Endocrine System: Glands, Hormones, and Regulation

Endocrine System Overview

The endocrine system is composed of glands that secrete hormones directly into the bloodstream, regulating various physiological processes throughout the body. This study guide covers the structure, function, and regulation of major endocrine glands, with a focus on the thyroid, parathyroid, adrenal glands, pancreas, pineal gland, and gonads.

Thyroid Gland

Structure and Location

• Butterfly-shaped gland located on the anterior trachea.

• Composed of hollow spherical follicles (see lab figures 1.4 & 1.6).

Thyroid Hormone (TH)

• Major metabolic hormone of the body.

• Consists of two hormones: triiodothyronine (T3) and thyroxine (T4).

• Release is controlled by TSH (thyroid-stimulating hormone) from the pituitary gland.

• Produced by follicular cells; synthesis requires iodine.

• Targets most body cells, increasing metabolic rate and protein synthesis.

Thyroid Disorders

• Hypothyroidism: Underproduction of TH. Symptoms include fatigue, weight gain, and cold intolerance. Often caused by inadequate iodine intake, leading to goiter (enlarged thyroid).

• Hyperthyroidism (Graves disease): Overproduction of TH due to autoimmune antibodies mimicking TSH. Symptoms include weight loss, heat intolerance, and exophthalmos (bulging eyes).

Calcitonin

• Produced by parafollicular cells of the thyroid.

• Release is humoral (triggered by high blood calcium levels).

• Targets osteoblasts to stimulate bone formation, lowering blood calcium.

• Counteracted by parathyroid hormone (PTH).

Parathyroid Glands

Structure and Location

• Typically four small glands on the posterior aspect of the thyroid.

Parathyroid Hormone (PTH)

• Increases blood calcium levels.

• Targets:

  • Kidneys: Reabsorb more calcium.

  • Osteoclasts: Break down bone to release calcium.

  • Intestines: Increase calcium absorption.

• Antagonistic to calcitonin.

Calcium Homeostasis and Disorders

• Blood calcium is essential for nerve impulses, muscle contraction, and blood clotting.

• Hyperparathyroidism: Causes fragile bones, weak muscles, and kidney stones.

• Hypoparathyroidism: Leads to muscle twitching, convulsions, and can cause respiratory paralysis if untreated.

Adrenal Glands

Structure

• Located atop the kidneys.

• Composed of an outer cortex (three zones) and an inner medulla.

Adrenal Cortex

• Produces corticosteroids (24+ hormones):

  • Zona glomerulosa: Produces mineralocorticoids (e.g., aldosterone) to regulate sodium and potassium balance.

  • Zona fasciculata: Produces glucocorticoids (e.g., cortisol) to regulate glucose metabolism and stress response.

  • Zona reticularis: Produces gonadocorticoids (androgens) for secondary sex characteristics.

Key Hormones and Disorders

• Aldosterone: Increases sodium reabsorption and potassium excretion in kidneys, raising blood volume and pressure.

• Cortisol: Increases glucose production, breaks down protein and fat, suppresses immune response. Excess: Cushing's disease; deficiency: Addison's disease.

• Androgens: Contribute to sex drive and secondary sex characteristics. Hypersecretion can cause early puberty in males and masculinization in females.

Adrenal Medulla

• Functions as part of the sympathetic nervous system.

• Secretes epinephrine (adrenaline) and norepinephrine:

  • Epinephrine: Increases heart rate, metabolism, and bronchiole dilation.

  • Norepinephrine: Causes vasoconstriction, increasing blood pressure.

Stress Response

• Short-term stress: Adrenal medulla hormones prepare the body for emergency (fight-or-flight).

• Long-term stress: Adrenal cortex hormones help the body adapt, but chronic exposure can be harmful (e.g., muscle loss, immune suppression).

Pancreas

Structure and Function

• Located between the spleen and small intestine.

• Mixed gland: Exocrine (acinar cells secrete digestive enzymes) and endocrine (islets of Langerhans secrete hormones).

Hormones of the Pancreas

• Insulin: Produced by beta (β) cells. Lowers blood glucose by promoting uptake into cells, especially muscle and fat.

• Glucagon: Produced by alpha (α) cells. Raises blood glucose by stimulating liver to release glucose.

Glucose Regulation Disorders

• Type 1 Diabetes Mellitus: Autoimmune destruction of β cells; requires insulin therapy. Onset usually before age 15.

• Type 2 Diabetes Mellitus: Insulin resistance in tissues; often associated with obesity. Onset usually after age 40.

• Hypoglycemia: Excessive insulin secretion; can cause dangerous drops in blood glucose.

Pineal Gland

• Secretes melatonin, which regulates circadian rhythms by acting on the hypothalamus.

• Overproduction in winter can cause seasonal affective disorder (SAD), with symptoms like depression and lethargy.

Gonads

Ovaries

• Produce estrogen and progesterone.

• Functions: Development of secondary sex characteristics, regulation of menstrual cycle, and maintenance of pregnancy.

Testes

• Produce testosterone.

• Functions: Development of secondary sex characteristics and sperm production.

Other Hormone-Producing Organs

• Placenta, adipose tissue, GI tract, heart, kidneys, skeleton, skin, thymus all contain hormone-producing cells with various regulatory functions.

Summary Table: Major Endocrine Glands and Hormones

Gland	Major Hormones	Main Functions
Thyroid	Thyroid hormone (T3, T4), Calcitonin	Regulates metabolism, lowers blood calcium
Parathyroid	Parathyroid hormone (PTH)	Raises blood calcium
Adrenal Cortex	Aldosterone, Cortisol, Androgens	Regulates minerals, stress response, sex characteristics
Adrenal Medulla	Epinephrine, Norepinephrine	Fight-or-flight response
Pancreas	Insulin, Glucagon	Regulates blood glucose
Pineal	Melatonin	Regulates circadian rhythm
Ovaries	Estrogen, Progesterone	Secondary sex characteristics, menstrual cycle
Testes	Testosterone	Secondary sex characteristics, sperm production

Key Equations

• Relationship between blood calcium and hormones: $\text{PTH} \uparrow \Rightarrow [\text{Ca}^{2+}]_{\text{blood}} \uparrow$ $\text{Calcitonin} \uparrow \Rightarrow [\text{Ca}^{2+}]_{\text{blood}} \downarrow$

• Glucose regulation: $\text{Insulin} \uparrow \Rightarrow [\text{Glucose}]_{\text{blood}} \downarrow$ $\text{Glucagon} \uparrow \Rightarrow [\text{Glucose}]_{\text{blood}} \uparrow$

Additional info: This guide expands on the provided notes with definitions, context, and a summary table for clarity and exam preparation.