Endocrine System in Humans

Endocrine Systemin Humans

INTRODUCTION

• The neural system is excellent for quick responses, it has its limitations:

- Short Duration: Neural responses are fast but do not last long.

- Limited Reach: Nerve fibers do not connect to every cell in the body, meaning not all cellular functions can be directly controlled by the neural system.

• To overcome these limitations, the body uses the endocrine system for continuous regulation of cellular functions. This system involves glands that secrete hormones directly into the bloodstream.
• Hormones are chemical messengers that travel throughout the body, influencing various physiological processes.
• The neural system provides rapid but short-term coordination through nerve impulses, while the endocrine system offers long-term regulation through hormones. Together, they ensure the body can respond swiftly to immediate challenges and maintain overall stability and function over time.
• Endocrine glands are specialized glands that do not have ducts, which is why they are called ductless glands. These glands produce and secrete hormones directly into the bloodstream.
• Chemical substances that act as messengers, coordinating various functions in the body.
• Traditionally, hormones were defined as chemicals produced by endocrine glands, released into the blood, and transported to distant target organs.
• The modern definition of hormones is broader:

- Non-Nutrient Chemicals: Hormones are non-nutrient chemicals that serve as intercellular messengers.

- Trace Amounts: They are effective in very small (trace) amounts.

• Differences Between Invertebrates and Vertebrates

- Invertebrates: These animals have simple endocrine systems with a limited number of hormones.

- Vertebrates: These animals, including humans, have complex endocrine systems with a wide variety of hormones that regulate numerous body functions.

Endocrine System in Humans

• The human endocrine system consists of both organized endocrine glands and hormone-producing tissues/cells spread throughout the body.
• This system plays a crucial role in regulating various physiological processes through the secretion of hormones.
• Major endocrine glands- pituitary gland, pineal gland, thyroid gland, parathyroid glands, adrenal glands, pancreas, gonads (testes and ovaries), thymus.
• In addition to the major endocrine glands, several hormone-producing tissues/cells organs produce hormones: gastrointestinal tract, liver, kidneys, heart.
• These hormones act as chemical messengers, influencing processes such as growth, metabolism, reproduction, and stress responses.

Hypothalamus

• The hypothalamus is a crucial part of the brain, specifically located in the basal part of the diencephalon in the forebrain.
• It plays a vital role in regulating various body functions by producing hormones that influence the pituitary gland.
• By releasing both stimulating and inhibiting hormones, the hypothalamus ensures that the pituitary gland releases the appropriate hormones necessary for various bodily functions.
• This precise control system helps maintain homeostasis and regulates critical processes such as growth, reproduction, and metabolism.
• Structure and Function

- Location: Basal part of the diencephalon in the forebrain.

- Neurosecretory Cells (Nuclei): Groups of cells in the hypothalamus that produce hormones.

• Types of Hormones Produced

- Releasing Hormones: Stimulate the secretion of specific hormones from the pituitary gland. Example: Gonadotropin-releasing hormone (GnRH) stimulates the pituitary to synthesize and release gonadotropins.

- Inhibiting Hormones: Inhibit the secretion of specific hormones from the pituitary gland. Example: Somatostatin inhibits the release of growth hormone (GH) from the pituitary.

• Hormone Pathway

- Production: Hormones are produced in the hypothalamic neurons.

- Transport: Hormones travel through the axons of these neurons and are released from their nerve endings.

- Portal Circulatory System: Hormones reach the anterior pituitary gland through a specialized blood vessel system known as the portal circulatory system.

- Regulation:

Anterior Pituitary: Regulated by the hypothalamic hormones delivered via the portal circulatory system.

Posterior Pituitary: Under direct neural regulation from the hypothalamus.

Pituitary Gland

• The pituitary gland, often called the "master gland," is essential for regulating various bodily functions through the secretion of hormones. Here's a detailed overview of its location, structure, functions, and related disorders.
• Location and Structure

- Location: The pituitary gland is located in a bony cavity at the base of the skull known as the sella turcica. This positioning helps protect the gland.

- Attachment: It is connected to the hypothalamus, a crucial brain region, via a stalk called the infundibulum.

- Divisions:

Adenohypophysis (Anterior Pituitary): This part consists of the pars distalis and pars intermedia.

Neurohypophysis (Posterior Pituitary): Also known as pars nervosa.

Adenohypophysis (Anterior Pituitary)

• Pars Distalis: Commonly known as the anterior pituitary.

Hormones Produced:

- Growth Hormone (GH): Stimulates overall body growth and development, especially in bones and muscles. Over-secretion during childhood can lead to gigantism, characterized by abnormal height and growth, while under-secretion can result in pituitary dwarfism, causing stunted growth.

- Prolactin (PRL): Essential for the development of mammary glands and the production of milk after childbirth. It also plays a role in regulating the immune system.

- Thyroid Stimulating Hormone (TSH): Stimulates the thyroid gland to produce thyroid hormones (T3 and T4), which regulate the body's metabolism, energy levels, and overall growth and development.

- Adrenocorticotropic Hormone (ACTH): Stimulates the adrenal cortex to produce glucocorticoids (like cortisol), which help manage stress, regulate metabolism, and control inflammation.

- Luteinizing Hormone (LH): In females, LH triggers ovulation and maintains the corpus luteum, which produces progesterone essential for pregnancy. In males, it stimulates the testes to produce testosterone, crucial for sperm production and secondary sexual characteristics.

- Follicle Stimulating Hormone (FSH): In females, FSH promotes the growth and maturation of ovarian follicles, which are vital for ovulation and fertility. In males, it works alongside testosterone to stimulate spermatogenesis, the process of sperm production.

• Pars Intermedia: Although almost merged with the pars distalis in humans, it produces melanocyte-stimulating hormone (MSH), which regulates the production and distribution of melanin, the pigment responsible for skin color.

Neurohypophysis (Posterior Pituitary)

• Hormones Stored and Released: The posterior pituitary does not produce hormones but stores and releases two crucial hormones synthesized by the hypothalamus.

- Oxytocin: Stimulates uterine contractions during childbirth and milk ejection from mammary glands.

- Vasopressin (Antidiuretic Hormone, ADH): Promotes water and electrolyte reabsorption in the kidneys, reducing urine output.

Disorders Related to Pituitary Gland

• Gigantism: Caused by an over-secretion of GH during childhood, leading to excessive growth and height.
• Pituitary Dwarfism: Results from a deficiency in GH during childhood, causing reduced growth and short stature.
• Acromegaly: Occurs in adults due to excess GH, resulting in abnormal growth of the hands, feet, and facial features. It can cause serious health complications if untreated.
• Diabetes Insipidus: Caused by insufficient ADH, leading to frequent urination and severe dehydration.

Pineal Gland

1. The pineal gland is a small endocrine gland located in the brain.

• Location and Structure

- Location: Situated on the dorsal (back) side of the forebrain.

- Structure: Small, pinecone-shaped gland.

• Hormone Produced

- Melatonin: The primary hormone produced by the pineal gland.

• Functions of Melatonin

- Regulation of Diurnal Rhythms: Melatonin is crucial for maintaining the body’s 24-hour (diurnal) rhythm, which includes:

1. Sleep-Wake Cycle: Melatonin levels rise in the evening, promoting sleep, and decrease in the morning, helping to wake up.

2. Body Temperature: Helps regulate the body's temperature throughout the day and night.

- Influence on Metabolism: Melatonin affects various metabolic processes, helping to regulate energy balance and body weight.

- Pigmentation: Plays a role in regulating skin pigmentation.

- Menstrual Cycle: Influences the menstrual cycle in females, potentially affecting fertility and reproductive health.

- Defense Capability: Impacts the immune system, helping to strengthen the body’s defense mechanisms against infections and diseases.

Thyroid Gland

• The thyroid gland is a vital endocrine gland responsible for producing hormones that regulate metabolism and various bodily functions.
• Location and Structure

- Location: The thyroid gland is located on either side of the trachea (windpipe).

- Structure: Composed of two lobes connected by a thin flap of connective tissue called the isthmus.

• Composition: Made up of follicles and stromal tissues. Each follicle contains follicular cells that surround a cavity.
• Hormones Produced

- Thyroxine (T4) and Triiodothyronine (T3): These hormones are synthesized by the follicular cells. Iodine is essential for their production.

- Thyroxine (T4): Contains four iodine atoms.

- Triiodothyronine (T3): Contains three iodine atoms.

- Thyrocalcitonin (TCT): A protein hormone also produced by the thyroid gland, which helps regulate blood calcium levels.

• Functions of Thyroid Hormones

- Regulation of Basal Metabolic Rate (BMR): Thyroid hormones play a crucial role in maintaining the body’s basal metabolic rate, which is the rate at which the body uses energy at rest.

- Metabolism: They control the metabolism of carbohydrates, proteins, and fats, ensuring efficient energy production and utilization.

- Red Blood Cell Formation: Support the production of red blood cells, which are essential for carrying oxygen throughout the body.

- Water and Electrolyte Balance: Help maintain proper water and electrolyte balance in the body.

• Disorders Related to Thyroid Gland

Hypothyroidism: A condition caused by insufficient production of thyroid hormones.

Common causes and effects include:

- Iodine Deficiency: Leads to reduced hormone synthesis, resulting in goiter (enlargement of the thyroid gland).

- During Pregnancy: Can cause cretinism in the baby, leading to stunted growth, mental retardation, low IQ, abnormal skin, and deaf-mutism.

- In Adult Women: May cause irregular menstrual cycles.

Hyperthyroidism: Caused by excessive production of thyroid hormones, which can lead to:

- Graves’ Disease (Exophthalmic Goiter): Characterized by an enlarged thyroid gland, protrusion of the eyeballs, increased basal metabolic rate, and weight loss. This condition is a form of hyperthyroidism.

• Importance of Iodine

Essential for Hormone Synthesis: Iodine is crucial for the production of T3 and T4. A deficiency can lead to hypothyroidism and goiter.

Parathyroid Gland

• The parathyroid glands are small endocrine glands located in the neck, behind the thyroid gland, and play a crucial role in regulating calcium levels in the blood.
• Location and Structure

- Location: Four parathyroid glands are located on the back side of the thyroid gland, with one pair in each lobe of the thyroid gland.

- Structure: These glands are small and typically arranged in pairs.

• Hormone Produced

- Parathyroid Hormone (PTH): A peptide hormone secreted by the parathyroid glands. The secretion of PTH is regulated by the circulating levels of calcium ions (Ca2+) in the blood.

• Functions of Parathyroid Hormone (PTH)

- Increases Blood Calcium Levels: PTH plays a key role in raising the levels of calcium in the blood (hypercalcemic effect). It achieves this through several mechanisms:

- Bone Resorption: PTH stimulates the breakdown of bone tissue (resorption), releasing calcium into the bloodstream.

- Renal Reabsorption: PTH enhances the reabsorption of calcium in the kidneys, reducing calcium loss through urine.

- Intestinal Absorption: PTH increases the absorption of calcium from the food in the intestines, further boosting blood calcium levels.

• Role in Calcium Balance

- Hypercalcemic Hormone: PTH is considered a hypercalcemic hormone because it increases blood calcium levels.

- Interaction with Thyrocalcitonin (TCT): PTH works in conjunction with thyrocalcitonin (TCT), a hormone produced by the thyroid gland that lowers blood calcium levels. Together, these hormones maintain a balanced level of calcium in the body, which is essential for various physiological functions, including nerve transmission, muscle contraction, and blood clotting.

Thymus

• The thymus gland is crucial for the development and function of the immune system.
• Location and Structure

- Location: The thymus is located between the lungs, behind the sternum, and on the ventral side of the aorta.

- Structure: It is a lobular structure, meaning it is divided into multiple lobes.

• Functions and Hormones

- Immune System Development: The thymus is vital for the development of the immune system, particularly during childhood.

- Hormones Produced: The thymus secretes peptide hormones called thymosins.

• Role of Thymosins

- T-Lymphocyte Differentiation: Thymosins play a major role in the differentiation and maturation of T-lymphocytes (T-cells). These cells are essential for cell-mediated immunity, which involves the direct attack of infected or cancerous cells by T-cells.

- Humoral Immunity: Thymosins also promote the production of antibodies by B-lymphocytes (B-cells), supporting humoral immunity. This type of immunity involves the production of antibodies that circulate in body fluids and target pathogens.

• Changes with Age

- Degeneration with Age: The thymus gland gradually degenerates with age, particularly after puberty. This degeneration leads to a decrease in the production of thymosins.

- Impact on Immune Response: As the production of thymosins decreases in older individuals, their immune responses become weaker. This decline contributes to the increased susceptibility to infections and diseases in the elderly.

Adrenal Gland

• The adrenal glands are essential endocrine glands involved in producing hormones that help regulate metabolism, immune response, blood pressure, and stress reactions.
• Location and Structure

- Location: There are two adrenal glands, each located on the anterior (front) part of the kidneys.

- Structure: The adrenal gland is composed of two distinct parts:

1. Adrenal Medulla: The inner, central part of the gland.

2. Adrenal Cortex: The outer part surrounding the medulla.

Hormones of the Adrenal Medulla

• Adrenaline (Epinephrine) and Noradrenaline (Norepinephrine): These hormones are collectively known as catecholamines.
• Function: They are rapidly secreted in response to stress and are known as emergency hormones or "fight or flight" hormones.
• Effects:

- Increase alertness.

- Cause pupil dilation.

- Induce piloerection (raising of hairs).

- Trigger sweating.

- Increase heart rate and strength of heart contractions.

- Increase respiratory rate.

- Stimulate glycogen breakdown, leading to increased blood glucose levels.

- Stimulate the breakdown of lipids and proteins.

Hormones of the Adrenal Cortex

• The adrenal cortex is divided into three layers, each producing different types of hormones:
• Zona Glomerulosa (Outer Layer):

- Mineralocorticoids: The main mineralocorticoid is aldosterone.

- Function: Regulates the balance of water and electrolytes in the body.

- Effects:

• Stimulates reabsorption of sodium (Na+) and water in the kidneys.
• Promotes excretion of potassium (K+) and phosphate ions.
• Helps maintain fluid volume, osmotic pressure, and blood pressure.
• Zona Fasciculata (Middle Layer):

- Glucocorticoids: The primary glucocorticoid is cortisol.

- Function: Involved in carbohydrate metabolism and stress response.

- Effects:

• Stimulates gluconeogenesis (production of glucose from non-carbohydrate sources).
• Promotes lipolysis (breakdown of fats) and proteolysis (breakdown of proteins).
• Inhibits cellular uptake and utilization of amino acids.
• Maintains cardiovascular and kidney functions.
• Exhibits anti-inflammatory effects and suppresses the immune response.
• Stimulates red blood cell production.
• Zona Reticularis (Inner Layer):

- Androgenic Steroids: Small amounts are secreted.

- Function: Contribute to the development of secondary sexual characteristics.

- Effects: Influence the growth of axial hair, pubic hair, and facial hair during puberty.

• Disorders Related to Adrenal Glands

- Addison's Disease: Caused by underproduction of hormones by the adrenal cortex.

- Symptoms: Includes acute weakness, fatigue, and altered carbohydrate metabolism.

Pancreas

• The pancreas is a unique organ that functions both as an exocrine and an endocrine gland, playing a crucial role in digestion and the regulation of blood glucose levels.
• Structure and Function

- Composite Gland: The pancreas has both exocrine and endocrine functions.

- Exocrine Function: Produces digestive enzymes.

- Endocrine Function: Regulates blood sugar levels through the secretion of hormones.

• Endocrine Pancreas

- Islets of Langerhans: The endocrine portion of the pancreas consists of clusters of cells known as the Islets of Langerhans. There are about 1 to 2 million islets in a normal human pancreas, making up only 1 to 2% of the pancreatic tissue.

- Cell Types and Hormones:

α-cells (Alpha Cells): Secrete glucagon.

β-cells (Beta Cells): Secrete insulin.

• Hormones and Their Functions

- Glucagon:

Type: Peptide hormone.

Function: Increases blood glucose levels (hyperglycemia).

Mechanism:

• Glycogenolysis: Stimulates the breakdown of glycogen into glucose in the liver.
• Gluconeogenesis: Promotes the formation of glucose from non-carbohydrate sources.
• Reduction in Glucose Uptake: Decreases cellular uptake and utilization of glucose.

- Insulin:

Type: Peptide hormone.

Function: Decreases blood glucose levels (hypoglycemia).

Mechanism:

• Glucose Uptake: Enhances the uptake and utilization of glucose by cells, particularly hepatocytes (liver cells) and adipocytes (fat cells).
• Glycogenesis: Stimulates the conversion of glucose to glycogen for storage in the liver and muscles.
• Regulation of Blood Glucose

- Glucose Homeostasis: The balance of blood glucose levels is maintained by the opposing actions of insulin and glucagon.

- Hyperglycemia: Elevated blood glucose levels trigger the release of insulin, which lowers glucose levels.

- Hypoglycemia: Low blood glucose levels trigger the release of glucagon, which raises glucose levels.

• Disorders

- Diabetes Mellitus: A chronic condition characterized by prolonged hyperglycemia.

- Symptoms: Loss of glucose through urine and formation of ketone bodies, which can be harmful.

- Treatment: Insulin therapy is commonly used to manage diabetes by helping regulate blood glucose levels.

Testes

• The testes, located in the scrotal sac outside the abdomen, serve as both primary reproductive organs and endocrine glands in males.
• Structure and Composition

- Seminiferous Tubules: Structures within the testes where sperm production occurs.

- Interstitial Tissue: Also known as stromal tissue, contains Leydig cells or interstitial cells.

• Hormones Produced

- Androgens: Mainly testosterone, produced by Leydig cells in the interstitial tissue.

• Functions of Androgens

- Regulation of Male Reproductive Organs: Androgens play a crucial role in the development, maturation, and function of male accessory sex organs, such as the epididymis, vas deferens, seminal vesicles, prostate gland, and urethra.

- Physical Characteristics: Androgens stimulate muscular growth and the development of facial and axillary hair. They also contribute to traits such as aggressiveness and a low-pitched voice.

- Spermatogenesis: Androgens are essential for the process of spermatogenesis, the formation of spermatozoa.

- Neural Effects: Androgens act on the central nervous system, influencing male sexual behavior and libido.

- Metabolic Effects: Androgens have anabolic effects on protein and carbohydrate metabolism, promoting tissue growth and repair.

Ovary

• The ovaries are a pair of primary female reproductive organs located in the abdomen.
• Structure and Function

- Ovum Production: Each ovary produces one ovum (egg) during each menstrual cycle.

• Steroid Hormones:

- Estrogen: Produced mainly by growing ovarian follicles.

- Progesterone: Secreted primarily by the corpus luteum after ovulation.

• Hormones and Their Functions

Estrogen:

- Stimulates growth and activity of female secondary sex organs.

- Promotes development of ovarian follicles.

- Contributes to the appearance of female secondary sex characteristics, such as a high-pitched voice.

- Regulates female sexual behavior.

Progesterone:

- Supports pregnancy by preparing the uterus for implantation of a fertilized egg.

- Stimulates the formation of alveoli in the mammary glands and promotes milk secretion.

Hormones of Heart, Kidney, and Gastrointestinal Tract

• Apart from the traditional endocrine glands, various tissues in the body also secrete hormones that play significant roles in physiological regulation.

Heart

• Atrial Natriuretic Factor (ANF):

- Source: Secreted by the atrial wall of the heart.

- Function: Decreases blood pressure by causing dilation of blood vessels.

- Mechanism: Released in response to increased blood pressure, ANF promotes vasodilation, reducing blood pressure.

Kidney

• Erythropoietin:

- Source: Produced by juxtaglomerular cells of the kidney.

- Function: Stimulates erythropoiesis, the formation of red blood cells (RBCs).

Gastrointestinal Tract

• Gastrin:

- Source: Produced by endocrine cells in the stomach.

- Function: Stimulates secretion of hydrochloric acid and pepsinogen from gastric glands.

• Secretin:

- Source: Released from endocrine cells in the small intestine.

- Function: Stimulates secretion of water and bicarbonate ions from the exocrine pancreas.

• Cholecystokinin (CCK):

- Source: Produced by endocrine cells in the small intestine.

- Function: Stimulates secretion of pancreatic enzymes and bile juice from the pancreas and gallbladder, respectively.

• Gastric Inhibitory Peptide (GIP):

- Source: Secreted by endocrine cells in the small intestine.

- Function: Inhibits gastric secretion and motility.

• Growth Factors

- Function: Essential for normal tissue growth, repair, and regeneration.

• Mechanism: These factors stimulate cellular growth, proliferation, and differentiation, promoting tissue development and maintenance.

Mechanism of Hormone Action

• Hormones exert their effects on target tissues by binding to specific proteins called hormone receptors, which are located either on the cell membrane (membrane-bound receptors) or inside the target cell (intracellular receptors).

Types of Hormone Receptors

• Membrane-Bound Receptors:

- Located on the cell membrane.

- Interact with peptide, polypeptide, and protein hormones.

- Activation leads to the generation of second messengers (e.g., cyclic AMP, IP3, Ca++) which regulate cellular metabolism.

• Intracellular Receptors:

- Mostly nuclear receptors, located inside the target cell.

- Interact with steroid hormones, iodothyronines, etc.

- Regulation of gene expression or chromosome function by the hormone-receptor complex interacting with the genome.

• Hormone-Receptor Complex Formation

- Specificity: Each hormone receptor is specific to one hormone only.

- Biochemical Changes: Binding of a hormone to its receptor leads to the formation of a hormone-receptor complex, initiating biochemical changes in the target tissue.

• Categories of Hormones

1. Peptide, Polypeptide, Protein Hormones:

- Examples include insulin, glucagon, pituitary hormones, and hypothalamic hormones.

2. Steroids:

- Examples include cortisol, testosterone, estradiol, and progesterone.

3. Iodothyronines:

- Thyroid hormones fall under this category.

4. Amino Acid Derivatives:

- Examples include epinephrine.

• Functional Effects

- Regulation of Metabolism: Hormone-receptor complex formation regulates target tissue metabolism and physiological functions.

- Developmental Effects: Cumulative biochemical actions of hormones result in physiological and developmental effects.