Endocrine System and Hormones Review (copy)

Review of Topics for Lecture Exam

Introduction

  • Review lectures 1 to 4:

    • Endocrine system

    • Chapter on blood (to be reviewed in another session)

Endocrine System

Definition of Hormone

  • Hormones are defined as potent chemical messengers in the body that have specific effects on target cells.

Signal Amplification

  • Signal amplification refers to the process by which a small amount of hormone can produce a large effect.

  • Mechanism involves intermediaries known as messengers, which include substances like cyclic AMP (cAMP) and protein kinases.

Main Organs Responsible for Hormone Secretion

  • Key organs responsible for hormone secretion include:

    • Hypothalamus

    • Pituitary gland

    • Thyroid gland

    • Parathyroid glands

    • Adrenal glands

    • Pancreas

    • Gonads (testes and ovaries)

    • Additional tissues: heart, kidneys, liver, adipose tissue, and digestive tract structures.

Hormonal Regulation Mechanisms

  • Most hormones operate via negative feedback mechanisms to maintain homeostasis.

    • Example: Regulation of thyroid hormones (T3 and T4)

    • When T3 and T4 levels drop:

      • Hypothalamus releases Thyrotropin-Releasing Hormone (TRH).

      • TRH stimulates the anterior pituitary to release Thyroid Stimulating Hormone (TSH).

      • TSH stimulates the thyroid gland to produce T3 and T4, restoring hormone levels and homeostasis.

    • Positive Feedback Example: Childbirth

    • Oxytocin release increases uterine contractions, leading to more oxytocin release until childbirth occurs.

Hypothalamus and Pituitary Gland

Overview

  • The hypothalamus and pituitary gland (including anterior and posterior lobes) play a critical role in endocrine regulation.

  • They serve as a link between the nervous system and endocrine system, maintaining homeostasis.

mechanisms of Hypothalamic Function

  • Regulatory Hormones:

    • Mostly releasing hormones (e.g., TRH) with an 'R' in their name.

    • Two inhibitory hormones: dopamine (inhibiting prolactin release) and somatostatin (inhibiting growth hormone release).

Key Releasing Hormones from the Hypothalamus

  1. Prolactin Releasing Factor (PRF): Stimulates milk production in mammary glands.

  2. Thyrotropin-Releasing Hormone (TRH): Stimulates release of TSH from anterior pituitary, affecting thyroid function.

  3. Corticotropin-Releasing Hormone (CRH): Stimulates release of ACTH.

  4. Growth Hormone-Releasing Hormone (GHRH): Triggers growth hormone release.

  5. Gonadotropin-Releasing Hormone (GnRH): Stimulates release of FSH and LH.

Anterior Pituitary Hormones

  • Hormones and Their Functions:

    • TSH (Thyroid Stimulating Hormone): Stimulates the thyroid to produce T3 and T4. Requires iodine for thyroid hormone production.

    • FSH (Follicle Stimulating Hormone): Promotes follicle maturation in females; sperm production in males.

    • LH (Luteinizing Hormone): Triggers ovulation in females; testosterone production in males.

    • ACTH (Adrenocorticotropic Hormone): Stimulates adrenal cortex to produce cortisol and other hormones.

    • Growth Hormone (GH): Stimulates growth and protein synthesis in tissues.

    • Prolactin (PRL): Promotes milk production in the mammary glands.

Posterior Pituitary Function

  • Hormones stored in the posterior pituitary include:

    • Oxytocin: Triggers milk ejection and uterine contractions.

    • ADH (Antidiuretic Hormone): Reduces urine output and raises blood pressure (via water retention).

Homeostatic Functions of Hormones

Thyroid Gland Function

  • Location: Anterior neck, consisting of two lobes and isthmus.

  • Follicular Cells: Produce T3 (triiodothyronine) and T4 (thyroxine) with the need for iodine.

  • Effects of T3 and T4:

    • Increase metabolic rate.

    • Calorigenic effect (heat production).

    • Regulate heart rate and impact growth of skin, hair, and nails.

    • Vital for nervous system health.

  • Parafollicular Cells: Produce calcitonin, which lowers blood calcium levels and is antagonistic to parathyroid hormone (PTH).

Adrenal Gland Function

  • Structure: Divided into cortex (outer) and medulla (inner).

    • Cortex Hormones:

    • Zona Glomerulosa: Aldosterone (mineralocorticoid).

    • Zona Fasciculata: Cortisol (glucocorticoid).

    • Zona Reticularis: DHEA (sex hormones).

    • Medulla Hormones: Secretes catecholamines (epinephrine, norepinephrine, dopamine) under sympathetic stimulation from the hypothalamus.

  • Stress Responses:

    • Short-term: Catecholamines stimulate fight-or-flight responses (increased heart rate, blood pressure).

    • Long-term: Cortisol influences metabolism and immune response over extended periods.

Gonadal Hormones

  • Ovaries:

    • FSH promotes follicle development; LH triggers ovulation.

    • Estrogen and progesterone produced from corpus luteum maintain endometrium during pregnancy.

  • Testes:

    • FSH stimulates sperm development; LH stimulates testosterone production.

Hormonal Homeostasis in Blood Glucose Level

  • Insulin (from beta cells of pancreas): Decreases blood glucose levels via glycogenesis and cell uptake of glucose.

  • Glucagon (from alpha cells of pancreas): Increases blood glucose levels via glycogenolysis and gluconeogenesis.

Disorders of the Endocrine System

Classification of Endocrine Disorders

  • Hyposecretion: Low hormone levels due to gland damage (e.g., Addison's disease, hypothyroidism).

  • Hypersecretion: Excess hormone levels often due to tumors (e.g., Cushing's syndrome, Graves' disease).

  • Receptor Insensitivity: Failure of target tissues to respond (e.g., type 2 diabetes).

Specific Disorders

  1. Hypothyroidism:

    • Causes: Hashimoto's disease, iodine deficiency.

    • Symptoms: Weight gain, cold intolerance, dry skin, fatigue.

  2. Hyperthyroidism:

    • Causes: Graves' disease, excessive thyroid hormone production.

    • Symptoms: Weight loss, heat intolerance, rapid heartbeat, nervousness.

  3. Diabetes Mellitus: Type 1 (autoimmune destruction of insulin-producing cells) and Type 2 (insulin resistance).

    • Symptoms: Polyuria, polydipsia, polyphagia.

  4. Adrenal Disorders:

    • Addison's Disease (hyposecretion of cortisol, aldosterone): Symptoms include fatigue, weight loss, hypotension.

    • Cushing's Syndrome (hypersecretion): Symptoms include hyperglycemia, obesity, hypertension.

  5. Acromegaly and Gigantism: Excess growth hormone production due to pituitary adenoma; acromegaly occurs after growth plates close, while gigantism occurs before.

Conclusion

  • Important concepts and connections between the endocrine system's hormones, their functions, mechanisms, homeostatic control, and disorders have been reviewed extensively for the upcoming exam.