Chemical Coordination and Integration

Endocrine Glands and Hormones

The neural system helps our organs work together quickly. But its effects don’t last long. Not all cells are connected by nerves, and some functions need to be regulated continuously. This is where hormones come in. The neural system and the endocrine system work together to regulate our body’s functions.

Endocrine Glands and Hormones

• Ductless Glands: Endocrine glands do not have ducts.
• Hormones: These glands release chemicals called hormones into the blood. Hormones travel to distant organs to perform their functions.
• Definition of Hormones: Hormones are chemicals produced in small amounts. They act as messengers between cells.

Examples:

• Invertebrates: Simple endocrine systems with few hormones.
• Vertebrates (like humans): Many chemicals act as hormones for coordination.

Human Endocrine System

• Components: The system includes glands and hormone-producing tissues/cells spread throughout the body.
• Major Endocrine Glands:
  • Pituitary
  • Pineal
  • Thyroid
  • Adrenal
  • Pancreas
  • Parathyroid
  • Thymus
  • Gonads: Testis in males, Ovary in females

Other Hormone-Producing Organs:

• Gastrointestinal Tract
• Liver
• Kidney
• Heart

Functions

Each gland has a specific structure and function, which will be explained in the following sections. The hypothalamus, a part of the brain, also plays a key role in controlling these glands.

1. Hypothalamus

• Location: Basal part of the diencephalon in the forebrain.
• Function: Regulates many body functions.
• Hormones: Produced by groups of neurosecretory cells called nuclei.
  • Releasing Hormones: Stimulate secretion of pituitary hormones.
    • Example: Gonadotrophin releasing hormone (GnRH) – stimulates gonadotrophin release.
  • Inhibiting Hormones: Inhibit secretion of pituitary hormones.
    • Example: Somatostatin – inhibits growth hormone release.
• Pathway: Hormones travel through axons, released from nerve endings, reach the pituitary gland via a portal circulatory system.
• Regulation:
  • Anterior Pituitary: Regulated by hypothalamic hormones.
  • Posterior Pituitary: Directly controlled by neural signals from the hypothalamus.

2. Pituitary Gland

• Location: In a bony cavity called sella turcica, attached to the hypothalamus by a stalk.
• Parts:
  • Adenohypophysis (Anterior Pituitary):
    • Pars Distalis:
      • Hormones:
        • Growth Hormone (GH): Promotes growth.
          • Over-secretion: Gigantism
          • Under-secretion: Pituitary dwarfism
        • Prolactin (PRL): Stimulates growth of mammary glands and milk production.
        • Thyroid Stimulating Hormone (TSH): Stimulates thyroid hormone production.
        • Adrenocorticotrophic Hormone (ACTH): Stimulates glucocorticoid production in adrenal cortex.
        • Luteinizing Hormone (LH): Stimulates gonadal activity.
          • Males: Androgen production
          • Females: Induces ovulation, maintains corpus luteum
        • Follicle Stimulating Hormone (FSH):
          • Males: Regulates spermatogenesis
          • Females: Stimulates ovarian follicle growth
    • Pars Intermedia:
      • Hormone: Melanocyte Stimulating Hormone (MSH) – Regulates skin pigmentation.
  • Neurohypophysis (Posterior Pituitary):
    • Hormones:
      • Oxytocin: Stimulates contraction of uterus during childbirth and milk ejection from mammary glands.
      • Vasopressin (ADH): Promotes water and electrolyte resorption in kidneys, reduces urine loss.
        • Impairment: Leads to Diabetes Insipidus (water loss and dehydration).

Important Points

• Growth Hormone (GH):
  • Excess in adults: Acromegaly (severe disfigurement, mainly of the face).
• LH and FSH: Known as gonadotrophins because they stimulate gonadal activity.
• Oxytocin: Important for childbirth and breastfeeding.
• Vasopressin (ADH): Essential for water conservation in the body

Pituitary Part	Hormones	Functions
Adenohypophysis	Growth Hormone (GH)	Promotes growth
(Anterior Pituitary)	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)	Stimulates gonadal activity in both males and females
	Follicle Stimulating Hormone (FSH)	Regulates spermatogenesis in males and ovarian follicle growth in females
Pars Intermedia	Melanocyte Stimulating Hormone (MSH)	Regulates skin pigmentation
Neurohypophysis	Oxytocin	Stimulates uterine contractions during childbirth and milk ejection from mammary glands
(Posterior Pituitary)	Vasopressin (ADH)	Promotes water and electrolyte resorption in kidneys, reduces urine loss

3. Pineal Gland

• Location: Dorsal side of the forebrain.
• Hormone: Melatonin.
• Functions:
  • Regulates 24-hour (diurnal) rhythms.
    • Sleep-Wake Cycle: Helps maintain normal sleep patterns.
    • Body Temperature: Keeps body temperature in check.
  • Influences other body functions:
    • Metabolism
    • Pigmentation
    • Menstrual Cycle
    • Defense Capability

4. Thyroid Gland

• Location:
  • Two lobes on either side of the trachea.
  • Connected by a thin tissue flap called the isthmus.
• Structure:
  • Made up of follicles and stromal tissues.
  • Follicles have follicular cells that make hormones.
• Hormones:
  • Tetraiodothyronine (T4) or Thyroxine.
  • Triiodothyronine (T3).
  • Need iodine to make these hormones.
• Disorders:
  • Hypothyroidism:
    • Caused by iodine deficiency.
    • Leads to goitre (enlarged thyroid).
    • In pregnancy, can cause cretinism (stunted growth, mental retardation).
    • In adults, can cause irregular menstrual cycles.
  • Hyperthyroidism:
    • Caused by thyroid cancer or nodules.
    • Leads to high hormone levels, affecting body functions.
    • Exophthalmic Goitre (Graves’ Disease):
      • Symptoms: Enlarged thyroid, protruding eyes, high metabolism, weight loss.
• Functions of Thyroid Hormones:
  • Regulate basal metabolic rate.
  • Help in red blood cell formation.
  • Control metabolism of carbs, proteins, and fats.
  • Maintain water and electrolyte balance.
• Thyrocalcitonin (TCT):
  • A protein hormone.
  • Regulates blood calcium levels.

5. Parathyroid Gland

• Location:
  • Four glands on the back of the thyroid.
  • One pair in each lobe.
• Hormone:
  • Parathyroid Hormone (PTH):
    • Peptide hormone.
    • Regulated by blood calcium levels.
• Functions of PTH:
  • Increases blood calcium levels.
  • Acts on bones to increase bone resorption.
  • Stimulates calcium reabsorption in kidneys.
  • Increases calcium absorption from food.
  • Works with TCT to balance calcium in the body.

6. Thymus

• Location:
  • Between the lungs, behind the sternum, in front of the aorta.
• Role:
  • Helps develop the immune system.
  • Secretes peptide hormones called thymosins.
• Functions of Thymosins:
  • Help T-lymphocytes mature for cell-mediated immunity.
  • Promote antibody production for humoral immunity.
  • Thymus shrinks in old age, reducing thymosin production and weakening the immune system.

7. Adrenal Gland

• Location:
  • One pair, one above each kidney.
• Structure:
  • Adrenal Medulla (inner part)
  • Adrenal Cortex (outer part)
• Adrenal Medulla Hormones:
  • Adrenaline (Epinephrine)
  • Noradrenaline (Norepinephrine)
  • Known as catecholamines, also called emergency hormones or Fight or Flight hormones.
• Functions of Catecholamines:
  • Increase alertness, pupil dilation, and sweating.
  • Raise heart rate and respiration.
  • Break down glycogen to increase blood glucose.
  • Break down lipids and proteins.
• Adrenal Cortex Hormones:
  • Glucocorticoids (like cortisol):
    • Involved in carbohydrate metabolism.
    • Stimulate gluconeogenesis, lipolysis, and proteolysis.
    • Inhibit amino acid uptake.
    • Help with cardiovascular and kidney functions.
    • Anti-inflammatory and suppress immune response.
    • Stimulate RBC production.
  • Mineralocorticoids (like aldosterone):
    • Regulate water and electrolyte balance.
    • Promote Na+ and water reabsorption, and K+ and phosphate excretion.
    • Maintain fluid volume, osmotic pressure, and blood pressure.
  • Androgenic Steroids:
    • Promote growth of body hair during puberty.
• Disorder:
  • Addison’s Disease:
    • Caused by underproduction of adrenal cortex hormones.
    • Leads to weakness and fatigue due to altered carbohydrate metabolism.

8. Pancreas

• Type:
  • Composite gland (both exocrine and endocrine).
• Endocrine Part:
  • Islets of Langerhans: 1-2 million in the pancreas, 1-2% of pancreatic tissue.
  • Cells:
    • α-cells: Secrete glucagon.
    • β-cells: Secrete insulin.
• Hormones:
  • Glucagon:
    • Peptide hormone.
    • Increases blood sugar (hyperglycemia).
    • Acts on liver cells to stimulate glycogen breakdown (glycogenolysis) and glucose production (gluconeogenesis).
    • Reduces glucose uptake and use by cells.
  • Insulin:
    • Peptide hormone.
    • Decreases blood sugar (hypoglycemia).
    • Acts on liver and fat cells to increase glucose uptake and use.
    • Stimulates glucose conversion to glycogen (glycogenesis).
• Disorder:
  • Diabetes Mellitus:
    • Prolonged high blood sugar (hyperglycemia).
    • Leads to glucose loss in urine and formation of harmful ketone bodies.
    • Treated with insulin therapy.

9. Testis

• Location:
  • In the scrotal sac outside the abdomen in males.
• Functions:
  • Primary sex organ.
  • Endocrine gland.
• Structure:
  • Seminiferous Tubules.
  • Leydig Cells (in intertubular spaces): Produce androgens, mainly testosterone.
• Functions of Androgens:
  • Develop and mature male sex organs (epididymis, vas deferens, seminal vesicles, prostate gland, urethra).
  • Stimulate muscular growth, facial and axillary hair growth.
  • Influence male traits like aggressiveness and deeper voice.
  • Stimulate sperm production (spermatogenesis).
  • Affect male sexual behavior (libido).
  • Promote protein and carbohydrate metabolism (anabolic effects).

10. Ovary

• Location:
  • In the abdomen of females.
• Functions:
  • Primary female sex organ.
  • Produces one ovum each menstrual cycle.
  • Produces steroid hormones: estrogen and progesterone.
• Structure:
  • Composed of ovarian follicles and stromal tissues.
• Hormones:
  • Estrogen:
    • Produced by growing ovarian follicles.
    • Stimulates growth and activity of female secondary sex organs.
    • Develops ovarian follicles.
    • Causes appearance of female secondary sex characteristics (e.g., high pitch voice).
    • Promotes mammary gland development.
    • Regulates female sexual behavior.
  • Progesterone:
    • Produced by corpus luteum (formed from ruptured follicle after ovulation).
    • Supports pregnancy.
    • Stimulates formation of alveoli in mammary glands and milk secretion.

Hormones of Heart, Kidney, and Gastrointestinal Tract

• Heart:
  • Atrial Natriuretic Factor (ANF):
    • Peptide hormone.
    • Decreases blood pressure.
    • Causes blood vessel dilation when blood pressure is high.
• Kidney:
  • Erythropoietin:
    • Peptide hormone.
    • Stimulates formation of red blood cells (erythropoiesis).
• Gastrointestinal Tract:
  • Gastrin:
    • Stimulates gastric glands to secrete hydrochloric acid and pepsinogen.
  • Secretin:
    • Stimulates exocrine pancreas to secrete water and bicarbonate ions.
  • Cholecystokinin (CCK):
    • Stimulates pancreas to secrete enzymes.
    • Stimulates gall bladder to secrete bile juice.
  • Gastric Inhibitory Peptide (GIP):
    • Inhibits gastric secretion and motility.
• Growth Factors:
  • Secreted by various non-endocrine tissues.
  • Essential for normal tissue growth and repair.

Mechanism of Hormone Action

• How Hormones Work:
  • Hormones affect target tissues by binding to specific proteins called hormone receptors.
  • Receptor Types:
    • Membrane-bound receptors: On the cell membrane.
    • Intracellular receptors: Inside the cell, often in the nucleus.
• Hormone-Receptor Complex:
  • When a hormone binds to its receptor, it forms a hormone-receptor complex.
  • This complex causes biochemical changes in the target tissue.
  • These changes regulate the metabolism and functions of the target tissue.
• Types of Hormones:
  • Peptide, Polypeptide, Protein Hormones:
    • Examples: Insulin, glucagon, pituitary hormones, hypothalamic hormones.
  • Steroids:
    • Examples: Cortisol, testosterone, estradiol, progesterone.
  • Iodothyronines:
    • Example: Thyroid hormones.
  • Amino-acid Derivatives:
    • Example: Epinephrine.
• Mechanism of Action:
  • Membrane-bound Receptors:
    • Hormones do not enter the cell.
    • Generate second messengers like cyclic AMP, IP3, Ca++.
    • These messengers regulate cellular metabolism.
  • Intracellular Receptors:
    • Hormones enter the cell.
    • Regulate gene expression or chromosome function by interacting with the genome.
    • Lead to physiological and developmental effects.

Chapter Summary:

• Chemical Coordination by Hormones:
  • Special chemicals act as hormones in the body.
  • They coordinate, integrate, and regulate body functions.
• Functions of Hormones:
  • Regulate metabolism, growth, and organ development.
  • Act on endocrine glands or specific cells.
• Components of Endocrine System:
  • The endocrine system includes:
    • Hypothalamus
    • Pituitary
    • Pineal
    • Thyroid
    • Adrenal
    • Pancreas
    • Parathyroid
    • Thymus
    • Gonads (testis and ovary)
  • Other organs like gastrointestinal tract, kidney, and heart also produce hormones.
• Pituitary Gland:
  • Divided into three parts: pars distalis, pars intermedia, pars nervosa.
    • Pars distalis produces six trophic hormones.
    • Pars intermedia secretes one hormone.
    • Pars nervosa secretes two hormones.
  • Pituitary hormones regulate growth, development, and peripheral endocrine glands.
• Pineal Gland:
  • Secretes melatonin, regulating 24-hour rhythms like sleep-wake cycle and body temperature.
• Thyroid Gland:
  • Regulates basal metabolic rate, neural system development, erythropoiesis, and menstrual cycle.
  • Regulates carbohydrate, protein, and fat metabolism.
  • Thyrocalcitonin regulates blood calcium levels, decreases blood calcium levels.
• Parathyroid Glands:
  • Secrete parathyroid hormone (PTH) to regulate blood calcium levels and calcium homeostasis.
• Thymus Gland:
  • Secretes thymosins for
    • Differentiating T-lymphocytes for cell-mediated immunity.
    • Increases antibody production for humoral immunity.
• Adrenal Gland:
  • Composed of adrenal medulla and cortex.
  • Medulla secretes epinephrine and norepinephrine, affecting various body functions like.
    • Increases alertness, pupil dilation, piloerection, sweating, heartbeat, respiration rate, glycogenolysis, lipolysis, proteolysis.
  • Cortex secretes glucocorticoids and mineralocorticoids regulating metabolism and electrolyte balance by following ways.
    • Glucocorticoids stimulate gluconeogenesis, lipolysis, proteolysis, erythropoiesis, cardiovascular system, blood pressure, glomerular filtration rate, and inhibit inflammation.
    • Mineralocorticoids regulate water and electrolyte balance.
• Pancreas:
  • Endocrine part secretes glucagon and insulin.
  • Glucagon raises blood sugar, while insulin lowers it.
    • Glucagon stimulates glycogenolysis and gluconeogenesis (hyperglycemia).
    • Insulin stimulates glucose uptake, utilisation, glycogenesis (hypoglycemia).
    • Insulin deficiency/resistance leads to diabetes mellitus.
• Testis:
  • Secretes androgens for male reproductive system development and functions.
    • Stimulates development and functions of male sex organs, secondary sex characteristics, spermatogenesis, male behavior, anabolic pathways, erythropoiesis.
• Ovary:
  • Secretes estrogen and progesterone for female reproductive system development and maintenance.
    • Estrogen stimulates growth of female sex organs and secondary sex characteristics.
    • Progesterone maintains pregnancy, develops mammary glands, and supports lactation.
• Heart and Kidney Hormones:
  • Heart produces atrial natriuretic factor to lower blood pressure.
  • Kidney produces erythropoietin for red blood cell production (erythropoiesis).
• Gastrointestinal Tract Hormones:
  • Secretes gastrin, secretin, cholecystokinin, and gastric inhibitory peptide to regulate digestion.