Overview of the Endocrine System

Overview

The endocrine system consists of glands and cells that secrete hormones, coordinating growth, metabolism, stress responses, reproduction, and homeostasis through long-distance chemical signaling.

Endocrine System Structure

• Endocrine glands secrete hormones directly into the bloodstream, without ducts.
• Main glands: pituitary, thyroid, parathyroid, adrenal, pineal, pancreas, and gonads.
• Some organs (hypothalamus, thymus, heart, kidneys, stomach, intestines, liver, skin, adipose tissue, bone) also have endocrine functions.
• Exocrine glands (such as sebaceous and sweat glands) release secretions through ducts and are separate from endocrine glands.

Hormone Signaling and Function

• Hormones are transported in the blood and bind to specific receptors on target cells, triggering particular responses.
• Endocrine responses are typically slower and less specific than nervous system responses.
• The same hormone can have different effects in various tissues.
• The nervous system manages rapid changes; the endocrine system maintains internal balance and controls reproduction.
• The fight-or-flight response shows how both systems interact, using adrenal hormones.

Types of Hormones

• Amine hormones are made from amino acids (e.g., melatonin, epinephrine).
• Peptide/protein hormones are short or long chains of amino acids (e.g., ADH, GH, FSH).
• Steroid hormones are derived from cholesterol and require transport proteins (e.g., testosterone, estrogens, cortisol).
• Hormone receptors can be located inside the cell or on its membrane, influencing the cell’s response.

Hormone Regulation

• Downregulation: Cells reduce their receptors if hormone levels remain high.
• Upregulation: Cells increase receptors if hormone levels are low.
• Hormone interactions can be permissive (one enables another), synergistic (effects are amplified), or antagonistic (effects oppose each other).
• Negative feedback loops stabilize hormone levels, while positive feedback is rare (such as oxytocin release during childbirth).

Pituitary Gland

• Anterior pituitary: Produces hormones regulated by the hypothalamus.
  • Growth hormone (GH): Stimulates growth, protein synthesis, and energy release.
  • Thyroid-stimulating hormone (TSH): Promotes thyroid hormone secretion.
  • Adrenocorticotropic hormone (ACTH): Stimulates the adrenal cortex to produce cortisol.
  • Follicle-stimulating hormone (FSH) and luteinizing hormone (LH): Control gamete production and sex hormone release.
  • Prolactin (PRL): Stimulates milk production; usually inhibited except during pregnancy.
  • Tropic hormones (TSH, ACTH, FSH, LH) regulate other endocrine glands.
• Posterior pituitary: Stores and releases hormones from the hypothalamus.
  • Oxytocin: Induces uterine contractions, milk release, and bonding.
  • Antidiuretic hormone (ADH): Increases water reabsorption in the kidneys and is released with high blood solute concentration.
  • ADH deficiency causes diabetes insipidus. Alcohol inhibits ADH, leading to increased urination.

Pineal Gland

• Produces melatonin, which regulates sleep cycles in response to light.
• High melatonin promotes drowsiness; low melatonin supports wakefulness.
• Influences circadian rhythms and may delay puberty.

Thyroid Gland

• Located in front of the trachea, shaped like a butterfly; produces T₃, T₄, and calcitonin.
• T₃ and T₄ production is stimulated by TSH and controlled by negative feedback.
• T₃ and T₄ control metabolic rate, while calcitonin lowers blood calcium.

Parathyroid Glands

• Small glands behind the thyroid; chief cells secrete parathyroid hormone (PTH) to raise blood calcium levels.

Adrenal Glands

• Located on top of the kidneys, with a rich blood supply.
• Adrenal cortex: Releases steroid hormones including cortisol (long-term stress) and aldosterone (fluid and electrolyte balance).
• Adrenal medulla: Releases epinephrine and norepinephrine in response to stress (fight-or-flight response).
• The general adaptation syndrome includes three stages: alarm (immediate), resistance (adaptation), and exhaustion (breakdown).

Adrenal Cortex Hormones

• Aldosterone: Regulates sodium, potassium, and blood pressure through the renin-angiotensin-aldosterone system (RAAS).
• Cortisol: Elevates glucose levels, breaks down tissues during stress, and suppresses the immune response.
• Androgens: Supplement sex hormones, mainly significant in women after menopause.

Adrenal Medulla Hormones

• Chromaffin cells secrete epinephrine and norepinephrine during stress response.
• Effects include increased glucose, heart rate, blood pressure, and redirection of blood flow to essential organs.

Pancreas

• Primarily an exocrine organ, but its pancreatic islets have endocrine functions.
• Islets secrete:
  • Alpha cells: Glucagon, raises blood glucose.
  • Beta cells: Insulin, lowers blood glucose.
  • Delta cells: Somatostatin, inhibits insulin and glucagon.
  • PP cells: Pancreatic polypeptide, regulates appetite and pancreatic secretions.
• Insulin and glucagon together maintain blood glucose balance.

Gonadal Glands

• Testes: Produce testosterone (male development, secondary sex traits) and inhibin (suppresses FSH).
• Ovaries: Produce estrogens and progesterone (female development, reproduction, pregnancy maintenance), and inhibin (reduces FSH). The placenta produces extra hormones during pregnancy.

Diseases and Disorders

• Acromegaly/Gigantism: Excess GH; acromegaly in adults (bone enlargement), gigantism in children (overall growth).
• Pituitary dwarfism: GH deficiency in children, causing impaired growth.
• Addison’s disease: Low cortisol and aldosterone; symptoms include weakness, low sodium, and low glucose.
• Cushing’s disease: Excess cortisol; symptoms include high blood glucose and fat accumulation.
• Hirsutism: Excess androgens, causing abnormal hair growth in women.
• Hyperthyroidism: High levels of thyroid hormones; symptoms include weight loss and sweating.
• Hypothyroidism: Low thyroid hormones; symptoms include weight gain and sluggishness.
• Graves’ disease: Autoimmune form of hyperthyroidism.
• Diabetes insipidus: ADH deficiency, leading to chronic dehydration.
• Diabetes mellitus: Pancreatic disorder, resulting in high blood glucose.

Medical Specialties and Procedures

• Endocrinologist: Medical doctor specializing in hormone disorders (e.g., thyroid disease, diabetes).
• Endocrine surgeon: Removes glands/tissue affected by disease; hormone therapy is often needed after surgery.
• Thyroid and diabetes specialists: Endocrinologists focused on those specific disorders.
• Thyroid scan: Uses a radioactive compound to evaluate thyroid status.
• Radioactive iodine uptake: Measures thyroid function by tracking iodine absorption.
• Blood serum testing: Analyzes hormone levels in the blood.
• Endocrine surgery: Surgical gland removal, usually requiring hormone replacement afterward.

Key Terms and Definitions

• Androgens: Sex hormones, mainly produced in males but also in females.
• Chromaffin cells: Adrenal medulla cells producing adrenaline.
• Endocrine gland: Ductless gland that secretes hormones into the blood.
• Epinephrine: Adrenal hormone involved in the stress response.
• Exocrine gland: Gland that releases substances through ducts (not hormones).
• Hormone: Chemical messenger traveling in the blood.
• Hormone receptor: Protein that receives hormone signals.
• Neurotransmitter: Molecule enabling signal transmission between nerves.
• Norepinephrine: Hormone and neurotransmitter for stress response.
• Osmoreceptors: Cells in the hypothalamus that detect changes in solute concentrations.
• Permeability: How easily substances move through a membrane.
• Synthesis: The creation of complex compounds from simpler ones.