Endocrine System

Pituitary Gland

Often called the "master gland," it regulates many other endocrine glands and produces hormones like growth hormone (GH), prolactin, and adrenocorticotropic hormone (ACTH).

Hormone

is a chemical messenger produced by endocrine glands that travels through the bloodstream to regulate various physiological processes in the body. Hormones help control functions such as metabolism, growth, reproduction, mood, and homeostasis.

Key Characteristics of Hormones:

• Produced by Endocrine Glands: Such as the pituitary, thyroid, adrenal glands, pancreas, and gonads.

• Travel Through the Bloodstream: Unlike neurotransmitters, which act locally, hormones have systemic effects.

• Bind to Specific Receptors: Each hormone affects only target cells that have the appropriate receptors.

• Regulate Body Functions: Including metabolism (e.g., insulin), stress response (e.g., cortisol), growth (e.g., growth hormone), and reproduction (e.g., estrogen and testosterone).

Target tissue

refers to the specific cells, tissues, or organs that a hormone acts upon. These tissues contain receptors that recognize and bind to a particular hormone, allowing the hormone to exert its effects.

Key Features of Target Tissue

• Specificity: Only cells with the correct receptors respond to a hormone.

• Hormone-Receptor Interaction: Once a hormone binds to its receptor, it triggers a biological response.

• Varied Effects: Different target tissues can respond differently to the same hormone.

Examples of Hormones and Their Target Tissues:

• Insulin → Targets liver, muscle, and fat cells to regulate blood sugar.

• Thyroid Hormones (T3 & T4) → Target almost all body cells to regulate metabolism.

• Adrenaline (Epinephrine) → Targets heart, lungs, and muscles for the "fight or flight" response.

• Estrogen & Testosterone → Target reproductive organs, bones, and brain for sexual development and function.

Negative feedback

is a regulatory mechanism in which a system responds to a stimulus by reducing or stopping its output to maintain homeostasis. It helps keep physiological processes stable by counteracting changes.

How Negative Feedback Works:

• A stimulus causes a change (e.g., an increase or decrease in a hormone level).

• A sensor detects the change (e.g., receptors in the body).

• A control center processes the information (e.g., the hypothalamus or endocrine glands).

• An effector counteracts the change (e.g., reducing or stopping hormone secretion).

• The system returns to balance, stopping further correction once normal levels are restored.

Examples of Negative Feedback in the Endocrine System:

• Blood Sugar Regulation (Insulin & Glucagon)

  • High blood sugar → Pancreas releases insulin → Cells absorb glucose → Blood sugar drops → Insulin secretion stops.

  • Low blood sugar → Pancreas releases glucagon → Liver releases glucose → Blood sugar rises → Glucagon secretion stops.

• Thyroid Hormone Regulation (Hypothalamic-Pituitary-Thyroid Axis)

  • Low thyroid hormone (T3/T4) → Hypothalamus releases TRH → Pituitary releases TSH → Thyroid releases T3/T4 → Normal levels reached → TRH & TSH release stops.

• Body Temperature Control

  • Body overheats → Sweating & blood vessel dilation → Heat loss → Temperature drops → Cooling response stops.

  • Body gets too cold → Shivering & blood vessel constriction → Heat is retained → Temperature rises → Heating response stops.

hypothalamus

Is small, but critical part of the brain located just below the thalamus and above the brainstem. It plays a significant role in maintaining homeostasis (internal balance) by linking the nervous system to the endocrine system.Regulation of the Autonomic Nervous System,Temperature Regulation,Hormone Production,Control of the Pituitary Gland,Control of the Pituitary Gland,Thirst and Hunger Regulation,Sleep-Wake Cycles,Emotional and Behavioral Responses:

Anterior pituitary

(also known as the adenohypophysis) is the front portion of the pituitary gland, located at the base of the brain. It plays a critical role in the regulation of many body functions through the release of hormones. The anterior pituitary is controlled by the hypothalamus, which sends signals to it to either stimulate or inhibit hormone release.

Key Functions of the Anterior Pituitary:

1. Growth Hormone (GH) – Stimulates growth, cell reproduction, and regeneration. It influences the growth of bones, muscles, and tissues. It also helps regulate metabolism.

2. Thyroid-Stimulating Hormone (TSH) – Stimulates the thyroid gland to produce and release thyroid hormones (T3 and T4), which regulate metabolism and energy production in the body.

3. Adrenocorticotropic Hormone (ACTH) – Stimulates the adrenal glands to produce cortisol, a hormone involved in the body’s stress response, immune system regulation, and metabolism.

4. Prolactin (PRL) – Stimulates milk production in the mammary glands after childbirth. It also plays a role in reproductive health.

5. Follicle-Stimulating Hormone (FSH) – In females, it stimulates the growth and maturation of ovarian follicles (which contain eggs) and the production of estrogen. In males, it stimulates sperm production in the testes.

6. Luteinizing Hormone (LH) – In females, it triggers ovulation (the release of an egg from the ovary) and the formation of the corpus luteum, which produces progesterone. In males, it stimulates the production of testosterone in the testes.

7. Melanocyte-Stimulating Hormone (MSH) – Influences the production of melanin in skin cells, affecting skin pigmentation. Though its exact role is still under investigation, it may also be involved in appetite regulation and sexual behavior.

Posterior pituitary

(also known as the neurohypophysis) is the back portion of the pituitary gland, located just beneath the hypothalamus in the brain. Unlike the anterior pituitary, the posterior pituitary doesn't produce its own hormones; instead, it stores and releases hormones that are produced by the hypothalamus. These hormones are transported down nerve fibers (axons) to the posterior pituitary for storage and release into the bloodstream.

Key Functions of the Posterior Pituitary:

Key Functions of the Posterior Pituitary:

The posterior pituitary is responsible for the release of two important hormones:

1. Oxytocin:

   • Role in Reproduction: It stimulates uterine contractions during childbirth, aiding in labor and delivery. It also plays a key role in the milk ejection reflex (or "let-down" reflex), helping to release milk from the mammary glands when the baby suckles.

   • Social and Emotional Functions: Oxytocin is also known as the "love hormone" or "bonding hormone" because it is involved in social bonding, trust, and emotional regulation. It is released during activities such as hugging, childbirth, and breastfeeding.

2. Antidiuretic Hormone (ADH) or Vasopressin:

   • Water Balance Regulation: ADH helps regulate the body’s water balance by promoting water reabsorption in the kidneys. This reduces urine output and helps the body retain water when it is dehydrated.

   • Blood Pressure Regulation: ADH can also cause blood vessels to constrict (vasoconstriction), which raises blood pressure, particularly during situations of low blood volume or dehydration.

Summary of the Differences Between the Anterior and Posterior Pituitary

• Anterior Pituitary: Produces its own hormones, which regulate other endocrine glands (e.g., thyroid, adrenal glands, gonads).

• Posterior Pituitary: Does not produce its own hormones; instead, it stores and releases hormones (oxytocin and ADH) produced by the hypothalamus.

Thyroid Gland

Produces thyroid hormones (T3 and T4) that regulate metabolism, growth, and energy levels.

A thyroid follicle

is the basic structural unit of the thyroid gland, which is located in the neck and plays a crucial role in regulating metabolism through the production of thyroid hormones. The thyroid follicles are small, spherical structures that are filled with a substance called colloid, and their walls are made up of follicular cells.

Parathyroid Glands

Secrete parathyroid hormone (PTH), which controls calcium levels in the blood.

Pineal Gland

Produces melatonin, which regulates sleep-wake cycles.

Produces thymosin, which is essential for immune system development, particularly in childhood.

Antidiuretic Hormone (ADH) or Vasopressin

• Secreting Organ: Posterior Pituitary (produced in the hypothalamus)

• Target Tissue: Kidneys, blood vessels

• Main Effects:

  • Promotes water retention by the kidneys, reducing urine output.

  • Increases blood pressure by causing blood vessels to constrict (vasoconstriction).

Oxytocin

• Secreting Organ: Posterior Pituitary (produced in the hypothalamus)

• Target Tissue: Uterus, mammary glands, brain

• Main Effects:

  • Stimulates uterine contractions during childbirth.

  • Promotes milk ejection during breastfeeding.

  • Influences bonding and social behaviors (affection, trust).

Thyroid-Stimulating Hormone (TSH)

• Secreting Organ: Anterior Pituitary

• Target Tissue: Thyroid Gland

• Main Effects:

  • Stimulates the thyroid gland to produce and release thyroid hormones (T3 and T4).

Adrenocorticotropic hormone (ACTH

is a hormone produced and secreted by the anterior pituitary gland. It plays a key role in the regulation of the adrenal glands, particularly in the production and release of cortisol. Here's a detailed look at ACTH:

Secreting Organ:

• Anterior Pituitary Gland (also known as the adenohypophysis)

Target Tissue:

• Adrenal Cortex (specifically, the zona fasciculata)

Main Effects:

• Stimulates Cortisol Production: ACTH primarily stimulates the adrenal cortex to produce and release cortisol, which is a hormone involved in stress response, metabolism, and immune function.

• Regulation of Stress Response: Cortisol helps the body respond to stress by increasing blood sugar levels, suppressing the immune system, and promoting the breakdown of fats and proteins for energy.

• Increases Aldosterone Secretion: While ACTH’s main role is to stimulate cortisol production, it can also mildly stimulate the release of aldosterone, a hormone that helps regulate blood pressure and sodium balance.

Growth Hormone (GH)

• Secreting Organ: Anterior Pituitary

• Target Tissue: Bones, muscles, liver, adipose tissue

• Main Effects:

  • Stimulates growth and development of bones and muscles.

  • Increases protein synthesis and promotes fat breakdown.

  • Stimulates the liver to produce IGF-1, which helps with growth and cell regeneration.

Prolactin

• Secreting Organ: Anterior Pituitary

• Target Tissue: Mammary glands

• Main Effects:

  • Stimulates milk production after childbirth.

  • Plays a role in reproductive health and may influence mood.

Melatonin

• Secreting Organ: Pineal Gland

• Target Tissue: Brain (especially the hypothalamus)

• Main Effects:

  • Regulates circadian rhythms (sleep-wake cycle).

  • Induces sleepiness and helps control the timing of the sleep-wake cycle.

Thyroid Hormones (T3 and T4)

• Secreting Organ: Thyroid Gland

• Target Tissue: Almost all body cells

• Main Effects:

  • Regulate metabolism by increasing oxygen consumption and heat production.

  • Stimulate growth and development.

  • Influence heart rate and cholesterol levels.

Calcitonin

a hormone produced by the thyroid gland, specifically by the parafollicular cells (also known as C cells) in the thyroid. It plays a key role in regulating calcium and phosphate balance in the body, though its effects are less pronounced compared to other hormones like parathyroid hormone (PTH).Secreting Organ: Thyroid gland (parafollicular cells/C cells)

• Target Tissue: Bones, kidneys, intestines

• Main Effects: Lowers blood calcium levels by inhibiting osteoclast activity, promoting calcium deposition in bones, increasing calcium excretion by the kidneys.

Thymosin

• Secreting Organ: Thymus

• Target Tissue: T lymphocytes (T cells), immune system cells

• Main Effects:

  • Maturation of T cells: Thymosin is involved in the maturation of T lymphocytes (T cells). These are a type of white blood cell that plays a critical role in immune responses. The thymus is the primary site where T cells mature before being released into the bloodstream.

  • Immune Function Regulation: Thymosin helps regulate the immune system by promoting the development of T cells that can recognize and respond to pathogens, such as viruses, bacteria, and cancer cells.

  • Thymosin Alpha-1: One of the forms of thymosin, thymosin alpha-1, has been studied for its potential to enhance immune system function, especially in conditions of immune suppression.

Thymopoietin

• Secreting Organ: Thymus

• Target Tissue: T lymphocytes (T cells), immune system cells

• Main Effects:

  • Regulation of T cell development: Thymopoietin plays a significant role in the maturation and differentiation of T cells in the thymus. It helps T cells acquire the ability to recognize specific foreign antigens presented by other cells.

  • Enhancement of T cell function: Thymopoietin also supports the function of T cells in recognizing and responding to infections and foreign invaders. It helps in the final stages of T cell maturation, including the expression of surface markers required for T cell activation.

  • Modulation of Immune Response: Thymopoietin has been suggested to influence the balance between immune tolerance and immune activation, which is crucial for preventing autoimmune diseases while still allowing immune responses to pathogens.

Parathyroid Hormone (PTH)

• Secreting Organ: Parathyroid Glands

• Target Tissue: Bones, kidneys, intestines

• Main Effects:

  • Increases blood calcium levels by stimulating calcium release from bones, enhancing calcium reabsorption by kidneys, and increasing calcium absorption from the intestines.

Cortisol

• Secreting Organ: Adrenal Glands (specifically, the adrenal cortex)

• Target Tissue: Liver, muscle, adipose tissue, immune system

• Main Effects:

  • Increases blood glucose levels by promoting gluconeogenesis (production of glucose from non-carbohydrate sources).

  • Suppresses the immune system and reduces inflammation.

  • Helps the body respond to stress.

Aldosterone

• Secreting Organ: Adrenal Glands (zona glomerulosa of the adrenal cortex)

• Target Tissue: Kidneys (mainly distal tubules and collecting ducts), sweat glands, salivary glands

• Main Effects: Regulates sodium and water balance, increases blood volume and blood pressure, promotes potassium excretion, and helps maintain acid-base balance.

• Regulation: Controlled by the renin-angiotensin-aldosterone system (RAAS), potassium levels, and ACTH.

Adrenaline (Epinephrine)

• Secreting Organ: Adrenal Glands (medulla)

• Target Tissue: Heart, lungs, muscles, blood vessels

• Main Effects:

  • Increases heart rate and force of contraction.

  • Dilates airways (bronchodilation) to improve oxygen intake.

  • Stimulates the release of glucose for immediate energy (fight or flight response).

Norepinephrine (also known as noradrenaline

• Secreting Organ: Adrenal Glands (adrenal medulla) and sympathetic nerve endings

• Target Tissues: Heart, blood vessels, lungs, liver, brain

• Main Effects: Increases heart rate and force of contraction, constricts blood vessels to raise blood pressure, dilates airways, promotes glucose release, enhances alertness, and dilates pupils.

• Clinical Relevance: Plays a role in stress response, hypertension, mood disorders, and is used therapeutically in shock to raise blood pressure.

Insulin

• Secreting Organ: Pancreas (beta cells in the islets of Langerhans)

• Target Tissue: Liver, muscle, fat cells

• Main Effects:

  • Lowers blood glucose levels by promoting glucose uptake into cells.

  • Stimulates glycogen synthesis in the liver and muscle.

  • Promotes fat storage and protein synthesis.

Glucagon

• Secreting Organ: Pancreas (alpha cells of the islets of Langerhans)

• Target Tissues: Liver, adipose tissue, muscle cells

• Main Effects: Raises blood glucose levels by promoting glycogen breakdown (glycogenolysis), gluconeogenesis, and fat breakdown. Inhibits glycogen storage.

• Regulation: Secretion is primarily stimulated by low blood glucose levels and inhibited by high blood glucose levels.

The pancreas

is a glandular organ located in the abdomen, behind the stomach, and plays a crucial role in both digestive and endocrine functions. It has both exocrine and endocrine functions, making it a mixed gland. The pancreas is both an exocrine (digestive) and endocrine (hormonal) organ.

• Exocrine function: Produces digestive enzymes and bicarbonate to aid in digestion.

• Endocrine function: Regulates blood glucose levels through hormones like insulin, glucagon, somatostatin, and pancreatic polypeptide.

Pancreatic Islets (Islets of Langerhans)

• are clusters of endocrine cells in the pancreas that secrete hormones regulating blood glucose levels.

• They contain alpha cells (glucagon), beta cells (insulin), delta cells (somatostatin), PP cells (pancreatic polypeptide), and epsilon cells (ghrelin).

• Their main function is glucose homeostasis, controlled by insulin (lowers blood sugar) and glucagon (raises blood sugar).

• Disorders like diabetes mellitus occur when the balance of insulin and glucagon is disrupted

Adrenal Cortex

• is the outer portion of the adrenal gland and produces steroid hormones.

• It has three layers, each responsible for different hormones:

  • Zona Glomerulosa → Aldosterone (regulates blood pressure & electrolytes).

  • Zona Fasciculata → Cortisol (regulates metabolism, stress, and immune response).

  • Zona Reticularis → Androgens (precursor for sex hormones).

• Hormone production is regulated by RAAS, ACTH, and stress responses.

• Disorders like Cushing’s syndrome, Addison’s disease, and hyperaldosteronism affect adrenal cortex function.

Adrenal medulla

• the inner part of the adrenal gland and functions as part of the sympathetic nervous system.

• It secretes epinephrine (adrenaline) and norepinephrine (noradrenaline), which help the body respond to stress and danger.

• These hormones increase heart rate, blood pressure, energy availability, and alertness.

• Regulation is controlled by the nervous system, not hormones.

• Disorders include pheochromocytoma (excess catecholamines) and adrenaline-related issues.

Ovaries

produce estrogen, progesterone, and inhibin.

• These hormones regulate menstruation, ovulation, pregnancy, and secondary sexual characteristics.

• The HPO axis controls ovarian function via FSH and LH.

• Disorders like PCOS, ovarian cysts, menopause, and ovarian cancer can affect hormone production and fertility.

Testes

• produce testosterone, inhibin, and androstenedione.

• Testosterone regulates male secondary sexual characteristics, sperm production, muscle mass, and libido.

• The HPG axis (Hypothalamus-Pituitary-Gonadal Axis) controls testosterone and sperm production.

• Disorders include hypogonadism, testicular cancer, cryptorchidism, and infertility.