Endocrine Structure and Function

Endocrine Structure and Function Notes

The Endocrine System Functions

  • Differentiation of systems during development: Important for growth and sexual differentiation.
  • Stimulation of growth and development: Hormonal regulation plays a key role in growth stages.
  • Coordination of the male and female reproductive systems: Hormones control sexual development and reproductive functions.
  • Maintenance of internal environment (homeostasis): Regulates critical physiological parameters:
    • Blood pressure (bp)
    • Heart rate
    • Respiratory rate
    • Body temperature
    • pH levels
    • Metabolism
  • Adaptation to emergency demands: Hormones mobilize energy and resources during stress responses.

Nine Glands of the Endocrine System

  • Hypothalamus: The control center of the endocrine system.
  • Pituitary Gland (Hypophysis): Known as the master gland, has anterior and posterior lobes.
  • Thyroid Gland: Regulates metabolism and growth.
  • Parathyroid Glands: Control calcium balance.
  • Adrenal Glands: Composed of the cortex and medulla, involved in stress responses and metabolism.
  • Testes: Produce male sex hormones and sperm.
  • Ovaries: Produce female sex hormones and ova.
  • Pancreas: Contains Islets of Langerhans that regulate blood glucose levels.
  • Pineal Gland: Regulates sleep-wake cycles via melatonin production.

Hormones

  • General Characteristics:

    • Secretion occurs at specific rates and rhythms (diurnal, pulsatile, and cyclic).
    • Operate within feedback systems (positive and negative feedback).
    • Affect only target cells that have specific receptors.
    • Are excreted by kidneys or deactivated by the liver/cellular mechanisms.

  • Types of Hormones:

    • Autocrine: Hormone acts on the same cell that secreted it.
    • Endocrine: Hormone travels through the bloodstream to distant target organs.
    • Paracrine: Hormone secreted to affect nearby cells.
    • Tropic: Hormone released from one gland influences another endocrine gland.

Hormone Receptors

  • Location:
    • Water-soluble hormones:
    • Cannot cross plasma membrane.
    • Receptors located on the plasma membrane.
    • Lipid-soluble hormones:
    • Can diffuse across plasma membrane.
    • Receptors located inside the target cell.
  • Transport in Blood:
    • Water-soluble hormones circulate freely in blood.
    • Lipid-soluble hormones require carrier proteins for transport.

Hormone Action Mechanism

  • When hormones bind to their receptors, they act as first or second messengers.
    • First Messenger: The hormone itself, which binds to the receptor.
    • Second Messenger: A molecule that transmits the signal inside the cell (e.g., cAMP).
  • This signaling can activate various intracellular enzymes, regulating cellular activities.

Regulation of Hormone Release

  • Hormones are released in response to changes in cellular environments to maintain homeostasis.
  • Release mechanisms include chemical, hormonal, neural inputs, and feedback factors.

Feedback Mechanisms

  • Positive Feedback Loop: Amplifies effects of a hormone, causing increased release of another hormone.
  • Negative Feedback Loop: Inhibits hormone release, promoting homeostasis.

Hormone Receptor Regulation

  • Up-Regulation: Increase in hormone receptors in response to low hormone levels.
  • Down-Regulation: Decrease in hormone receptors in response to high hormone levels.

Systems Covered in This Unit

  • Hypothalamic/Pituitary Axis (HPA)
  • Thyroid and Parathyroid Glands
  • Pancreas
  • Adrenal Glands
  • Dysfunction of Hormonal Systems

Hypothalamic-Pituitary System Anatomy

  • Hypothalamus: Located below the thalamus and above the pituitary; size of an almond.
  • Pituitary Gland: Located below the hypothalamus. Connected to it via a capillary portal system for the anterior lobe and neural connections for the posterior lobe.
Hypothalamic Hormones
  • GnRH: Gonadotropin-releasing hormone that stimulates LH & FSH.
  • CRH: Corticotropin-releasing hormone that stimulates ACTH.
  • TRH: Thyroid-releasing hormone that stimulates TSH.
  • GHRH: Growth hormone-releasing hormone that stimulates GH, influencing growth of long bones and muscles.
  • PIF: Prolactin inhibitory factor; continuously secreted to regulate prolactin release.

Anterior Pituitary Hormones - "FLAT"

  • FSH: Follicle-stimulating hormone; stimulates ovarian and testicular functions.
  • LH: Luteinizing hormone; affects ovulation and testosterone production.
  • ACTH: Adrenocorticotropic hormone; stimulates adrenal cortex to release cortisol.
  • TSH: Thyroid-stimulating hormone; stimulates the thyroid to produce hormones.

Anterior Pituitary Hormones - "PEG"

  • Prolactin: Stimulates breast tissue growth and milk production.
  • (Beta) Endorphins: Modulate pain perception and produce a feeling of euphoria.
  • GH: Growth hormone; stimulates long bone growth and metabolism directly.

Thyroid Gland

  • Hormones:
    • Triiodothyronine (T3) & Thyroxine (T4): Regulate metabolic rate and increase protein synthesis.
    • Calcitonin: Affects calcium-phosphorus balance, primarily regulated by feedback from the hypothalamus and pituitary.

Parathyroid Glands

  • Parathyroid Hormone (PTH): Regulates calcium and phosphorous balance, crucial for bone health and metabolism.

Adrenal Glands

  • Cortex Hormones:
    • Glucocorticoids: Primarily cortisol, regulates metabolism and stress responses.
    • Mineralocorticoids: Aldosterone, regulates sodium and potassium balance.
    • Adrenal androgens and estrogens: Influence secondary sexual characteristics.
  • Medulla Hormones: Epinephrine and norepinephrine, essential for fight-or-flight responses.

Pancreas Hormones

  • Insulin: Lowers blood glucose levels by facilitating cellular uptake.
  • Glucagon: Increases blood glucose levels by promoting hepatic glucose production.
  • Amylin: Works alongside insulin in glucose control and regulating appetite.