The endocrine system_08Dec2024_155

KUWAIT UNIVERSITY The Endocrine System

  • Dr. Eman Alenezi, Department of Hearing and Speech Sciences

Overview of the Endocrine System

  • Works in conjunction with the nervous system.

  • Endocrine system:

    • Hormones are released into the bloodstream.

    • Effects may take hours but last longer than nervous responses.

  • Nervous system:

    • Various parts release hormones into blood and neurotransmitters that act rapidly.

    • Effects are milliseconds long and brief.

General Functions of Hormones

  • Regulate critical physiological functions:

    • Extracellular fluid balance

    • Metabolism

    • Biological clock regulation

    • Contraction of cardiac and smooth muscle

    • Glandular secretion

    • Some immune functions

  • Involved in growth, development, and reproduction.

Types of Glands

Exocrine Glands

  • Secrete products via ducts into body cavities or surfaces.

    • Examples: Sweat, oil, mucous, and digestive glands.

Endocrine Glands

  • Secrete hormones directly into the bloodstream.

    • Key glands include: Pituitary, thyroid, parathyroid, adrenal, and pineal glands.

    • Secondary hormone-secreting organs include: Hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach, liver, small intestine, skin, heart, and placenta.

Hormone Receptors

  • Hormones affect only target cells with specific membrane proteins called receptors.

  • Constant synthesis and degradation of 2000-100,000 receptors per target cell.

  • Two types of regulation:

    • Down-regulation: Excess hormone leads to decreased receptor sensitivity.

    • Up-regulation: Hormone deficiency increases receptor sensitivity.

Mechanisms of Hormone Action

  • Hormones bind to receptors on or inside target cells, leading to:

    • Synthesis of new molecules,

    • Altered membrane permeability,

    • Changed reaction rates.

  • The response of target cells varies:

    • Insulin stimulates glycogen synthesis in liver cells.

    • Insulin stimulates triglyceride synthesis in adipocytes.

Hypothalamus and Pituitary Gland

  • Major integration point between the endocrine and nervous systems.

  • Receives input from various brain regions and internal organs.

  • Controls pituitary gland via 9 releasing and inhibiting hormones.

  • Both are master glands controlling other endocrine gland functions.

Anatomy of the Hypothalamus and Pituitary

  • Infundibulum connects hypothalamus to posterior pituitary.

  • Anterior pituitary comprises 75% of the gland and develops from oral cavity tissues.

  • Posterior lobe consists of axon endings from hypothalamic neurons (neurosecretory cells).

Blood Flow to Anterior Pituitary

  • Controlling hormones travel through portal veins, entering capillaries in the anterior pituitary.

Human Growth Hormone (hGH)

  • Secreted by somatotroph cells.

  • Increases insulin-like growth factors which promote cell growth by:

    • Enhancing amino acid uptake,

    • Stimulating lipolysis in adipose tissue for ATP production,

    • Reducing glucose use to maintain levels for brain support.

Regulation of hGH

  • Low blood sugar triggers gonadotropin-releasing hormone (GnRH) genesis from hypothalamus, increasing hGH release, promoting glycogen breakdown.

  • High blood sugar triggers growth hormone inhibitory hormone (GHIH), decreasing hGH and glycogen breakdown.

Diabetogenic Effect of hGH

  • Excess hGH raises blood glucose levels, causing continual insulin release and potential Type II diabetes if pancreatic function declines.

Thyroid Stimulating Hormone (TSH)

  • Regulated by hypothalamus and stimulates triiodothyronine (T3) and thyroxine (T4) production, enhancing metabolic rate.

Follicle Stimulating Hormone (FSH)

  • Stimulated by hypothalamic releasing hormones and promotes:

    • Follicle formation in ovaries,

    • Estrogen secretion by follicle cells,

    • Sperm production in testes.

Luteinizing Hormone (LH)

  • Regulated by releasing hormones from hypothalamus, promoting:

    • Estrogen secretion,

    • Ovulation in females,

    • Testosterone secretion in males.

Prolactin (PRL)

  • Produced by lactotroph cells, responsible for mammary gland development and milk production during breastfeeding.

Adrenocorticotrophic Hormone (ACTH)

  • Stimulated by hypothalamic releasing hormones to promote cortisol production in adrenal cortex.

Melanocyte-Stimulating Hormone (MSH)

  • Produced by corticotroph cells, its function in humans is unclear, but affects pigmentation in other species.

Posterior Pituitary Gland

  • Does not produce hormones but stores and releases:

    • Antidiuretic hormone (ADH)

    • Oxytocin

Oxytocin

  • Facilitates uterine contractions during labor and milk ejection during breastfeeding.

Antidiuretic Hormone (ADH)

  • Regulates water retention in kidneys, blood pressure, and sweat production.

Regulation of ADH

  1. High blood osmotic pressure: Stimulates hypothalamic osmoreceptors, sending signals for ADH release.

  2. Low blood osmotic pressure: Inhibits osmoreceptors, halting ADH production.

Thyroid Gland

  • Located beside the trachea, containing follicles that synthesize and store hormones (T3 and T4).

Histology of the Thyroid

  • Composed of follicles and parafollicular cells.

Parathyroid Glands

  • Locate behind the thyroid; principal cells produce parathyroid hormone (PTH), increasing blood calcium levels.

Regulation of Calcium Levels

  • High/low Ca+2 levels trigger the release of PTH or calcitonin to maintain balance.

Adrenal Glands

  • Located atop each kidney, producing hormones in three distinct zones (mineralocorticoids, glucocorticoids, androgens).

Adrenal Medulla

  • Secretes epinephrine and norepinephrine, enhancing sympathetic nervous responses.

Pancreas Anatomy

  • Consists of endocrine (islets) and exocrine (acini) cells with specific hormonal functions including:

    • Glucagon (from alpha cells),

    • Insulin (from beta cells),

    • Somatostatin (from delta cells).

Ovaries and Testes

  • Ovaries produce estrogen and progesterone regulating the reproductive cycle.

  • Testes produce testosterone affecting sperm production and male secondary characteristics.

Pineal Gland

  • Attached to the brain, produces melatonin, regulating sleep cycles and circadian rhythms.

Seasonal Affective Disorder & Jet Lag

  • Conditions caused by melatonin production affecting mood and sleep quality.

Thymus Gland

  • Involved in T cell maturation, producing thymosin and other factors.

Miscellaneous Hormones and Eicosanoids

  • Local hormones like leukotrienes and prostaglandins influence various physiological processes such as inflammation and metabolism.

Development of the Endocrine System

  • Various glands develop from specific embryonic structures, impacting endocrine function from early stages.

Aging and the Endocrine System

  • Age-related decreases in hormone production and receptor sensitivity affect metabolic processes, and increase risks for conditions like osteoporosis.

Pituitary Gland Disorders

  • Disorders include growth hormone deficiencies resulting in dwarfism and excesses leading to giantism or acromegaly.

Thyroid Gland Disorders

  • Includes hypothyroidism leading to cretinism, and hyperthyroidism, such as Graves' disease, leading to various systemic symptoms.

Cushing’s Syndrome

  • Caused by glucocorticoid overload leading to muscle loss, fat redistribution, and poor wound healing.

Addison’s Disease

  • Results in glucocorticoid deficits, causing hypoglycemia and low blood pressure with severe effects.

Diabetes Mellitus

  • Characterized by hyperglycemia, excessive urination, thirst, and hunger, with Type I and Type II forms differing in onset and treatment protocols.