endocrine system

ENDROCRINE SYSTEM

Overview of the Endocrine System

  • The endocrine system works alongside the nervous system to coordinate and integrate bodily functions.

  • The hypothalamus links both systems.

  • Ductless glands secrete hormones directly into the bloodstream to target organs.

Principal Endocrine Glands

  1. Pituitary Gland (Anterior & Posterior)

  2. Thyroid Gland

  3. Parathyroid Glands

  4. Adrenal Gland (Cortex & Medulla)

  5. Islets of Langerhans (in the pancreas)

  6. Gonads (Testes and Ovaries)

HORMONES

Definition

  • Hormones are chemical substances synthesized and secreted by specific cells, transported via circulation, and elicit specific responses in target tissues.

Chemistry of Hormones

  • Types of Hormones:

    1. Steroid Hormones: Derived from cholesterol (e.g., cortisol, aldosterone).

    2. Amino Acid Derivatives: E.g., Thyroid hormones, catecholamines (from tyrosine).

    3. Peptide/Protein Hormones: E.g., Anterior and posterior pituitary hormones, parathyroid hormones.

GROWTH HORMONE (Somatotrophic Hormone)

  • Structure: Small protein molecule.

  • Function: Promotes growth in all tissues (bones, muscles, viscera).

Actions of Growth Hormone

  1. Effect on Cartilage and Bone:

    • Acts indirectly to stimulate the liver and other tissues to produce Somatomedins that promote cartilage and bone growth.

  2. Metabolic Functions:

    • Protein: Increases amino acid transport into cells.

    • Fat: Stimulates lipolysis, releasing fatty acids for energy.

    • Carbohydrates: Decreases cellular glucose uptake, increasing blood glucose levels.

  3. Other Functions:

    • Stimulates red blood cell production (erythropoiesis).

    • Increases calcium absorption in the GI tract and reduces urinary excretion of sodium and potassium.

Disorders of Growth Hormone Secretion

  • Deficiency (Pituitary Hypofunction): Leads to dwarfism; short stature with proportional body growth but no mental retardation.

  • Excess (Pituitary Hyperfunction): Leads to gigantism (before epiphyseal fusion) and acromegaly (after fusion).

    • Gigantism: Rapid growth of all body tissues; increased height, muscle size; hyperglycemia.

    • Acromegaly: Soft tissue overgrowth; enlargement of hands, feet, skull features, potentially involving diabetes.

POSTERIOR PITUITARY GLAND

Hormones

  1. Antidiuretic Hormone (ADH)

  2. Oxytocin

Functions of Antidiuretic Hormone (ADH)

  • Kidneys: Increases water reabsorption in nephrons; reduces urine volume.

  • Blood Vessels: Potent vasoconstrictor; raises blood pressure.

  • Deficiency: Causes Diabetes Insipidus; leads to polyuria and polydipsia.

Function of Oxytocin

  • Milk ejection during lactation and uterine contraction during childbirth.

THYROID GLAND

Structure

  • Composed of two lobes connected by an isthmus, located inferior to the larynx.

Hormones Secreted

  1. Thyroxine (T4)

  2. Triiodothyronine (T3)

  3. Calcitonin

Functions of Thyroid Hormones (T3 & T4)

  1. Calorigenic Effect: Stimulates metabolism and oxygen consumption.

  2. Metabolic Effects:

    • Increases protein synthesis; promotes growth.

    • Increases glucose absorption and metabolism.

    • Enhances fat metabolism, reducing blood lipid levels.

  3. Effects on Growth & Development: Critical for normal growth and nervous system development in early life.

Regulation of Thyroid Hormones

  • Controlled by Thyroid Stimulating Hormone (TSH) from the anterior pituitary; feedback system maintains balance.

Abnormalities

  • Hyperthyroidism: Overproduction of T3/T4, leading to weight loss, heat intolerance, and possible exophthalmos (Graves' disease).

  • Hypothyroidism: Low hormone secretion; in children, leads to cretinism (growth failure), and in adults can cause myxedema (low metabolic rate).

PARATHYROID GLANDS

Structure

  • Four glands behind the thyroid.

Hormones

  • Parathyroid Hormone (PTH)

Actions of PTH

  • Elevates calcium levels in blood, reduces phosphate concentration.

  • Actions on bones (increases resorption), kidneys (increases calcium reabsorption), and intestines (enhances absorption).

SUPRARENAL (ADRENAL) GLAND

Structure

  • Located on superior kidneys, composed of cortex and medulla.

Medulla Function

  • Secretes catecholamines (adrenaline and noradrenaline) in response to stress.

Actions of Catecholamines

  • Increase metabolic rate, blood glucose, respiratory rate; assist in 'fight or flight' response.

Cortex Function

  • Secretes corticosteroids:

    1. Mineralocorticoids (Aldosterone): Regulate sodium and potassium balance.

    2. Glucocorticoids (Cortisol): Influence glucose metabolism, exhibit anti-inflammatory and immunosuppressive actions.

Abnormalities

  • Addison's Disease: Adrenal insufficiency; causes weight loss, hypoglycemia, and hyperpigmentation.

  • Cushing's Syndrome: Excess cortisol; features include 'moon face', hypertension, and protein loss leading to weakness.

PANCREAS

Structure

  • Composed of acini (digestive enzyme secretion) and islets of Langerhans (insulin and glucagon secretion).

Hormones

  • Insulin: Lowers blood glucose; facilitates glucose entry into cells, glycogen synthesis.

  • Glucagon: Raises blood glucose; promotes glycogen breakdown and gluconeogenesis.

Diabetes Mellitus (DM)

  • Result from inadequate insulin production; characterized by hyperglycemia, polyuria, polydipsia, and polyphagia.