Chapter 9: The Endocrine System

Overview

• Second controlling system of the body after the nervous system.
• Uses chemical messengers called hormones, released into the blood.
• Hormones regulate several major processes:
  • Reproduction
  • Growth and development
  • Mobilization of body defenses
  • Maintenance of homeostasis
  • Regulation of metabolism

Hormones

• Produced by specialized cells, secreted into extracellular fluids.
• Blood transfers hormones to target sites, regulating the activity of other cells.
• Endocrinology: the study of hormones and endocrine organs.

Chemistry of Hormones

• Amino acid-based hormones:
  • Proteins
  • Peptides
  • Amines
• Steroids: made from cholesterol
• Prostaglandins: highly active lipids acting as local hormones

Hormone Action Mechanisms

• Hormones affect specific tissues or organs (target cells or organs) with specific protein receptors.
• Hormone binding alters cellular activity in various ways:
  1. Change plasma membrane permeability or potential
  2. Activate or inactivate enzymes
  3. Stimulate or inhibit cell division
  4. Promote or inhibit product secretion
  5. Turn on/off gene transcription

Mechanisms of Action

• Direct gene activation: Used by steroid and thyroid hormones.
• Second-messenger system: Used by protein and peptide hormones.

Control of Hormone Release

• Hormone levels maintained by negative feedback mechanisms.
• Stimuli for hormone release:
  1. Hormonal
  2. Humoral
  3. Neural

Types of Stimuli

• Hormonal: Most common, endocrine organs activated by other hormones (e.g., hypothalamus and anterior pituitary).
• Humoral: Changes in blood ion/nutrient levels (e.g., calcium, glucose).
• Neural: Nerve fibers stimulate hormone release (e.g., adrenal medulla by the sympathetic nervous system).

Major Endocrine Organs

• Hypothalamus
• Pituitary gland
• Pineal gland
• Thyroid gland
• Parathyroid glands
• Thymus
• Adrenal glands
• Pancreas
• Gonads (testes and ovaries)

Pituitary Gland

• Pea-sized gland with anterior and posterior lobes.
• Anterior: glandular tissue, often called master gland.
• Posterior: nervous tissue, stores hormones made by the hypothalamus.

Hypothalamus

• Produces releasing/inhibiting hormones for anterior pituitary.
• Produces oxytocin and antidiuretic hormone (ADH) stored in the posterior pituitary.

Hormones of the Pituitary Gland

• Posterior Pituitary:
  • Oxytocin: Uterine contractions, milk ejection.
  • ADH: Water reabsorption in kidneys, increases blood pressure.
• Anterior Pituitary:
  • Includes growth hormone, prolactin, FSH, LH, TSH, ACTH.

Thyroid Gland

• Produces thyroid hormone (T3 and T4) and calcitonin.
• Thyroid disorders: goiters, cretinism, myxedema, Graves' disease.

Parathyroid Glands

• Secrete parathyroid hormone (PTH), regulating calcium ion homeostasis.

Adrenal Glands

• Adrenal Cortex: Produces corticosteroids (mineralocorticoids, glucocorticoids, sex hormones).
• Adrenal Medulla: Produces catecholamines (epinephrine and norepinephrine).

Pancreatic Islets

• Produce insulin (from beta cells) and glucagon (from alpha cells), maintaining blood sugar homeostasis.

Gonads

• Ovaries: Produce estrogen and progesterone.
• Testes: Produce testosterone and other androgens.

Other Hormone-Producing Tissues

• Includes stomach, small intestine, kidneys, heart, and placenta.

Developmental Aspects

• Endocrine efficiency decreases with age, affecting conditions like diabetes, immune function, and metabolic rate.