Chapter 16: The Endocrine System – Anatomy & Physiology Study Notes

The Endocrine System

Overview and Primary Functions

The endocrine system is one of the body's two major control systems, working alongside the nervous system to regulate physiological processes. It uses chemical messengers called hormones that are secreted into the extracellular fluid and travel through the bloodstream to target cells throughout the body.

• Reproduction

• Growth and development

• Maintenance of electrolyte, water, and nutrient balance

• Regulation of cellular metabolism and energy balance

• Mobilization of body defenses

Endocrine vs. Nervous System

• Nervous System: Uses neurotransmitters released locally at synapses; effects are rapid (milliseconds) and short-lived.

• Endocrine System: Uses hormones delivered via the bloodstream to distant tissues; effects are slower (seconds to days) and longer-lasting.

Major Endocrine Organs

Endocrine Glands and Their Functions

• Pituitary gland: Master gland controlling other endocrine glands.

• Pineal gland: Secretes melatonin, regulating sleep-wake cycles.

• Thyroid gland: Controls protein production and energy use.

• Parathyroid glands: Regulate calcium levels in blood and bone.

• Adrenal glands: Produce hormones involved in stress response and kidney function.

• Pancreas: Monitors blood glucose levels.

• Gonads (testes/ovaries): Produce sex hormones (testosterone, estrogen, progesterone).

• Neuroendocrine organ: Hypothalamus (produces hormones and controls pituitary gland).

Other tissues/organs with endocrine function: Thymus, adipose tissue, stomach, kidneys, heart, small intestine, skin.

Hormone Structure and Mechanisms

Major Structural Classes of Hormones

• Amino acid-based hormones: Most hormones; water-soluble.

• Steroid hormones: Synthesized from cholesterol (e.g., gonadal and adrenocortical hormones); lipid-soluble.

Mechanisms of Hormone Action

• Water-soluble hormones: (All amino acid-based except thyroid hormone)

  • Act on plasma membrane receptors (cannot enter cell).

  • Exert effects via intracellular second-messenger systems (e.g., cAMP).

• Lipid-soluble hormones: (Steroid and thyroid hormones)

  • Act on intracellular receptors (can enter cell).

  • Directly activate genes, causing synthesis of new proteins.

Control of Hormone Release

Types of Stimuli

• Humoral stimulus: Changes in blood levels of ions/nutrients trigger hormone release.

• Neural stimulus: Nerve fibers stimulate hormone release.

• Hormonal stimulus: Hormones stimulate other endocrine glands to release hormones.

Most hormone release is regulated by negative feedback mechanisms.

Hypothalamus and Pituitary Gland

Structural and Functional Relationships

• Hypothalamus: Part of the diencephalon; regulates homeostasis (blood pressure, hunger, thirst, temperature, reproduction).

• Produces releasing/inhibiting hormones that control the anterior pituitary.

• Produces antidiuretic hormone (ADH) and oxytocin, stored in the posterior pituitary.

Pituitary Gland

• Posterior pituitary (neurohypophysis): Releases ADH and oxytocin received from hypothalamus.

• Anterior pituitary (adenohypophysis): Manufactures and releases six hormones:

  • Growth Hormone (GH)

  • Thyroid Stimulating Hormone (TSH)

  • Adrenocorticotropic Hormone (ACTH)

  • Follicle Stimulating Hormone (FSH)

  • Luteinizing Hormone (LH)

  • Prolactin (PRL)

Pituitary Hormones: Functions and Regulation

Hormone	Regulation of Release	Target Organ/Effect
ADH	Stimulated by hypothalamic neurons	Kidneys: water reabsorption
Oxytocin	Stimulated by hypothalamic neurons	Uterus: contractions; Breast: milk ejection
GH	Stimulated by GHRH; inhibited by GHIH	Liver, muscle, bone: growth, protein synthesis
TSH	Stimulated by TRH; inhibited by feedback from thyroid hormones	Thyroid gland: release of thyroid hormones
ACTH	Stimulated by CRH; inhibited by feedback from glucocorticoids	Adrenal cortex: release of glucocorticoids
FSH/LH	Stimulated by GnRH; inhibited by feedback from sex hormones	Gonads: gamete production, sex hormone release
PRL	Stimulated by PRH; inhibited by PIH	Breast: milk production

Imbalances: Excess GH in childhood causes gigantism; in adults, acromegaly. Deficiency causes pituitary dwarfism. ADH deficiency leads to diabetes insipidus.

Thyroid and Parathyroid Glands

Thyroid Hormones

• Thyroxine (T4) and Triiodothyronine (T3): Increase basal metabolic rate, regulate tissue growth, development, and metabolism.

• Calcitonin: Lowers blood calcium by inhibiting osteoclasts and stimulating calcium uptake in bone.

Thyroid Hormone Synthesis and Regulation

• Thyroid hormone is produced from iodinated thyroglobulin stored in follicles.

• Regulated by negative feedback: low TH stimulates TRH and TSH release; high TH inhibits them.

Imbalances: Iodine deficiency causes goiter; excess TH (Grave's disease) causes hypermetabolism and exophthalmos.

Parathyroid Hormone (PTH)

• Increases blood calcium by stimulating osteoclasts, increasing kidney reabsorption, and activating vitamin D for intestinal absorption.

Adrenal Glands

Hormones and Effects

Region	Hormone	Main Effects
Zona glomerulosa	Mineralocorticoids (e.g., aldosterone)	Regulate sodium and potassium balance
Zona fasciculata	Glucocorticoids (e.g., cortisol)	Regulate metabolism, stress response
Zona reticularis	Gonadocorticoids (e.g., androgens)	Sex hormone effects
Adrenal medulla	Catecholamines (epinephrine, norepinephrine)	Fight-or-flight response

Imbalances: Excess cortisol causes Cushing's syndrome; deficiency causes Addison's disease.

Pancreas and Blood Glucose Regulation

Pancreatic Hormones

• Alpha cells: Produce glucagon (raises blood glucose).

• Beta cells: Produce insulin (lowers blood glucose).

Insulin and Glucagon: Comparison

Hormone	Main Effect	Mechanism
Insulin	Lowers blood glucose	Promotes glucose uptake by cells, glycogen synthesis
Glucagon	Raises blood glucose	Stimulates glycogen breakdown, gluconeogenesis

Imbalances: Insulin deficiency or resistance leads to diabetes mellitus (DM), characterized by hyperglycemia, polyuria, polydipsia, and polyphagia. Excess insulin causes hypoglycemia, which can result in confusion, coma, or death if untreated.

Key Equations and Concepts

• Negative Feedback Regulation: $\text{Falling TH} \rightarrow \text{TRH} \rightarrow \text{TSH} \rightarrow \text{TH release}$ $\text{Rising TH} \rightarrow \text{inhibits TRH and TSH}$

• Blood Glucose Regulation: $\text{Insulin} \uparrow \Rightarrow \text{Blood Glucose} \downarrow$ $\text{Glucagon} \uparrow \Rightarrow \text{Blood Glucose} \uparrow$

Additional info: The thymus, adipose tissue, and other organs also secrete hormones with local or systemic effects, but are not considered classic endocrine glands.