Endocrine System & Hormones: Homeostasis and Regulation

Endocrine System Overview

• The endocrine system is a major regulating system responsible for maintaining body functions and homeostasis.
• Endocrine glands are ductless, secreting hormones directly into the bloodstream.
• Hormones are chemical messengers that regulate various bodily functions. Different types include amines, proteins, and steroids.
• Examples of hormones: thyroxine, epinephrine, norepinephrine, insulin, growth hormone, calcitonin, antidiuretic hormone, oxytocin, cortisol, aldosterone, estrogen, progesterone, and testosterone.

Endocrine Glands & Hormones

• Specific glands and their hormones:
  • Hypothalamus: releasing hormones for anterior pituitary, ADH, oxytocin
  • Pituitary (hypophysis): GH, TSH, ACTH, FSH, LH, prolactin, ADH, oxytocin
  • Thyroid: thyroxine, T3, calcitonin
  • Pineal: melatonin
  • Parathyroid: parathyroid hormone
  • Thymus: immune hormones
  • Pancreas: insulin, glucagon
  • Adrenal (suprarenal): cortex (aldosterone, cortisol, sex hormones), medulla (epinephrine, norepinephrine)
  • Ovaries: estrogen, progesterone, inhibin
  • Testes: testosterone, inhibin

Regulation of Hormone Secretion

• Primarily regulated by negative feedback mechanisms.
• Information about the hormone's effects is "fed back" to the gland, resulting in decreased hormone secretion when appropriate.
• Hormone effects reverse the stimulus and decrease hormone secretion.

Pituitary Gland

• Also known as the hypophysis.
• Plays a role in regulating several body functions.
• Hangs from the hypothalamus by a short stalk (infundibulum).
• Two parts: posterior and anterior pituitary.

Posterior Pituitary Gland

• Stores hormones produced in the hypothalamus (ADH and oxytocin).
• Release is stimulated by neural impulses from the hypothalamus.
• ADH (antidiuretic hormone):
  • Increases water reabsorption in the kidneys.
  • Decreases sweating.
  • Causes vasoconstriction (in large amounts).
• Oxytocin:
  • Stimulates contractions of the uterus during childbirth and milk release during breastfeeding.

Anterior Pituitary Gland

• Separate glandular tissue from the hypothalamus.
• Hormones are regulated by releasing hormones from the hypothalamus.
• Specific anterior pituitary hormones include: growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), prolactin, follicle-stimulating hormone (FSH), and luteinizing hormone (LH).

Specific Anterior Pituitary Hormones

• Growth Hormone (GH): promotes growth, protein repair, and replacement. Secretion is regulated by GHRH (growth hormone releasing hormone) and GHIH (growth hormone inhibiting hormone).

• Thyroid-Stimulating Hormone (TSH): stimulates thyroxine (T4) and triiodothyronine (T3) secretion in the thyroid gland. TRH (thyrotropin-releasing hormone) regulates TSH.

• Adrenocorticotropic Hormone (ACTH): increases cortisol secretion in the adrenal cortex. Released in response to stress signals by the hypothalamus.

• Prolactin: initiates and maintains milk production in mammary glands. Regulated by prolactin-releasing hormone (PRH) and prolactin-inhibiting hormone (PIH).

• Follicle-Stimulating Hormone (FSH) & Luteinizing Hormone (LH): regulate the ovaries (egg development, ovulation) and testes (sperm production, testosterone secretion).

Thyroid Gland

• Located on the front and sides of the trachea, just below the larynx.
• Structural units are thyroid follicles made of cuboidal epithelium.
• Follicles produce thyroxine (T4) and triiodothyronine (T3). Iodine is needed for synthesis of these hormones.
• Thyroid also produces calcitonin.

Thyroid Hormones (T₃ & T₄)

• Stimulated by TSH from the anterior pituitary.
• Regulate energy production and protein synthesis.
• Contribute to growth and normal body functions throughout life.
• Effects are reflected in the function of various organs (brain, muscles, heart).
• Increase cell respiration and metabolism of carbs, fats, and excess amino acids.
• Increase rate of protein synthesis within cells.

Thyroid Gland: Calcitonin

• Plays a role in maintaining normal blood levels of calcium (Ca2+) and phosphate.
• Decreases reabsorption of calcium and phosphate from bones to the blood, thereby lowering blood levels of these ions.
• Stimulus for secretion is hypercalcemia (high blood calcium).

Parathyroid Glands

• Four glands located on the back of each thyroid lobe.
• Secrete parathyroid hormone (PTH).
• Antagonistic to calcitonin.
• Important for maintaining normal blood levels of calcium (Ca2+) and phosphate.

Parathyroid Hormone (PTH)

• Raises blood calcium (Ca2+) levels and lowers blood phosphate levels.
• Secretion is stimulated by hypocalcemia (low blood calcium).
• The target organs are the bones, small intestine, and kidneys.
• Increases calcium reabsorption from bones and increases calcium absorption in the small intestine. Vitamin D is required for the latter.
• Increases calcium reabsorption in kidneys and phosphate excretion in kidneys.

Pancreas

• Located in the upper left quadrant of the abdominal cavity.

• Islets of Langerhans are specialized cells that produce hormones (insulin and glucagon).

• Alpha cells produce glucagon

• Beta cells produce insulin

Glucagon

• Stimulated by hypoglycemia (low blood sugar).
• Stimulates liver to convert glycogen to glucose (glycogenolysis).
• Increases use of fats and excess amino acids to make glucose (gluconeogenesis).
• Overall effect: Raises blood glucose levels and provides more energy molecules.

Insulin

• Stimulated by hyperglycemia (high blood sugar).
• Increases the permeability of cells to glucose, allowing glucose to enter cells for energy production.
• Increases conversion of excess glucose to glycogen in the liver and muscles (glycogenesis).
• Increases amino acid and fatty acid transport into cells, facilitating synthesis processes.

Adrenal Glands

• Two glands located on top of each kidney.
• Composed of two parts: adrenal medulla and adrenal cortex.

Adrenal Medulla

• Secretes epinephrine and norepinephrine (catecholamines).
• Both hormones are sympathomimetic (mimic the effects of the sympathetic nervous system).
• Secretion is stimulated by sympathetic nervous system impulses.
• Functions of the hormones often duplicate or prolong those of the sympathetic division of the ANS.
• Epinephrine: Released in larger amounts. Has effects on increasing HR, force of contraction, vasoconstriction in some areas and vasodilation in others, dilates bronchioles, and increases cell respiration rate
• Norepinephrine: Released in smaller amounts. Primarily responsible for vasoconstriction in skin, viscera, and skeletal muscle, elevating blood pressure.

Adrenal Cortex

• Secretes steroid hormones, including mineralocorticoids (aldosterone), glucocorticoids (cortisol), and sex hormones (estrogen and androgens).

Aldosterone

• Release is stimulated by low blood sodium (Na+), low blood volume, or low blood pressure; and high blood potassium (K+).
• Increases the reabsorption of sodium (Na+) ions by the kidneys, increasing blood sodium levels and thus blood volume and pressure. Conversely it increases excretion of potassium (K+) in urine.

Cortisol

• Released in response to ACTH (adrenocorticotropic hormone) from the anterior pituitary.
• Primarily involved during stressful conditions such as injury, disease, exercise, and severe hemorrhage.
• Functions include increased use of fats and amino acids for energy (gluconeogenesis), decreases use of glucose in a sparing fashion.
• Involved in the anti-inflammatory response, protecting the body by stabilizing lysosomes and blocking histamine effects.

Ovaries

• Secrete estrogen, progesterone, and inhibin.
• Estrogen: stimulated by FSH, promotes ovum maturation, stimulates blood vessel growth in the endometrium, develops secondary sexual characteristics.
• Progesterone: stimulated by LH, support pregnancy, essential in lining development, supports growth of blood vessels and placenta development.
• Inhibin: decreases FSH secretion from the anterior pituitary.

Testes

• Secrete testosterone and inhibin.
• Testosterone: stimulated by LH, promotes sperm maturation and development of male secondary sexual characteristics, brings about closure of epiphyseal plates.
• Inhibin: decreases FSH from the anterior pituitary, regulates spermatogenesis.

Mechanisms of Hormone Action

• Two-messenger mechanism (protein hormones): Hormone binds to membrane receptor, initiating a cascade that eventually leads to changing cellular processes. Cyclic AMP (cAMP) is a key secondary messenger.

• Steroid hormone action: Hormones diffuse through the cell membrane and bind to cytoplasmic receptors. Forming a receptor-hormone complex that enters the nucleus. The complex activates specific genes, leading to increased protein synthesis and changes in cellular activity.

Test your knowledge of the endocrine system. This quiz covers hormone classes, secretion mechanisms, and the relationship between the hypothalamus and pituitary gland. Questions explore hormone action and regulation of blood glucose levels.