Endocrine System Overview

Endocrine System Overview

• The endocrine system is a system of glands that produce and secrete hormones, chemical messengers that regulate various bodily functions.
• The system is comprised of endocrine glands that synthesize and secrete hormones.
• Hormones interact and alter the activity of target cells.
• Functions include regulating development, growth, metabolism, maintaining blood composition and volume, and controlling digestive and reproductive processes.

Intercellular Communication

• Direct: Rare, occurs via gap junctions, involves ions, small solutes, lipid-soluble molecules, and between adjacent cells of the same type.
• Paracrine: Most common, uses local hormone-like substances (e.g., prostaglandins, growth factors) to affect nearby cells. Receptors are needed on the target cells for paracrine factors to exert their effect.
• Endocrine: Uses hormones delivered via the bloodstream to target cells in many tissues and organs. This method increases or decreases the synthesis or metabolic activity of enzymes in target cells.
• Synaptic: Specific area of effect using neurotransmitters and acts quickly, good for crisis management. Receptors on target cells are required.

Hormones

• Chemical messengers released into tissues or blood.
• Bind to target cell receptors, altering target cell/tissue/organ activity.

Types of Hormones

• Hormones are derived from either cholesterol or amino acids
• Divided into three types:
  • Steroids: Derived from cholesterol, lipid-soluble, include androgens (testosterone), estrogens, progesterone (gonads), aldosterone (mineralocorticoids) - adrenal glands, cortisol (glucocorticoids) - adrenal glands, and calcitriol (kidneys).
  • Biogenic Amines: Water-soluble; derive from amino acids, include catecholamines (dopamine, epinephrine, norepinephrine) - adrenal glands, thyroid hormone (T3 and T4) - thyroid gland, and melatonin - pineal gland.
  • Peptide/Protein Hormones: Water-soluble, chains of amino acids; include antidiuretic hormone (ADH) - hypothalamus, insulin and glucagon - pancreas, growth hormone - anterior pituitary, and erythropoietin - kidneys.

Local Hormones (eicosanoids)

• Derived from fatty acids.
• Released from cells and act on the same cell (autocrine stimulation) or nearby cells (paracrine stimulation).
• Examples include prostaglandins, found in most tissues, stimulating pain receptors, fever, and increasing the inflammatory response.

Hormone Transport

• Hormones can travel freely in the body or bound to carrier proteins.
• Lipid-soluble hormones need carrier molecules produced by the liver.
• Water-soluble hormones dissolve in the blood and do not need carrier molecules.

Hormone Receptors

• Intracellular receptors: Located inside target cells, bind to lipid-soluble hormones.
• Extracellular receptors: Located on the plasma membrane, bind to water-soluble hormones, catecholamines, peptides, and eicosanoids.

Intracellular Receptors (Steroid Hormones)

• Diffuse through the cell membrane.
• Bind to receptors in the cytoplasm or nucleus.
• Hormone-receptor complex binds to DNA, activating specific genes and altering protein synthesis.
• Altering protein synthesis affects the metabolic activity and structure of the target cell.

Extracellular Receptors (Proteins and Biogenic Amines)

• Polar molecules require receptors.
• Hormones bind to the receptor on the plasma membrane.
• Activating a G-protein intermediary.
• Activate or inhibit one of two plasma membrane enzymes (adenylate cyclase or phospholipase C) to start a cascade.

Activities of Second Messengers (Intracellular)

• Activated: Increase in metabolic activity and opening of ion channels.
• Inactive: The cell plateaus or decreases in metabolic activity with ion channels remaining closed.

Control of Hormonal Secretion

• Hormonal stimulation: Arrival/removal of a hormone causes another hormone release. (e.g., thyroid hormone)
• Humoral stimulation: Changes in nutrient or ion levels trigger hormone release. (e.g., calcium, glucose)
• Nervous system stimulation: Neurotransmitters (NT) at neuroglandular junctions trigger hormone release.
• Negative feedback: Hypothalamus regulates hormones through negative feedback loops. It produces and stores ADH and oxytocin in the posterior pituitary and releases regulatory hormones to control the anterior pituitary. It also controls sympathetic output to the adrenal medulla.

Negative Feedback Control

• The hypothalamus monitors blood concentrations of hormones and adjusts production accordingly. It stops releasing CRH when adequate levels of glucocorticoids are detected in the bloodstream.

Hormone Inactivation

• Hormones are inactivated by diffusion from blood into tissues, binding to target cell receptors, absorption and breakdown in the kidneys and liver, or breakdown in blood/interstitial fluid due to enzymes.