17.1 An Overview of the Endocrine System

The endocrine system uses chemical signals to regulate bodily functions. Hormones, released by glands, travel through the bloodstream to target specific cells. This system works alongside the nervous system to maintain homeostasis and control long-term processes like growth and metabolism.

Major endocrine glands include the pituitary, thyroid, and adrenals. Each gland produces unique hormones that affect different parts of the body. Understanding how these glands work together helps explain how the body maintains balance and responds to changes.

Endocrine System Overview

Electrical vs chemical signaling

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• Nervous system signaling
  • Utilizes electrical signaling for rapid communication
    • Action potentials propagate along neurons enabling swift transmission of signals
    • Neurotransmitters are released at synapses facilitating chemical signaling between neurons (acetylcholine, dopamine)
  • Signals are typically short-lived and localized to specific neural pathways
• Endocrine system signaling
  • Employs chemical signaling via hormones for slower, longer-lasting effects
    • Hormones are released into the bloodstream by endocrine glands (thyroid hormone, insulin)
    • Hormones travel throughout the body to target cells expressing specific receptors
  • Signals can have widespread, long-lasting effects on multiple organ systems (growth, metabolism, reproduction)

Major endocrine glands and locations

• Hypothalamus
  • Situated in the brain, inferior to the thalamus
  • Produces releasing and inhibiting hormones that control the anterior pituitary gland (TRH, CRH, GnRH)
• Pituitary gland
  • Positioned inferior to the hypothalamus, in the sella turcica of the sphenoid bone
  • Anterior pituitary produces hormones that regulate growth (GH), metabolism (TSH, ACTH), and reproduction (FSH, LH)
  • Posterior pituitary stores and releases hormones produced by the hypothalamus (ADH, oxytocin)
• Thyroid gland
  • Found in the neck, inferior to the larynx
  • Produces thyroid hormones that regulate metabolism (T3, T4) and growth
• Parathyroid glands
  • Four small glands located posterior to the thyroid gland
  • Produce ###parathyroid_hormone_(PTH)_0###, which regulates calcium homeostasis
• Adrenal glands
  • Situated superior to the kidneys
  • Adrenal cortex produces glucocorticoids (cortisol), mineralocorticoids (aldosterone), and androgens (DHEA)
  • Adrenal medulla produces catecholamines (epinephrine and norepinephrine) involved in the stress response
• Pancreas
  • Located in the abdominal cavity, posterior to the stomach
  • Endocrine portion (islets of Langerhans) produces insulin and glucagon to regulate blood glucose levels
• Gonads
  • Ovaries in females, located in the pelvic cavity
    • Produce estrogens and progesterone involved in female reproductive function
  • Testes in males, located in the scrotum
    • Produce androgens, primarily testosterone, essential for male reproductive function and development

Types of cell signaling

• Endocrine signaling
  • Hormones are released into the bloodstream by endocrine glands (thyroid, adrenals, pancreas)
  • Hormones travel throughout the body to target cells expressing specific receptors
  • Signaling can have widespread, long-lasting effects on multiple organ systems (growth, metabolism, reproduction)
• Autocrine signaling
  • Signaling molecules are released by a cell and bind to receptors on the same cell
  • Allows a cell to regulate its own activity and function
  • Examples include immune cells (cytokines) and cancer cells (growth factors)
• Paracrine signaling
  • Signaling molecules are released by a cell and diffuse locally to target cells in close proximity
  • Enables local regulation of cellular activity within a tissue or organ
  • Examples include neurotransmitters (acetylcholine), growth factors (VEGF), and inflammatory mediators (histamine)

Endocrine System Regulation and Disorders

• Homeostasis
  • The endocrine system plays a crucial role in maintaining internal balance
  • Hormones help regulate various physiological processes to maintain stability
• Feedback mechanisms
  • Negative feedback: The most common regulatory mechanism in the endocrine system
    • Helps maintain hormone levels within a normal range by inhibiting further hormone production
  • Positive feedback: Less common, but important in certain processes
    • Amplifies hormone production to achieve a specific physiological outcome
• Cellular response to hormones
  • Target cells: Specific cells that respond to particular hormones due to the presence of hormone receptors
  • Second messengers: Intracellular molecules that relay and amplify hormone signals within target cells
• Endocrine disorders
  • Result from hormonal imbalances or dysfunctional endocrine glands
  • Can affect various body systems and lead to a wide range of symptoms