Endocrine System Overview

Overview of the Endocrine System

  • The endocrine system relies on hormones, which are chemicals released into the bloodstream that regulate various physiological processes.
  • Hormones are characterized by their slow action, long-lasting effects, and ability to travel great distances within the body compared to the fast and specific actions of the nervous system.

Key Components of the Endocrine System

Glands and Their Hormones

  • Hormones, Glands, Target Organs, and Functions
    • Students must memorize the following attributes for each hormone:
    • The hormone itself
    • The gland that releases it
    • The target organ(s)
    • The function of the hormone
    • Whether it uses positive or negative feedback control.

Comparisons with the Nervous System

  • Endocrine hormones:
    • Slow-acting but long-lasting effects.
    • Use chemical signals to communicate.
    • Can affect a variety of organs and systems, creating general and wide-reaching responses.
  • Nervous system:
    • Fast and localized signals via electrical impulses and neurotransmitters.
    • Responses are immediate and specific.

Major and Minor Glands in the Endocrine System

Major Endocrine Glands

  • Hypothalamus: Controls the pituitary gland.
  • Pituitary Gland: Known as the "master gland", it regulates many endocrine functions.
  • Thyroid Gland: Found in the neck, it regulates metabolism.
  • Parathyroid Glands: Located on the back of the thyroid, regulate calcium levels.
  • Adrenal Glands: Rest on top of the kidneys; involved in stress responses.
  • Pancreas: Functions in both digestion and hormone regulation (insulin/glucagon).
  • Gonads: Ovaries (female) and testes (male) produce sex hormones.

Secondary Endocrine Structures

  • Heart, kidneys, adipose tissue, skin, and bone also release hormones but primarily function in other systems.

Mechanisms of Hormone Action

  • Hormones are secreted into the bloodstream where they travel to target organs and bind to specific receptors (lock-and-key mechanism).
  • These receptors can be either on the cell membrane (for water-soluble hormones) or inside the cell (for lipid-soluble hormones).

Types of Signaling

  • Autocrine signaling: Hormones that act on the same cell that secreted them.
  • Paracrine signaling: Hormones that act on nearby cells.
  • Prostaglandins: Local hormones that mediate these actions; can act as both autocrine and paracrine signals.

Hormone Families

Types of Hormones

  • Amine Hormones: Examples include norepinephrine; these are typically water-soluble and derived from amino acids.
  • Peptide Hormones: Examples include oxytocin; made of chains of amino acids and are also water-soluble.
  • Steroid Hormones: Derived from cholesterol and lipid-soluble (e.g., testosterone, estrogen).

Specific Hormones and Glands

Hypothalamus and Pituitary Gland

  • Hormones from Hypothalamus:
    • Oxytocin: Produced in hypothalamus, stored in the posterior pituitary; stimulates uterine contractions and milk ejection (positive feedback).
    • Antidiuretic Hormone (ADH): Regulates water retention in kidneys (negative feedback).

Anterior Pituitary Hormones (TP-FLAG)

  • Thyroid Stimulating Hormone (TSH): Stimulates thyroid hormone release (negative feedback).
  • Prolactin (PRL): Promotes milk production (positive feedback with nursing).
  • Follicle Stimulating Hormone (FSH): Stimulates egg and sperm production (negative feedback).
  • Luteinizing Hormone (LH): Stimulates ovulation and testosterone production (negative feedback).
  • Adrenocorticotropic Hormone (ACTH): Stimulates adrenal cortex to release cortisol (negative feedback).
  • Growth Hormone (GH): Stimulates growth and protein synthesis (negative feedback).

Thyroid Gland Hormones

  • Thyroid Hormones: Increase metabolic rate; regulated by TSH from the pituitary (negative feedback).
  • Calcitonin: Decreases blood calcium levels by promoting bone growth (negative feedback).

Parathyroid Hormones

  • Parathyroid Hormone (PTH): Increases blood calcium levels by acting on bones and kidneys (negative feedback).
    • Increases osteoclast activity to release calcium.

Adrenal Gland Hormones

  • Adrenal Cortex: Produces hormones like aldosterone (regulates sodium), cortisol (stress hormone), and androgens (supplement sex hormones).
  • Adrenal Medulla: Produces epinephrine and norepinephrine for short-term stress responses; increases heart rate and blood sugar.

Testes and Ovaries

  • Testosterone: Produced by testes; responsible for male secondary characteristics and sperm production (negative feedback).
  • Estrogen and Progesterone: Produced by ovaries; regulate female reproductive cycle and secondary characteristics (negative feedback).

Feedback Mechanisms

  • Negative Feedback: Common mechanism where high hormone levels lead to decreased secretion of that hormone.
  • Positive Feedback: Rare in the endocrine system; occurs in cases like oxytocin during childbirth.

Additional Notes

  • Hormones are subject to degradation by enzymes and are eventually cleared from the body via the liver and kidneys.

  • Different hormones can interact synergistically or antagonistically at target cells, affecting physiological outcomes significantly.

  • Permissiveness: One hormone requires the presence of another to exert its effects (e.g., thyroid hormone with reproductive hormones).

  • Synergism: Two hormones amplify each other's effect (e.g., glucagon and epinephrine on glucose release).

  • Antagonism: One hormone opposes the action of another (e.g., calcitonin vs parathyroid hormone on calcium levels).