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The Endocrine System Overview

  • Comprises glands that secrete hormones to regulate various body functions.

  • Hormones are chemical messengers that influence numerous bodily processes.

Nervous and Endocrine Systems

  • Both systems collaborate to coordinate body functions.

  • Nervous System:

    • Utilizes nerve impulses and neurotransmitters.

    • Allows quick responses with shorter effects, acting on specific targets.

  • Endocrine System:

    • Releases hormones into the bloodstream.

    • Slower responses with prolonged effects and broader influence.

Endocrine Glands

  • Two categories of glands:

    • Exocrine Glands: Secretes products via ducts to the outside.

    • Endocrine Glands: Ductless; release hormones into surrounding fluid, diffusing into the bloodstream.

  • Major endocrine glands include:

    • Pituitary, thyroid, parathyroid, adrenal, pineal glands.

    • Other hormone-secreting structures: hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach, liver, small intestine, skin, heart, adipose tissue, and placenta.

Hormone Activity

  • Hormones target specific tissues with corresponding receptors.

  • Receptors are continuously synthesized and broken down.

  • Types of Hormones:

    • Circulating Hormones: travel within the bloodstream.

    • Local Hormones: exert effects in nearby tissues.

Chemical Classes of Hormones

  • Lipid-soluble Hormones:

    • Pass through cell membranes and bind to receptors inside cells.

    • Examples: Steroids (testosterone, estrogen, progesterone, aldosterone, cortisol).

  • Water-soluble Hormones:

    • Bind to receptors on cell membranes, requiring a second messenger system for action.

    • Examples: Insulin, thyroid-stimulating hormone, follicle-stimulating hormone, luteinizing hormone.

Factors Affecting Hormone Action

  • Hormone effect depends on hormone and target cell responsiveness:

    • Concentration of the hormone.

    • Number of receptors present on target cells.

    • Influence from other hormones.

Control of Hormone Secretion

  • Regulated by:

    • Nervous system signals.

    • Chemical blood changes.

    • Other hormones.

  • Major regulatory mechanism: Negative Feedback

    • Example: Oxytocin and uterine contractions during childbirth represent Positive Feedback.

Hypothalamus and Pituitary Gland

  • Hypothalamus connects the nervous and endocrine systems.

  • Pituitary gland: two lobes:

    • Anterior Pituitary (Adenohypophysis): Regulated by hypothalamus via releasing hormones, controlling stress, growth, reproduction, and lactation.

    • Posterior Pituitary (Neurohypophysis): Stores and releases oxytocin and antidiuretic hormone (ADH).

Functions of Anterior Pituitary Hormones

  • Adrenocorticotropic Hormone (ACTH): Stimulates adrenal cortex.

  • Thyroid-Stimulating Hormone (TSH): Stimulates thyroid hormone secretion.

  • Follicle-Stimulating Hormone (FSH): Targets gonads, influencing reproductive system growth.

  • Luteinizing Hormone (LH): Affects sex hormone production in gonads.

  • Human Growth Hormone (hGH): Promotes growth and metabolism regulation.

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

  • Beta-Endorphin: Inhibits pain perception.

Posterior Pituitary Hormones

  • Stores hormones produced by the hypothalamus:

    • Oxytocin (OT): Initiates uterine contractions.

    • Antidiuretic Hormone (ADH): Reduces water loss in kidneys; regulates blood pressure.

Thyroid Gland

  • Located below the larynx; regulates metabolism.

  • Hormones produced:

    • Thyroxine (T4) & Triiodothyronine (T3): Increase basal metabolic rate, protein synthesis, and energy usage.

    • Calcitonin: Reduces blood calcium levels.

Parathyroid Glands

  • Typically four glands behind the thyroid.

  • Produce Parathyroid Hormone (PTH): Increases blood calcium by activating osteoclasts.

  • Works in concert with calcitonin for calcium homeostasis.

Adrenal Glands

  • Located atop each kidney; consist of:

    • Adrenal Cortex: Secrets cortisol (regulates metabolic processes) and aldosterone (controls sodium and potassium levels).

    • Adrenal Medulla: Produces epinephrine and norepinephrine (fight-or-flight response).

The Pancreas

  • Contains both exocrine and endocrine functions.

  • Endocrine portion (Islets of Langerhans):

    • Alpha Cells: Secrete glucagon (raises blood sugar).

    • Beta Cells: Secrete insulin (lowers blood sugar).

    • Delta Cells: Secrete somatostatin (inhibits insulin and glucagon).

    • F Cells: Produce pancreatic polypeptide (regulates pancreatic enzyme activity).

Gonads: Ovaries and Testes

  • Produce gametes and hormones.

  • Ovaries: Secrete estrogens (regulate menstrual cycle, pregnancy) and progesterone.

  • Testes: Produce testosterone (regulates sperm production and male characteristics).

Pineal Gland

  • Located in midbrain; regulates biological clock via melatonin production in response to light.

  • More melatonin produced during darkness.

Thymus

  • Located behind sternum; involved in immune system regulation.

  • Produces thymosin and other factors for T cell maturation.