Ch18 Endocrine System

Chapter 18: The Endocrine System

Overview of the Endocrine System

• Endocrine System

  • Composed of endocrine cells and tissues that produce approximately 30 hormones (chemical messengers).

  • Controls and coordinates various body processes in conjunction with the nervous system.

  • Key terms:

    • Endo-: within

    • Para-: next to

    • -Crine: secrete

Endocrine Cells and Tissues

• Definition: All endocrine cells and tissues secrete hormones or paracrines into extracellular fluid.

• Endocrine organs are distributed throughout the body, contributing to system-wide regulatory functions.

The Pituitary Gland (Hypophysis)

• Location: Sits within the sella turcica, a bony cavity at the base of the skull.

• Structure: Composed of a bilobed gland connected to the hypothalamus via the infundibulum.

• Function:

  • Releases nine important peptide hormones that bind to extracellular receptors and utilize cAMP as a second messenger.

Regulation by the Hypothalamus

• The hypothalamus controls the pituitary gland's functions:

  • Transports hormones (ADH and OXT) to the posterior pituitary for circulatory entry.

  • Releases regulatory hormones that modulate anterior pituitary secretion.

  • Contains autonomic centers providing direct influence over the adrenal medulla.

Hypophyseal Portal System

• Definition: A portal system consisting of two capillary beds connected by portal vessels.

• Ensures rapid access of hypothalamic regulatory hormones to anterior pituitary before general circulation.

Pituitary Hormones

• Hypothalamic Control: Released hormones include:

  • Releasing Hormones (RH): Stimulate secretion of anterior lobe hormones.

  • Inhibiting Hormones (IH): Prevent hormone formation in the anterior lobe.

• Secretion follows a negative feedback mechanism, ensuring homeostasis.

Atypical Hormonal Regulation

• Prolactin (PRL): Uniquely regulated by both Prolactin Releasing Hormone and Prolactin Inhibiting Hormone.

• Growth Hormone: Controlled by Growth Hormone-Releasing Hormone (GH–RH) and Growth Hormone-Inhibiting Hormone (GH–IH).

Anterior and Posterior Pituitary Hormones

• Hormones acting directly include:

  • Anterior Pituitary: TSH, ACTH, FSH, LH, PRL, MSH, GH

  • Posterior Pituitary: ADH (water conservation) and OXT (uterine contractions).

The Thyroid Gland

• Location: Below the thyroid cartilage of the larynx, comprising two lobes connected by an isthmus.

• Structure: Contains thyroid follicles housing thyroglobulin, a precursor to thyroid hormones (T3 and T4).

• Hormones: Thyroid hormones regulate metabolic processes; T4 (Thyroxine) has four iodine atoms, while T3 (Triiodothyronine) has three.

Thyroid Hormone Action

• T3 is the most biologically active form, influencing gene expression related to metabolism and heat production.

• Thyroid hormones play roles in growth and development; deficits during early life damage neural development.

Thyroid Disorders

• Goiter: Enlargement due to iodine deficiency.

• Hyperthyroidism: Over-activity (Graves Disease), characterized by symptoms including exophthalmos.

Parathyroid Glands

• Located on the posterior surface of the thyroid gland; secrete Parathyroid Hormone (PTH) to regulate calcium levels via osteoclasts and kidney absorption.

Adrenal Glands

• Location: Positioned atop each kidney, comprised of cortex (steroid hormone production) and medulla (catecholamine secretion).

• Adrenal Cortex: Divided into zones (glomerulosa, fasciculata, reticularis) producing corticosteroids for various physiological effects.

Pineal Gland

• Located in the third ventricle; synthesizes melatonin to regulate circadian rhythms and inhibit reproductive functions.

Pancreas

• Functions as both an endocrine and exocrine gland; regulates blood glucose levels through glucagon (alpha cells) and insulin (beta cells).

Hormone Regulation and Actions

• Secretion Control: Regulated mainly by negative feedback influenced by physiological stimuli, including humoral and neural triggers.

• Hormone Interactions: Hormones can exert antagonistic, synergistic, or permissive effects on each other.

General Adaptation Syndrome (GAS)

• Stress Response: Divided into alarm, resistance, and exhaustion phases, involving hormonal adjustments to maintain homeostasis under stress.

Hormones and Growth

• Hormones supporting growth include Growth Hormone, thyroid hormones, insulin, and reproductive hormones, influencing various developmental aspects.