6.2 Endocrine System of Man

The endocrine system is a glandular system composed of approximately 20 ductless glands that secrete chemical messengers called hormones directly into the bloodstream. These hormones travel throughout the body to target cells and regulate various physiological processes such as growth, metabolism, reproduction, and stress response.

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Major Endocrine Glands and Locations

The primary endocrine glands are located throughout the body as shown below.

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1. Pituitary Gland

The pituitary gland is located below the hypothalamus and is often called the "master gland" because its hormones regulate many other endocrine glands. It is divided into three lobes: anterior, median, and posterior.

Anterior Lobe

This lobe produces and secretes six major hormones, many of which are regulated by releasing and inhibiting factors from the hypothalamus.

• Growth Hormone (GH) or Somatotrophic Hormone (STH):

  • Function: Stimulates cell growth, division, protein synthesis, and uptake of amino acids.
  • Regulation: Stimulated by GHRF and inhibited by somatostatin from the hypothalamus.
  • Disorders:
    • Dwarfism: Deficiency in childhood leads to short stature with proportional body parts.
    • Gigantism: Oversecretion in childhood leads to abnormal height increase.
    • Acromegaly: Oversecretion in adulthood causes bones (hands, feet, jaw) to thicken.

• Thyroid Stimulating Hormone (TSH):

  • Function: Regulates the synthesis and release of hormones from the thyroid gland.
  • Regulation: Stimulated by TRF from the hypothalamus.
  • Disorders: Oversecretion causes hyperthyroidism; undersecretion causes hypothyroidism.

• Adrenocorticotropic Hormone (ACTH):

  • Function: Stimulates the adrenal cortex to secrete corticosteroids (e.g., cortisone).
  • Regulation: Stimulated by CRF from the hypothalamus, often in response to stress.

• Gonadotropic Hormones: These hormones regulate the reproductive system.

  • Follicle-Stimulating Hormone (FSH): Controls egg and sperm production.
  • Luteinising Hormone (LH) / Interstitial Cell Stimulating Hormone (ICSH): Controls sex hormone production (estrogen, testosterone).
  • Prolactin (LTH): Initiates and maintains milk production in breasts.

Median Lobe

In humans, this is a thin, less prominent layer of cells.

• Melanocyte-Stimulating Hormone (MSH):
  • Function: Stimulates the production and release of melanin in skin and hair, influencing pigmentation. Secretion increases during pregnancy.

Posterior Lobe

This lobe does not synthesize hormones; it stores and releases two hormones produced by the hypothalamus.

• Antidiuretic Hormone (ADH) or Vasopressin:

  • Function: Promotes water reabsorption in the kidneys to reduce urine output, thus conserving body water. High concentrations can cause vasoconstriction, increasing blood pressure.
  • Regulation: Released in response to dehydration or low blood volume.
  • Disorders: Undersecretion leads to Diabetes Insipidus (excessive thirst and production of dilute urine).
  • Functions of Kidney→

• Oxytocin:

  • Function:
    • Stimulates powerful uterine contractions during childbirth.
    • Promotes milk ejection ("let-down") from mammary glands during nursing.
  • Disorders: Undersecretion can inhibit the normal labor process.

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2. Thyroid Gland

Located in the neck, on either side of the trachea.

• Thyroxine (T4) and Triiodothyronine (T3):

  • Function: Iodine-containing hormones that regulate the body's basal metabolic rate, promote nervous system development, and support skeletal growth.
  • Regulation: Stimulated by TSH from the pituitary.
  • Disorders:
    • Hyperthyroidism: Oversecretion. Can lead to Graves' Disease, an autoimmune disorder with symptoms like high metabolic rate, weight loss, and protruding eyeballs (exophthalmia).
    • Hypothyroidism: Undersecretion.
      • In adults: Myxedema, characterized by low metabolic rate, goiter (enlarged thyroid), and edema.
      • In infants: Cretinism, characterized by mental retardation and poor physical growth.

• Calcitonin:

  • Function: Lowers blood calcium levels by promoting calcium deposition in bones and increasing its excretion by the kidneys.
  • Regulation: Released when blood calcium levels are high.

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3. Parathyroid Glands

Four small glands located on the posterior surface of the thyroid.

• Parathormone (PTH):
  • Function: The single most important hormone for controlling blood calcium. It raises blood calcium levels by stimulating calcium release from bones. It works antagonistically to calcitonin.
  • Regulation: Released when blood calcium levels are low.
  • Disorders:
    • Oversecretion (Hypercalcemia): Bones soften and fracture easily; nervous system is depressed; kidney stones may form.
    • Undersecretion (Hypocalcemia): Leads to increased neuron excitability and tetany (prolonged muscle contraction).

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4. Pancreas

Has both exocrine (digestive enzymes) and endocrine functions. The endocrine cells are located in patches called the Islets of Langerhans.

• Insulin: Secreted by Beta (β) cells.

  • Function: Lowers blood glucose levels by increasing glucose uptake by cells and promoting its conversion to glycogen (glycogenesis).
  • Regulation: Released when blood glucose is high (e.g., after a meal).
  • Disorders: Undersecretion causes Diabetes Mellitus, characterized by high blood glucose.

• Glucagon: Secreted by Alpha (α) cells.

  • Function: Raises blood glucose levels, acting antagonistically to insulin.
  • Regulation: Released when blood glucose is low.

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5. Adrenal Glands

Located on top of each kidney, consisting of an outer cortex and inner medulla.

Adrenal Medulla (Inner)

• Epinephrine (Adrenaline) & Norepinephrine:
  • Function: "Fight-or-flight" hormones released during stress. They increase heart rate, blood pressure, and blood flow to muscles. Epinephrine has a stronger effect on metabolic rate, while norepinephrine is more influential on peripheral vasoconstriction.
  • Baroreceptors and Volume Receptors→

Adrenal Cortex (Outer)

Produces corticosteroids regulated by ACTH.

• Glucocorticoids (e.g., Cortisone): Help regulate blood glucose levels.
• Mineralocorticoids (e.g., Aldosterone): Regulate salt and water balance.
• Disorders:
  • Addison's Disease: Undersecretion, causing muscle weakness and metabolic disturbance.
  • Cushing's Disease: Oversecretion, causing protein breakdown and bone/muscle weakness.
• Androgens (e.g., Testosterone): Produced in small amounts in both sexes.

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6. Gonads (Ovaries and Testes)

Produce both gametes (eggs/sperm) and sex hormones.

• Ovaries (Female):

  • Estrogen: Responsible for female secondary sex characteristics and repair of the uterine wall.
  • Progesterone: Prepares and maintains the uterus for pregnancy and suppresses further ovulation.

• Testes (Male):

  • Testosterone: Responsible for male secondary sex characteristics, maturation of reproductive organs, and sperm production.

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Comprehensive Hormone Summary Table

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Possible Questions/Answers

• Q: What is the difference between endocrine and exocrine glands? A: Endocrine glands are ductless and secrete hormones directly into the bloodstream. Exocrine glands (e.g., sweat, salivary glands) release substances through ducts onto body surfaces or into cavities.

• Q: Why is the pituitary gland called the "master gland"? A: Because it secretes several hormones (like TSH, ACTH, and gonadotropins) that control the function of other endocrine glands throughout the body.

• Q: How do insulin and glucagon work together to maintain blood sugar homeostasis? A: They work as an antagonistic pair. When blood sugar is high, the pancreas releases insulin to lower it. When blood sugar is low, the pancreas releases glucagon to raise it, ensuring the level stays within a normal range.

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