Overview of the Endocrine System

  • The Endocrine System plays a vital role in the regulation of various physiological functions in the human body through the secretion of hormones.

Endocrine vs. Exocrine

  • Definition:
    • Endocrine:
    • Product: Always hormone
    • Ducts: No
    • Destination: Bloodstream
    • Exocrine:
    • Product: Never hormone
    • Ducts: Yes
    • Destination: Covering/lining epithelium

Endocrine Organs

  • Distribution:
    • Scattered throughout the body
  • Pure endocrine organs:
    • Pituitary gland
    • Pineal gland
    • Thyroid gland
    • Parathyroid glands
    • Adrenal glands
  • Organs with endocrine cells:
    • Pancreas
    • Thymus
    • Gonads (ovaries and testes)
    • Hypothalamus
  • Characteristics:
    • Richly vascularized to facilitate hormone distribution

Location of Major Endocrine Glands

  • Pineal gland
  • Hypothalamus
  • Pituitary gland
  • Parathyroid glands: Located on the dorsal aspect of the thyroid gland
  • Thymus
  • Thyroid gland
  • Adrenal glands
  • Pancreas
  • Ovary (female)
  • Testis (male)

Hormones

  • Basic Hormone Action:
    • Hormones circulate throughout the body via blood vessels
    • They affect only specific tissues called target cells
    • A single hormone can have varying effects on different target cells

Control of Hormone Release: 3 Mechanisms

  • (a) Humoral stimulus:
    • Example: Low calcium levels in the blood stimulate the parathyroid glands
  • (b) Neural stimulus:
    • Example: Preganglionic sympathetic fibers stimulate adrenal medulla cells leading to catecholamine release (epinephrine and norepinephrine)
  • (c) Hormonal stimulus:
    • Example: Hormones from the hypothalamus stimulate secretion from the anterior pituitary gland

Control of Hormone Secretion

  • Hormone secretion is regulated by feedback loops:
    • If blood concentration of a hormone declines below a threshold, more hormone is secreted
    • If blood concentration exceeds a maximum, hormone production is halted

The Pituitary Gland

  • Location and Structure:
    • Located in the hypophyseal fossa in the sella turcica of the sphenoid bone
    • Divided into two lobes:
    • Adenohypophysis (anterior lobe): Contains pars tuberalis, pars intermedia, and pars distalis
    • Neurohypophysis (posterior lobe): Contains mammillary bodies, median eminence, infundibular stalk, and pars nervosa
  • Relationship with Hypothalamus:
    • Hypothalamic neurons release hormones into the primary capillary plexus, traveling via portal veins to the anterior pituitary, regulating hormone release.

Anterior Pituitary Gland

  • Under stimulation, it releases hormones such as:
    • TSH (Thyroid Stimulating Hormone)
    • FSH (Follicle Stimulating Hormone)
    • LH (Luteinizing Hormone)
    • ACTH (Adrenocorticotropic Hormone)
    • GH (Growth Hormone)
    • PRL (Prolactin)
  • Capillary Systems:
    • Primary capillary plexus is where releasing and inhibiting hormones from the hypothalamus act
    • Secondary capillary plexus is where anterior pituitary hormones are secreted into circulation

Posterior Pituitary Gland

  • Hormones Synthesized:
    • Oxytocin and Antidiuretic Hormone (ADH)
  • Stored and released in response to hypothalamic neuron activity
  • Transported via the hypothalamic-hypophyseal tract

Glands and Hormones

  • Overview of Hormones by Gland:
    • Pituitary:
    • Anterior Pituitary Hormones: GH, FSH, LH, TSH, ACTH, PRL
    • Posterior Pituitary Hormones: Oxytocin, ADH
    • Thyroid Gland:
    • Thyroid Hormones: TH, Calcitonin
    • Parathyroids:
    • PTH (Parathyroid Hormone)
    • Adrenal Glands:
    • Medulla: NE, Epinephrine
    • Cortex: Aldosterone, Cortisol
    • Pancreas:
    • Insulin, Glucagon
    • Testes:
    • Testosterone
    • Ovaries:
    • Estrogen, Progesterone

Other Endocrine Structures

  • Endocrine cells located in:
    • Atria of the heart: Secretes Atrial Natriuretic Peptide (ANP)
    • Gastrointestinal (GI) tract: Contains enteroendocrine cells
    • Placenta: Secretes several steroid and protein hormones, including Estrogen and Progesterone
    • Kidneys: Juxtaglomerular apparatus cells secrete Renin (regulates Aldosterone for sodium reabsorption)
    • Endothelial cells: Secrete Erythropoietin (stimulates RBC production)
    • Skin: Converts modified cholesterol molecules to a precursor of Vitamin D

Pituitary Disorders

  • Gigantism:
    • Result of hypersecretion of Growth Hormone (GH) during childhood
  • Pituitary Dwarfism:
    • Result of hyposecretion of GH
  • Acromegaly:
    • Hypersecretion of GH after the growth plates have closed
  • Diabetes Insipidus:
    • Condition where the posterior lobe does not produce enough ADH

Disorders of the Pancreas: Diabetes Mellitus

  • Causes:
    • Insufficient secretion of insulin
    • Resistance of body cells to insulin effects
  • Type I Diabetes:
    • Sudden onset; often develops before age 15
    • T cell-mediated autoimmune response destroys beta cells
  • Type II Diabetes:
    • Adult onset; usually after age 40
    • Characterized by reduced sensitivity of cells to insulin
    • Management through dietary changes and regular exercise