Endocrine System: Hormones and Glands

• The endocrine system consists of glands producing hormones that regulate body functions.

Hormones

• Hormones regulate the activity of specific cells or tissues.
• They are transported to target cells via the bloodstream.

Hormone Types

• Amines: Synthesized from amino acids; can be water or lipid soluble.
• Polypeptides: Proteins; mostly water-soluble.
• Steroids: Fused rings; mostly lipid-soluble.
• Water-soluble hormones cannot pass through cell membranes and bind to hormone receptors.
• Lipid-soluble hormones can pass through cell membranes.

Endocrine vs. Exocrine Glands

• Exocrine glands secrete fluids to the surface via ducts.
• Endocrine glands secrete hormones into the bloodstream via capillary beds.

Glands and Their Functions

• Pineal Gland: Produces melatonin, regulating the sleep cycle.
• Hypothalamus: Produces hormones that control the pituitary gland.
• Pituitary Gland: Secretes various hormones with multiple functions.
• Thyroid Gland: Produces hormones regulating metabolism.
• Adrenal Glands: Located above the kidneys; produce hormones involved in stress response.
• Pancreas: Produces insulin and glucagon, regulating blood-glucose levels,.
• Ovaries: Female sex organ; Produces hormones aiding gamete development.
• Testes: Male sex organ; Produces hormones aiding gamete development.

Endocrine Signaling

• Endocrine cells respond directly to a stimulus.

Neuroendocrine Signaling

• Sensory neurons receive a stimulus and send signals to neurosecretory cells, which then respond.

Negative and Positive Feedback Controls

• Negative feedback reduces the stimulus to maintain homeostasis.
  • Examples: Thermoregulation, regulation of blood glucose levels.
• Positive feedback increases the stimulus to drive a process to completion.
  • Examples: Uterine contraction, lactation.

Blood Glucose Levels

• Regulated by negative feedback to maintain homeostasis.
• Essential for ATP production; imbalances can lead to osmotic and solubility issues.
• Antagonistic hormones exert opposite effects to achieve homeostasis.

Insulin and Glucagon

• Insulin: Secreted by the pancreas, lowers blood glucose.
  • Stimulates cells to absorb glucose from the blood.
  • Promotes storage as glycogen in the liver and muscles.
• Glucagon: Secreted by the pancreas, raises blood glucose.
  • Stimulates the breakdown of glycogen into glucose in the liver.
  • Releases glucose into the bloodstream.

Diabetes

• A chronic condition where the body cannot properly regulate blood glucose levels due to:
  • Insufficient insulin production (Type 1).
  • Cells not responding to insulin properly (Type 2).

Hypothalamus

• Located at the rear base of the brain.
• Controls bodily functions
• Receives stimuli.
• Regulates hormone release from the pituitary gland.

Pituitary Gland

• Neurohypophysis (Posterior Pituitary):
  • Contains axons of neurosecretory cells.
  • Stores hormones produced by the hypothalamus, such as ADH and oxytocin.
  • ADH (Antidiuretic Hormone): Aids in osmoregulation.
  • Oxytocin: Stimulates milk production and uterine contraction.
• Adenohypophysis (Anterior Pituitary):
  • Composed of glandular tissue.
  • Produces its own hormones, regulated by hypothalamic hormones and the FLAT PEG hormones.

Anterior Pituitary Hormones (FLAT PEG)

• F – Follicle-Stimulating Hormone (FSH):
  • Stimulates gamete production (sperm in males, eggs in females).
• L – Luteinizing Hormone (LH):
  • Triggers ovulation and hormone production (testosterone, estrogen).
• A – Adrenocorticotropic Hormone (ACTH):
  • Stimulates the adrenal cortex to release cortisol.
• T – Thyroid-Stimulating Hormone (TSH):
  • Stimulates the thyroid gland to release thyroid hormones.
• P – Prolactin (PRL):
  • Stimulates milk production in the breasts of mammals.
• E – Endorphins:
  • Act as natural painkillers.
• G – Growth Hormone (GH):
  • Stimulates growth, cell reproduction, and regeneration.

Tropic Hormones

• Regulate the activity of other endocrine glands.
• FLAT hormones (FSH, LH, ACTH, TSH) are tropic; PEG hormones (Prolactin, Endorphins, Growth Hormone) are not.

Hormone Signaling Pathway

• Hormone Release: Endocrine gland secretes a hormone into the bloodstream.
• Transport in Blood: Hormone travels to target cells.
• Receptor Binding: Hormone binds to a specific receptor on or in the target cell.
• Signal Transduction: Binding activates a signal inside the cell.
• Cellular Response: The cell changes its behavior.
• Feedback Regulation: Helps control the hormone's effects.

Anterior Pituitary Hormones Pathway

• Stimulus causes neurosecretory cells to secrete hypothalamic hormone into the blood.
• This hormone travels to the anterior pituitary, triggering the secretion of the corresponding anterior pituitary hormone.
• That hormone travels to target organs.

Thyroid Stimulating Hormone (TSH)

• Triggers the thyroid to secrete triiodothyronine (T3) and thyroxine (T4).
  • T3 contains 3 iodine atoms.
  • T4 contains 4 iodine atoms.
• Regulates metabolism, heart rate, respiration rate, muscle tone, growth during childhood, reproduction, and digestion.

Calcitonin

• Regulates blood calcium levels.
• Inhibits the release of Ca+2 from bone.
• Stimulates excretion of Ca+2 from kidneys.

Parathyroid Hormone (PTH)

• Secreted by the parathyroid, regulates calcium levels in opposition to calcitonin.

Adrenocorticotropic Hormone (ACTH)

• Stimulates the adrenal glands in response to stress.

Follicle Stimulating (FSH) and Luteinizing Hormone (LH)

• Gonadotropins.
• Stimulate the ovaries and testes to produce and secrete sex hormones like androgens (testosterone), estrogens, and progestins.
  • Androgens (Testosterone): Promotes development of male reproductive structures and secondary sex characteristics.
  • Estrogens: Maintains the female reproductive system and develops female secondary sex characteristics.
  • Progestins: Maintains the uterus for pregnancy.

Prolactin

• Stimulates milk production in mammals.
• Affects fat metabolism in birds, delays metamorphosis in amphibians, and regulates salt/water balance in fishes.

Oxytocin

• Stimulates smooth muscle in the breast for milk production.

Growth Hormone (GH)

• Stimulates growth, targeting the liver.
• Stimulates the release of insulin-like growth factors (IGF), promoting bone and cartilage growth.
  • Hypersecretion of GH leads to gigantism.
  • Hyposecretion of GH leads to pituitary dwarfism.

Melanocyte Stimulating Hormone (MSH)

• Functions primarily in skin darkening.
• UV exposure stimulates the anterior pituitary to secrete MSH.
• MSH binds to receptors on melanocytes, increasing melanin pigment concentration.