	Growth Hormone (GH)	Regulates body growth.
		Prolactin (PRL)	Regulates mammary gland growth and milk formation.
		Thyroid Stimulating Hormone (TSH)	Stimulates thyroid gland to synthesize and secrete thyroid hormones.
		Adrenocorticotrophic Hormone (ACTH)	Stimulates adrenal cortex to synthesize and secrete glucocorticoids.
		Luteinizing Hormone (LH)	Males: Stimulates testis to synthesize and secrete androgens. Females: Induces ovulation and maintains corpus luteum.
		Follicle Stimulating Hormone (FSH)	Males: Regulates spermatogenesis (with androgens). Females: Stimulates growth and development of ovarian follicles.
Pars Intermedia		Melanocyte Stimulating Hormone (MSH)	Acts on melanocytes to regulate skin pigmentation. (In humans, it is almost merged with pars distalis).
Neurohypophysis	Posterior Pituitary	Oxytocin	Stimulates smooth muscle contraction (e.g., uterus during childbirth, milk ejection).
		Vasopressin (Anti-diuretic Hormone, ADH)	Stimulates kidney tubules to resorb water and electrolytes, reducing water loss via urine (diuresis).

• Disorders Related to Pituitary Hormones:
  • Gigantism: Caused by over-secretion of GH, leading to abnormal body growth.
  • Pituitary Dwarfism: Caused by low secretion of GH, resulting in stunted growth.
  • Acromegaly: Caused by excess secretion of GH in adults (especially middle age), leading to severe disfigurement (especially of the face). It can lead to serious complications and premature death if unchecked.
  • Diabetes Insipidus: Caused by impaired synthesis or release of ADH. The kidney’s ability to conserve water is diminished, leading to water loss and dehydration.

3.3. The Pineal Gland

• Location: On the dorsal side of the forebrain.
• Hormone: Secretes melatonin.
• Functions:
  • Regulates the 24-hour (diurnal) rhythm of the body.
  • Maintains the normal rhythm of the sleep-wake cycle and body temperature.
  • Influences metabolism, pigmentation, the menstrual cycle, and defence capability.

3.4. The Thyroid Gland

• Location: Composed of two lobes on either side of the trachea, interconnected by a thin flap of connective tissue called the isthmus.
• Structure: Composed of follicles and stromal tissues. Follicles are made of follicular cells enclosing a cavity.
• Hormones:
  • Thyroxine (T₄) and Triiodothyronine (T₃): Synthesized by follicular cells. Iodine is essential for their normal synthesis.
  • Thyrocalcitonin (TCT): A protein hormone that regulates blood calcium levels.
• Functions of T₃ and T₄:
  • Regulate the basal metabolic rate (BMR).
  • Support the process of red blood cell (RBC) formation.
  • Control the metabolism of carbohydrates, proteins, and fats.
  • Influence the maintenance of water and electrolyte balance.
• Disorders:
  • Hypothyroidism (Iodine Deficiency):
    • Goitre: Enlargement of the thyroid gland.
    • Cretinism: In babies of mothers with hypothyroidism during pregnancy, causes defective development leading to stunted growth, mental retardation, low IQ, abnormal skin, and deaf-mutism.
    • In adult women, can cause an irregular menstrual cycle.
  • Hyperthyroidism: Caused by cancer or nodules in the thyroid gland, leading to an abnormally high rate of hormone synthesis and secretion.
    • Exophthalmic Goitre (Graves’ disease): A form of hyperthyroidism characterized by thyroid enlargement, protrusion of eyeballs, increased BMR, and weight loss.

3.5. The Parathyroid Gland

• Location: Four glands located on the back side of the thyroid gland (one pair in each lobe).
• Hormone: Secretes Parathyroid Hormone (PTH), a peptide hormone.
• Function:
  • PTH is a hypercalcemic hormone, meaning it increases blood Ca²⁺ levels.
  • It acts on bones to stimulate bone resorption (demineralization).
  • It stimulates the reabsorption of Ca²⁺ by renal tubules.
  • It increases Ca²⁺ absorption from digested food.
  • Along with TCT from the thyroid, it plays a significant role in maintaining calcium balance in the body.

3.6. The Thymus

• Location: A lobular structure located between the lungs, behind the sternum, on the ventral side of the aorta.
• Hormone: Secretes peptide hormones called thymosins.
• Function:
  • Plays a major role in the development of the immune system.
  • Thymosins aid in the differentiation of T-lymphocytes, which provide cell-mediated immunity.
  • Thymosins also promote the production of antibodies to provide humoral immunity.
• Degeneration: The thymus degenerates in old individuals, leading to decreased production of thymosins and a weaker immune response.

3.7. The Adrenal Gland

• Location: A pair of glands, one located above each kidney.
• Structure: Composed of an outer adrenal cortex and a central adrenal medulla.

Adrenal Medulla:

• Hormones: Secretes adrenaline (epinephrine) and noradrenaline (norepinephrine), collectively known as catecholamines.
• Function: These are “emergency hormones” or “hormones of Fight or Flight,” rapidly secreted in response to stress. They:
  • Increase alertness, pupilary dilation, piloerection (raising of hairs), and sweating.
  • Increase heart rate, strength of heart contraction, and rate of respiration.
  • Stimulate the breakdown of glycogen (increasing blood glucose), lipids, and proteins.

Adrenal Cortex:

• Structure: Divided into three layers: zona glomerulosa (outer), zona fasciculata (middle), and zona reticularis (inner).
• Hormones: Secretes hormones called corticoids.
  • Glucocorticoids: Involved in carbohydrate metabolism. The main one is cortisol.
    • Functions: Stimulate gluconeogenesis, lipolysis, and proteolysis; inhibit cellular uptake of amino acids; maintain cardiovascular system and kidney functions; produce anti-inflammatory reactions; suppress the immune response; stimulate RBC production.
  • Mineralocorticoids: Regulate the balance of water and electrolytes. The main one is aldosterone.
    • Functions: Acts on renal tubules to stimulate reabsorption of Na⁺ and water and excretion of K⁺ and phosphate ions; helps maintain electrolytes, body fluid volume, osmotic pressure, and blood pressure.
  • Androgenic Steroids: Secreted in small amounts; play a role in the growth of axial, pubic, and facial hair during puberty.
• Disorder:
  • Addison’s disease: Caused by underproduction of hormones by the adrenal cortex. It alters carbohydrate metabolism, causing acute weakness and fatigue.

3.8. The Pancreas

• Type: A composite gland, acting as both an exocrine and endocrine gland.
• Endocrine Structure: The Islets of Langerhans (1 to 2 million, comprising 1-2% of pancreatic tissue).
• Cell Types and Hormones:
  • α-cells: Secrete glucagon (a peptide hormone).
  • β-cells: Secrete insulin (a peptide hormone).
• Functions:
  • Glucagon (Hyperglycemic hormone): Increases blood glucose levels by acting on liver cells (hepatocytes) to stimulate glycogenolysis and gluconeogenesis. It also reduces cellular glucose uptake.
  • Insulin (Hypoglycemic hormone): Decreases blood glucose levels by acting on hepatocytes and adipocytes to enhance cellular glucose uptake and utilization. It also stimulates the conversion of glucose to glycogen (glycogenesis).
• Glucose Homeostasis: Maintained jointly by insulin and glucagon.
• Disorder:
  • Diabetes Mellitus: A complex disorder caused by prolonged hyperglycemia. It is associated with the loss of glucose through urine and the formation of harmful compounds called ketone bodies. Patients can be treated with insulin therapy.

3.9. Gonads

Testis (in males):

• Location: A pair of testes present in the scrotal sac.
• Function: Acts as a primary sex organ and an endocrine gland.
• Hormone Production: The Leydig cells (interstitial cells) in the intertubular spaces produce a group of hormones called androgens, mainly testosterone.
• Functions of Androgens:
  • Regulate development and function of male accessory sex organs.
  • Stimulate muscular growth, facial and axillary hair growth, aggressiveness, and low pitch of voice.
  • Play a major stimulatory role in spermatogenesis.
  • Influence male sexual behaviour (libido).
  • Produce anabolic effects on protein and carbohydrate metabolism.

Ovary (in females):

• Location: A pair of ovaries located in the abdomen.
• Function: Primary female sex organ, produces one ovum per menstrual cycle.
• Hormone Production: Produces two groups of steroid hormones: estrogen and progesterone.
  • Estrogen: Synthesized and secreted mainly by growing ovarian follicles.
  • Progesterone: Secreted mainly by the corpus luteum, which forms from the ruptured follicle after ovulation.
• Functions of Hormones:
  • Estrogen: Stimulates growth of female secondary sex organs, development of ovarian follicles, appearance of female secondary sex characters (e.g., high pitch of voice), and mammary gland development; regulates female sexual behaviour.
  • Progesterone: Supports pregnancy; acts on mammary glands to stimulate alveoli formation and milk secretion.

4. Hormones of Heart, Kidney and Gastrointestinal Tract

Tissues that are not traditionally considered endocrine glands also secrete hormones.

Organ/Tissue	Cells	Hormone	Chemical Nature	Function
Heart	Atrial wall	Atrial Natriuretic Factor (ANF)	Peptide	Decreases blood pressure by causing dilation of blood vessels when blood pressure is high.
Kidney	Juxtaglomerular cells	Erythropoietin	Peptide	Stimulates erythropoiesis (formation of RBCs).
Gastrointestinal Tract	Endocrine cells	Gastrin	Peptide	Stimulates gastric glands to secrete hydrochloric acid and pepsinogen.
		Secretin	Peptide	Stimulates exocrine pancreas to secrete water and bicarbonate ions.
		Cholecystokinin (CCK)	Peptide	Stimulates pancreas to secrete enzymes and gall bladder to release bile juice.
		Gastric Inhibitory Peptide (GIP)	Peptide	Inhibits gastric secretion and motility.

Additionally, various non-endocrine tissues secrete growth factors essential for normal tissue growth and repair.

5. Mechanism of Hormone Action

Hormones affect target tissues by binding to specific proteins called hormone receptors. Each receptor is specific to one hormone.

• Hormone-Receptor Complex: The binding of a hormone to its receptor forms this complex, which leads to biochemical changes in the target tissue.

Types of Receptors:

1. Membrane-Bound Receptors: Located on the cell membrane of target cells.
2. Intracellular Receptors: Located inside the target cell, mostly in the nucleus (nuclear receptors).

Classification and Mechanism of Hormones:

Chemical Class	Examples	Receptor Type	Mechanism of Action
Peptide, Polypeptide, Protein Hormones	Insulin, glucagon, pituitary hormones, hypothalamic hormones	Membrane-bound	Do not enter the target cell. They generate second messengers (e.g., cyclic AMP, IP₃, Ca²⁺) which regulate cellular metabolism.
Steroids	Cortisol, testosterone, estradiol, progesterone	Intracellular	Enter the target cell and bind to intracellular receptors. The hormone-receptor complex interacts with the genome to regulate gene expression or chromosome function.
Iodothyronines	Thyroid hormones (T₃, T₄)	Intracellular	Same as steroids.
Amino-acid derivatives	Epinephrine	Membrane-bound	Same as peptide hormones.

The cumulative biochemical actions initiated by hormone-receptor complexes result in physiological and developmental effects.

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Q&A Section

Short-Answer Questions (Answer in 2-3 sentences)

1. Compare and contrast the neural and endocrine systems of coordination.
2. Provide the current scientific definition of a hormone.
3. What are the two types of hormones produced by the hypothalamus, and what is their general function?
4. Describe the anatomical location of the pituitary gland.
5. What is Acromegaly, and what causes it?
6. Name the two hormones released by the neurohypophysis and state where they are synthesized.
7. What is the primary function of melatonin, secreted by the pineal gland?
8. Explain the cause and primary symptom of the condition known as goitre.
9. What is cretinism, and what are its characteristic features?
10. Describe the role of Parathyroid Hormone (PTH) in calcium homeostasis.
11. How do thymosins contribute to the body’s immune response?
12. What happens to the thymus gland in old individuals, and what is the consequence?
13. Why are adrenaline and noradrenaline called “hormones of Fight or Flight”?
14. Distinguish between glucocorticoids and mineralocorticoids secreted by the adrenal cortex.
15. What is Addison’s disease?
16. Describe the endocrine function of the pancreas, naming the cells and the hormones they secrete.
17. Explain how insulin and glucagon work together to maintain glucose homeostasis.
18. What is Diabetes Mellitus?
19. What are androgens, and which cells produce them in males?
20. List three major functions of estrogens in the female body.
21. What is the primary role of progesterone?
22. How does Atrial Natriuretic Factor (ANF) regulate blood pressure?
23. What is the function of erythropoietin, and which organ produces it?
24. Explain the difference in the mechanism of action between protein hormones and steroid hormones.
25. What is a “second messenger” in the context of hormone action?

Multiple-Choice Questions (MCQs)

1. Which of the following is NOT an organized endocrine gland? a) Pituitary b) Thyroid c) Kidney d) Adrenal
2. Somatostatin from the hypothalamus has what effect on the pituitary? a) Stimulates release of gonadotrophins b) Inhibits release of growth hormone c) Stimulates release of prolactin d) Inhibits release of TSH
3. Which hormone is responsible for the regulation of the 24-hour (diurnal) rhythm? a) Cortisol b) Melatonin c) Adrenaline d) Thyroxine
4. Graves’ disease is a form of: a) Hypothyroidism b) Hyperthyroidism c) Diabetes Insipidus d) Addison’s disease
5. Which hormone is considered hypercalcemic? a) Thyrocalcitonin (TCT) b) Parathyroid hormone (PTH) c) Aldosterone d) Insulin
6. T-lymphocyte differentiation is a major role of which hormone? a) Thymosins b) Cortisol c) Epinephrine d) Estrogen
7. The adrenal cortex is composed of three layers. Which is the inner layer? a) Zona glomerulosa b) Zona fasciculata c) Zona reticularis d) Adrenal medulla
8. Which hormone stimulates gluconeogenesis, lipolysis, and proteolysis? a) Aldosterone b) Insulin c) Cortisol d) Glucagon
9. The Islets of Langerhans contain α-cells and β-cells, which secrete: a) Insulin and glucagon, respectively b) Glucagon and insulin, respectively c) Gastrin and secretin, respectively d) Aldosterone and cortisol, respectively
10. Prolonged hyperglycemia is associated with which disorder? a) Diabetes Insipidus b) Acromegaly c) Diabetes Mellitus d) Goitre
11. Leydig cells are responsible for producing: a) Progesterone b) Estrogen c) Androgens d) Oxytocin
12. The structure formed from the ruptured ovarian follicle after ovulation is the: a) Graafian follicle b) Corpus luteum c) Isthmus d) Stroma
13. Which hormone is secreted by the heart to decrease blood pressure? a) Erythropoietin b) Gastrin c) Aldosterone d) Atrial Natriuretic Factor (ANF)
14. CCK (Cholecystokinin) acts on which two organs? a) Stomach and liver b) Pancreas and gall bladder c) Kidney and heart d) Small intestine and stomach
15. Steroid hormones typically bind to: a) Membrane-bound receptors b) Second messengers c) Intracellular receptors d) Ion channels
16. Which hormone is NOT a peptide, polypeptide, or protein hormone? a) Insulin b) Glucagon c) Pituitary hormones d) Testosterone
17. A deficiency in ADH leads to: a) Pituitary dwarfism b) Diabetes Mellitus c) Diabetes Insipidus d) Cretinism
18. The hormone that stimulates uterine contraction during childbirth is: a) Prolactin b) Progesterone c) Oxytocin d) Estrogen
19. Spermatogenesis is stimulated by: a) LH and estrogen b) FSH and androgens c) TSH and thyroxine d) ACTH and cortisol
20. The primary role of FSH in females is to: a) Induce ovulation b) Maintain the corpus luteum c) Stimulate growth of ovarian follicles d) Support pregnancy

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Answer Keys

Short-Answer Questions Key

1. The neural system provides rapid, point-to-point coordination that is short-lived. The endocrine system uses chemical messengers (hormones) for slower, more widespread, and longer-lasting regulation of cellular functions.
2. Hormones are defined as non-nutrient chemicals produced in trace amounts that act as intercellular messengers. This definition includes chemicals from organized endocrine glands as well as other tissues.
3. The hypothalamus produces releasing hormones and inhibiting hormones. Releasing hormones stimulate the secretion of pituitary hormones, while inhibiting hormones prevent their secretion.
4. The pituitary gland is located in a bony cavity of the skull called the sella tursica. It is attached to the basal part of the forebrain (hypothalamus) by a stalk.
5. Acromegaly is a condition caused by the excess secretion of growth hormone in adults. It results in severe disfigurement, especially of the face, and can lead to serious complications and premature death if untreated.
6. The neurohypophysis (posterior pituitary) releases oxytocin and vasopressin (ADH). These hormones are actually synthesized by the neurosecretory cells of the hypothalamus.
7. Melatonin plays a very important role in regulating the 24-hour (diurnal) rhythm of the body. It helps maintain the normal rhythms of the sleep-wake cycle and body temperature.
8. Goitre is the enlargement of the thyroid gland. It is commonly caused by a deficiency of iodine in the diet, which is essential for the normal synthesis of thyroid hormones.
9. Cretinism is a condition caused by hypothyroidism during pregnancy, which leads to defective development of the growing baby. It is characterized by stunted growth, mental retardation, low IQ, abnormal skin, and deaf-mutism.
10. PTH is a hypercalcemic hormone that increases blood Ca²⁺ levels. It does this by stimulating bone resorption, promoting Ca²⁺ reabsorption in the kidneys, and increasing Ca²⁺ absorption from digested food.
11. Thymosins play a major role in the differentiation of T-lymphocytes, which provide cell-mediated immunity. They also promote the production of antibodies, which provide humoral immunity.
12. The thymus gland degenerates with age, resulting in decreased production of thymosins. This causes the immune responses of old persons to become weak.
13. They are called “hormones of Fight or Flight” because they are rapidly secreted during emergency or stressful situations. They prepare the body for an intense physical response by increasing heart rate, respiration, alertness, and energy availability.
14. Glucocorticoids, like cortisol, are primarily involved in carbohydrate metabolism and have anti-inflammatory and immune-suppressing functions. Mineralocorticoids, like aldosterone, regulate the balance of water and electrolytes in the body.
15. Addison’s disease is a disorder caused by the underproduction of hormones from the adrenal cortex. It results in altered carbohydrate metabolism, leading to acute weakness and fatigue.
16. The endocrine pancreas consists of the Islets of Langerhans. Its α-cells secrete glucagon, which raises blood sugar, and its β-cells secrete insulin, which lowers blood sugar.
17. When blood glucose is high, insulin is secreted to promote its uptake and storage, lowering levels. When blood glucose is low, glucagon is secreted to stimulate the release of glucose into the blood from liver stores, raising levels.
18. Diabetes Mellitus is a complex disorder caused by prolonged high blood glucose levels (hyperglycemia). It is associated with the loss of glucose in urine and the formation of harmful ketone bodies.
19. Androgens are a group of male sex hormones, with testosterone being the main one. They are produced by the Leydig cells, or interstitial cells, which are located in the spaces between the seminiferous tubules in the testes.
20. Estrogens stimulate the growth and activities of female secondary sex organs, promote the development of growing ovarian follicles, and are responsible for female secondary sex characters like high pitch of voice.
21. Progesterone’s primary role is to support pregnancy. It also acts on the mammary glands to stimulate the formation of milk-storing alveoli and milk secretion.
22. When blood pressure increases, the atrial wall of the heart secretes ANF. ANF causes the dilation of blood vessels, which in turn reduces blood pressure.
23. Erythropoietin stimulates erythropoiesis, which is the formation of red blood cells. It is produced by the juxtaglomerular cells of the kidney.
24. Protein hormones bind to receptors on the cell membrane and generate second messengers inside the cell to alter metabolism. Steroid hormones enter the cell, bind to intracellular receptors, and the resulting complex directly interacts with the genome to regulate gene expression.
25. A second messenger (e.g., cyclic AMP, Ca²⁺) is a molecule generated inside a target cell when a hormone (the first messenger) binds to a membrane-bound receptor. This second messenger then relays the signal and triggers metabolic changes within the cell.

MCQ Key

1. c) Kidney
2. b) Inhibits release of growth hormone
3. b) Melatonin
4. b) Hyperthyroidism
5. b) Parathyroid hormone (PTH)
6. a) Thymosins
7. c) Zona reticularis
8. c) Cortisol
9. b) Glucagon and insulin, respectively
10. c) Diabetes Mellitus
11. c) Androgens
12. b) Corpus luteum
13. d) Atrial Natriuretic Factor (ANF)
14. b) Pancreas and gall bladder
15. c) Intracellular receptors
16. d) Testosterone
17. c) Diabetes Insipidus
18. c) Oxytocin
19. b) FSH and androgens
20. c) Stimulate growth of ovarian follicles

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Essay Questions with Answers

1. Describe the relationship between the hypothalamus and the pituitary gland, detailing how the hypothalamus controls both the anterior and posterior pituitary.

Answer: The hypothalamus and pituitary gland form a critical neuro-endocrine axis that regulates many physiological functions. The hypothalamus, located in the forebrain, controls the pituitary gland through two distinct mechanisms. It controls the anterior pituitary (adenohypophysis) through a hormonal-vascular link. Neurosecretory cells in the hypothalamus produce releasing hormones (like GnRH) and inhibiting hormones (like somatostatin). These hormones are released into a portal circulatory system that transports them directly to the anterior pituitary, where they stimulate or inhibit the synthesis and secretion of anterior pituitary hormones like LH, FSH, and GH. The control of the posterior pituitary (neurohypophysis) is purely neural. The hormones oxytocin and vasopressin (ADH) are synthesized in the cell bodies of hypothalamic neurons. These hormones are then transported down the axons of these neurons and stored in their nerve endings within the posterior pituitary. Their release into the bloodstream is triggered by nerve impulses originating from the hypothalamus. Thus, the posterior pituitary does not produce its own hormones but serves as a storage and release site for hypothalamic hormones.

2. Explain the roles of the thyroid and parathyroid glands in maintaining calcium homeostasis in the body.

Answer: Calcium homeostasis, the maintenance of stable calcium levels in the blood, is a critical physiological process regulated by a delicate balance between hormones from the thyroid and parathyroid glands. The parathyroid glands secrete Parathyroid Hormone (PTH), which is a hypercalcemic hormone. When blood calcium levels are low, PTH is released. It acts on three main targets to raise calcium levels: it stimulates the dissolution of bone (bone resorption) to release calcium into the blood, it increases the reabsorption of calcium by the tubules in the kidneys, and it enhances the absorption of calcium from digested food in the intestines. Conversely, the thyroid gland secretes a hormone called Thyrocalcitonin (TCT). TCT has the opposite effect of PTH; it is a hypocalcemic agent (though its role is less significant than PTH in humans). When blood calcium levels are high, TCT is thought to decrease these levels, primarily by inhibiting bone resorption. Together, PTH and TCT work in an antagonistic fashion to ensure that blood calcium ion concentration is kept within a narrow, optimal range, which is vital for nerve function, muscle contraction, and blood clotting.

3. Detail the structure and function of the adrenal gland, distinguishing between the roles of the adrenal medulla and the adrenal cortex.

Answer: The adrenal gland is a paired endocrine gland located superior to each kidney. It is structurally and functionally divided into two distinct parts: the outer adrenal cortex and the inner adrenal medulla. The adrenal medulla is the central part of the gland and is part of the sympathetic nervous system. It secretes two hormones called catecholamines: adrenaline (epinephrine) and noradrenaline (norepinephrine). These are known as emergency hormones because they are released in response to stress or excitement, preparing the body for “fight or flight.” They increase alertness, heart rate, respiration rate, and stimulate the breakdown of glycogen, fats, and proteins to provide a rapid source of energy. The adrenal cortex is the outer region and is divided into three layers: the zona glomerulosa, zona fasciculata, and zona reticularis. It secretes a group of steroid hormones called corticoids. These are categorized into mineralocorticoids (e.g., aldosterone), which regulate water and electrolyte balance, and glucocorticoids (e.g., cortisol), which are involved in carbohydrate metabolism, suppressing the immune response, and producing anti-inflammatory reactions. The adrenal cortex also secretes small amounts of androgenic steroids. Underproduction of these hormones leads to Addison’s disease.

4. Compare the hyperglycemic and hypoglycemic hormones produced by the pancreas. Describe their mechanisms of action and how they lead to the regulation of blood glucose.

Answer: The pancreas plays a central role in blood glucose regulation through the secretion of two antagonistic peptide hormones from its endocrine portion, the Islets of Langerhans. The hyperglycemic hormone is glucagon, secreted by the α-cells. Glucagon’s primary function is to raise blood glucose levels when they fall too low. It acts mainly on liver cells (hepatocytes), stimulating two processes: glycogenolysis (the breakdown of stored glycogen into glucose) and gluconeogenesis (the synthesis of glucose from non-carbohydrate sources like amino acids). By promoting the release of glucose into the bloodstream, glucagon ensures the body has a steady supply of energy. The hypoglycemic hormone is insulin, secreted by the β-cells. Insulin’s function is to lower blood glucose levels when they are too high, such as after a meal. It acts on hepatocytes and adipocytes (fat cells), enhancing their ability to take up and utilize glucose from the blood. Insulin also stimulates glycogenesis, the process of converting excess glucose into glycogen for storage in the liver and muscles. The coordinated and opposing actions of insulin and glucagon maintain glucose homeostasis. A failure in this system, particularly a deficiency in insulin or resistance to its effects, results in diabetes mellitus.

5. Discuss the primary hormones produced by the male and female gonads and outline their major physiological functions.

Answer: The gonads (testis in males, ovary in females) are the primary sex organs that also function as endocrine glands, producing steroid hormones essential for reproduction and development. In males, the testis contains Leydig cells, which produce a group of hormones called androgens, with testosterone being the most important. Androgens are responsible for the development, maturation, and function of the male accessory sex organs like the prostate and seminal vesicles. They stimulate the development of male secondary sexual characteristics such as a low-pitched voice, facial and axillary hair, and increased muscle mass. Furthermore, androgens are crucial for spermatogenesis (sperm production) and influence male sexual behaviour (libido). In females, the ovary produces two main groups of hormones: estrogens and progesterone. Estrogens are primarily synthesized by the growing ovarian follicles. They stimulate the growth and activities of female secondary sex organs and the development of secondary sexual characteristics like a high-pitched voice and breast development. They also regulate the menstrual cycle and female sexual behaviour. Progesterone is secreted mainly by the corpus luteum after ovulation. Its primary role is to support and maintain pregnancy. It also prepares the mammary glands for lactation by stimulating the formation of alveoli.

6. What are endocrine glands? List the major organized endocrine glands in the human body and describe the hormones of any one of them in detail.

Answer: Endocrine glands are specialized, ductless glands whose secretions, called hormones, are released directly into the bloodstream. These hormones then travel through the circulatory system to target organs or tissues, where they regulate various physiological processes. The major organized endocrine glands in the human body are the pituitary, pineal, thyroid, parathyroid, adrenal, pancreas, thymus, and gonads (testis and ovary). Let’s detail the hormones of the pituitary gland. It is divided into the adenohypophysis and neurohypophysis. The adenohypophysis produces several key hormones: Growth Hormone (GH) regulates body growth; Prolactin (PRL) controls mammary gland development and milk formation; Thyroid Stimulating Hormone (TSH) stimulates the thyroid gland; Adrenocorticotrophic Hormone (ACTH) acts on the adrenal cortex; and Gonadotrophins (LH and FSH) regulate gonadal activity. The neurohypophysis stores and releases two hormones made by the hypothalamus: Oxytocin, which stimulates uterine contractions and milk ejection, and Vasopressin (ADH), which helps the kidneys conserve water.

7. Explain the mechanism of hormone action, focusing on the differences between hormones that use membrane-bound receptors and those that use intracellular receptors. Provide examples for each type.

Answer: Hormones exert their effects by binding to specific hormone receptors on or in target cells. The mechanism of action differs based on the chemical nature of the hormone and the location of its receptor. Hormones that use membrane-bound receptors are typically water-soluble, like peptide/protein hormones (e.g., insulin, pituitary hormones) and catecholamines (e.g., epinephrine). These hormones cannot pass through the cell membrane. They bind to receptors on the cell surface, forming a hormone-receptor complex. This binding triggers the generation of intracellular “second messengers” like cyclic AMP (cAMP), IP₃, or Ca²⁺ ions. These second messengers then amplify the signal and activate a cascade of biochemical reactions within the cell, leading to a change in cellular metabolism. Hormones that use intracellular receptors are lipid-soluble, such as steroid hormones (e.g., cortisol, testosterone, estrogen) and iodothyronines (thyroid hormones). These hormones can easily diffuse across the cell membrane and bind to receptors located inside the cytoplasm or nucleus. The hormone-receptor complex then enters the nucleus and binds to specific regions of the DNA. This interaction with the genome regulates gene expression, either by activating or inhibiting the transcription of specific genes into mRNA, which in turn alters protein synthesis and leads to long-term physiological and developmental effects.

8. What is hypothyroidism and hyperthyroidism? Describe the causes and symptoms associated with both conditions.

Answer: Hypothyroidism and hyperthyroidism are disorders resulting from abnormal levels of thyroid hormone secretion. Hypothyroidism is a condition caused by the under-secretion of thyroid hormones (T₃ and T₄). A common cause is a dietary deficiency of iodine, which is essential for hormone synthesis. This deficiency leads to an enlargement of the thyroid gland, known as goitre. In pregnant women, hypothyroidism can cause defective development in the fetus, leading to cretinism, a condition marked by stunted growth and mental retardation. In adult women, it can cause the menstrual cycle to become irregular. Hyperthyroidism is the opposite condition, caused by an abnormally high rate of synthesis and secretion of thyroid hormones. This can be due to a cancer of the thyroid gland or the development of nodules. A common form of hyperthyroidism is Exophthalmic goitre, also known as Graves’ disease. It is characterized by an enlarged thyroid gland, protrusion of the eyeballs, an increased basal metabolic rate (BMR), and significant weight loss. This condition adversely affects the body’s overall physiology.

9. Hormones are also secreted by tissues that are not traditionally considered endocrine glands. Discuss the hormonal roles of the heart, kidneys, and gastrointestinal tract.

Answer: Several organs whose primary functions are not endocrine in nature also produce and secrete important hormones. The atrial wall of the heart secretes a peptide hormone called Atrial Natriuretic Factor (ANF). When blood pressure is increased, ANF is released. It causes the dilation of blood vessels, which leads to a decrease in blood pressure, thus playing a key role in cardiovascular regulation. The juxtaglomerular cells of the kidney produce a peptide hormone called erythropoietin. This hormone is critical for hematopoiesis as it stimulates erythropoiesis, the process of red blood cell (RBC) formation in the bone marrow. The gastrointestinal (GI) tract contains endocrine cells scattered throughout its lining that secrete several major peptide hormones to regulate digestion. Gastrin stimulates the secretion of hydrochloric acid and pepsinogen in the stomach. Secretin acts on the pancreas to stimulate the release of water and bicarbonate ions. Cholecystokinin (CCK) stimulates the pancreas to release digestive enzymes and the gall bladder to release bile. Gastric Inhibitory Peptide (GIP) inhibits gastric secretion and motility.

10. Describe the dual function of the pancreas as both an exocrine and endocrine gland.

Answer: The pancreas is a composite gland, meaning it has both exocrine and endocrine functions, which are carried out by different cell populations within the organ. Its exocrine function is related to digestion. The vast majority of the pancreatic tissue consists of acinar cells, which synthesize and secrete pancreatic juice. This juice contains a variety of digestive enzymes (like amylase, lipase, and trypsinogen) that are released into a system of ducts that eventually lead to the small intestine. There, these enzymes play a crucial role in breaking down carbohydrates, fats, and proteins from food. Its endocrine function is to regulate blood glucose levels. This is carried out by small clusters of cells called the Islets of Langerhans, which are scattered throughout the pancreatic tissue. These islets contain two main cell types: α-cells, which secrete the hormone glucagon to raise blood glucose, and β-cells, which secrete the hormone insulin to lower blood glucose. These hormones are released directly into the bloodstream and are essential for maintaining glucose homeostasis. The failure of this endocrine function leads to diabetes mellitus.

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Glossary of Key Terms

Term	Definition
Acromegaly	A condition caused by excess secretion of growth hormone in adults, leading to severe disfigurement, especially of the face.
Addison’s disease	A disease resulting from the underproduction of hormones by the adrenal cortex, causing acute weakness and fatigue.
Adenohypophysis	The anterior part of the pituitary gland, consisting of the pars distalis and pars intermedia.
Adrenal Cortex	The outer tissue of the adrenal gland, which secretes corticoid hormones.
Adrenal Medulla	The centrally located tissue of the adrenal gland, which secretes catecholamines (adrenaline and noradrenaline).
Adrenaline (Epinephrine)	An emergency hormone secreted by the adrenal medulla that increases alertness, heart rate, and respiration.
Aldosterone	The main mineralocorticoid hormone, which regulates the balance of water and electrolytes.
Androgens	A group of hormones, mainly testosterone, produced by the testis that regulate male sexual development and functions.
Anti-diuretic Hormone (ADH)	Also known as vasopressin; a hormone that stimulates the resorption of water by the kidney, reducing water loss.
Atrial Natriuretic Factor (ANF)	A peptide hormone secreted by the heart’s atrial wall that decreases blood pressure.
Catecholamines	Hormones secreted by the adrenal medulla (adrenaline and noradrenaline), also known as emergency hormones.
Cholecystokinin (CCK)	A hormone from the GI tract that stimulates the pancreas and gall bladder to secrete enzymes and bile, respectively.
Corpus Luteum	A structure formed in the ovary from a ruptured follicle after ovulation, which secretes progesterone.
Corticoids	Hormones secreted by the adrenal cortex, including glucocorticoids and mineralocorticoids.
Cortisol	The main glucocorticoid hormone, involved in carbohydrate metabolism and suppressing the immune response.
Cretinism	A condition of stunted growth and mental retardation caused by hypothyroidism during fetal development.
Diabetes Insipidus	A condition caused by a deficiency of ADH, leading to excessive water loss and dehydration.
Diabetes Mellitus	A disorder caused by prolonged hyperglycemia, associated with the loss of glucose through urine.
Endocrine Glands	Ductless glands that secrete hormones directly into the bloodstream.
Erythropoietin	A peptide hormone produced by the kidney that stimulates the formation of red blood cells (erythropoiesis).
Estrogen	A group of steroid hormones produced by the ovary that regulate female sexual development and functions.
Exophthalmic Goitre	Also called Graves’ disease; a form of hyperthyroidism characterized by an enlarged thyroid and protruding eyeballs.
Follicle Stimulating Hormone (FSH)	A gonadotrophin that stimulates ovarian follicle growth in females and regulates spermatogenesis in males.
Gastrin	A hormone from the GI tract that stimulates the secretion of hydrochloric acid and pepsinogen.
Glucagon	A hyperglycemic peptide hormone from the pancreatic α-cells that increases blood glucose levels.
Glucocorticoids	A group of corticoids involved in carbohydrate metabolism.
Goitre	An enlargement of the thyroid gland, often due to iodine deficiency.
Gonadotrophins	Hormones (LH and FSH) that stimulate gonadal activity.
Hormone	Non-nutrient chemicals that act as intercellular messengers and are produced in trace amounts.
Hormone Receptors	Specific proteins located in target tissues that bind to hormones to initiate a response.
Hypercalcemic Hormone	A hormone, like PTH, that increases the level of calcium in the blood.
Hyperglycemia	An increased concentration of glucose in the blood.
Hypoglycemia	Decreased blood glucose levels.
Hypothalamus	The basal part of the diencephalon (forebrain) that regulates the pituitary gland and many body functions.
Insulin	A hypoglycemic peptide hormone from the pancreatic β-cells that decreases blood glucose levels.
Islets of Langerhans	The endocrine part of the pancreas, consisting of α-cells and β-cells.
Leydig cells	Interstitial cells in the testis that produce androgens.
Luteinizing Hormone (LH)	A gonadotrophin that induces ovulation in females and stimulates androgen synthesis in males.
Melatonin	A hormone secreted by the pineal gland that regulates the body’s diurnal (24-hour) rhythm.
Mineralocorticoids	A group of corticoids that regulate water and electrolyte balance.
Neurohypophysis	The posterior part of the pituitary gland (pars nervosa) that stores and releases oxytocin and vasopressin.
Noradrenaline (Norepinephrine)	An emergency hormone secreted by the adrenal medulla.
Ovary	The primary female sex organ, which produces ova and the hormones estrogen and progesterone.
Oxytocin	A hormone that stimulates contraction of smooth muscles, such as the uterus during childbirth.
Pancreas	A composite gland that functions as both an exocrine (digestive juices) and endocrine (insulin, glucagon) gland.
Parathyroid Hormone (PTH)	A peptide hormone from the parathyroid glands that increases blood calcium levels.
Pineal Gland	An endocrine gland located on the dorsal side of the forebrain that secretes melatonin.
Pituitary Gland	A major endocrine gland located in the sella tursica, controlling many other endocrine glands.
Progesterone	A steroid hormone secreted by the corpus luteum that supports pregnancy.
Prolactin (PRL)	A hormone from the anterior pituitary that regulates mammary gland growth and milk formation.
Secretin	A hormone from the GI tract that stimulates the pancreas to secrete water and bicarbonate ions.
Testis	The primary male sex organ, which produces spermatozoa and androgens.
Testosterone	The main androgenic hormone produced by the testis.
Thymosins	Peptide hormones secreted by the thymus gland that are essential for the development of the immune system.
Thymus	A lobular gland located near the heart that plays a major role in the immune system.
Thyrocalcitonin (TCT)	A protein hormone from the thyroid gland that regulates (decreases) blood calcium levels.
Thyroid Gland	An endocrine gland located in the neck that secretes thyroid hormones (T₃, T₄) and TCT.
Thyroxine (T₄)	The main iodine-containing hormone produced by the thyroid gland.
Vasopressin	Another name for Anti-diuretic Hormone (ADH).