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

Chapter 16: The Endocrine System

Overview of the Endocrine System

The endocrine system is one of the body's two major control systems, working alongside the nervous system to coordinate and regulate physiological processes. It achieves this through the secretion of hormones, which are chemical messengers that influence the activity of various organs and tissues.

• Hormones are secreted by endocrine glands and travel through the bloodstream to target cells.

• The study of hormones and endocrine organs is called Endocrinology.

• Endocrine responses are generally slower but longer-lasting than nervous system responses.

Main Functions of the Endocrine System

• Regulation of growth and development

• Maintenance of electrolyte, water, and nutrient balance in the blood

• Control of cellular metabolism and energy balance

• Mobilization of body defenses

Types of Glands

• Exocrine glands: Secrete products into ducts (e.g., sweat, saliva)

• Endocrine glands: Ductless; secrete hormones directly into the bloodstream (e.g., pituitary, thyroid, parathyroid, adrenal, pineal glands)

• Other organs with endocrine function: gonads (testes and ovaries), pancreas, stomach, intestine, kidney, heart, hypothalamus

Types of Chemical Messengers

• Autocrine: Act on the same cell that secretes them

• Paracrine: Affect nearby cells

• Endocrine: Affect distant target cells via the bloodstream

Example: Prostaglandins can act as autocrine or paracrine messengers.

Chemical Structure of Hormones

The chemical structure of a hormone determines its mechanism of action and solubility.

• Amino acid-based hormones: Include simple amino acid derivatives (e.g., thyroxine), peptides, and proteins; generally water-soluble.

• Steroid hormones: Derived from cholesterol; lipid-soluble (e.g., sex hormones such as estrogen and testosterone).

• Eicosanoids: Includes leukotrienes and prostaglandins; involved in local (paracrine/autocrine) signaling, especially in immune and allergic responses.

Mechanisms of Hormone Action

Hormones influence target cells by binding to specific receptors, leading to changes in cell activity. The response depends on the hormone's chemical nature and receptor location.

• Target cells must have specific receptors for a hormone to respond.

• Hormones can:

  • Alter plasma membrane permeability or membrane potential (e.g., by opening/closing ion channels)

  • Stimulate synthesis of enzymes and other proteins

  • Activate or deactivate enzymes

  • Induce secretory activity

  • Stimulate mitosis

Hormone Signaling Pathways

• Water-soluble hormones (all amino acid-based except thyroid hormone): Act on plasma membrane receptors; use second messenger systems (e.g., cAMP, PIP2-Ca2+ mechanisms).

• Lipid-soluble hormones (steroid and thyroid hormones): Act on intracellular receptors; directly activate genes.

Second Messenger Systems

• cAMP (cyclic AMP) Signaling Mechanism:

  • Hormone (first messenger) binds to receptor → activates G protein → activates adenylate cyclase → converts ATP to cAMP (second messenger)

  • cAMP activates protein kinases, which phosphorylate proteins and alter cell activity

  • Example hormones: ACTH, TSH, FSH, LH, PTH

• PIP2-Ca2+ Signaling Mechanism:

  • Hormone binds to receptor → activates phospholipase C → splits PIP2 into DAG and IP3

  • DAG activates protein kinase C; IP3 releases Ca2+ from endoplasmic reticulum

  • Ca2+ acts as a second messenger

  • Example hormones: Epinephrine, norepinephrine, ADH

• Direct Gene Activation (for lipid-soluble hormones):

  • Hormone diffuses into cell → binds to intracellular receptor → hormone-receptor complex binds DNA → initiates transcription of specific genes → mRNA directs protein synthesis

  • Example hormones: Estrogens, progesterone, testosterone, thyroid hormone

Regulation of Hormone Release

Hormone secretion is controlled by three types of stimuli:

• Humoral stimuli: Changes in blood levels of ions/nutrients (e.g., PTH release in response to low Ca2+)

• Neural stimuli: Nerve fibers stimulate hormone release (e.g., sympathetic stimulation of adrenal medulla)

• Hormonal stimuli: Hormones stimulate other endocrine glands to release hormones (e.g., pituitary hormones)

Hormone Receptors and Target Cell Response

• Target cells must have specific receptors for a hormone to respond.

• Receptor number and affinity can change (up-regulation and down-regulation).

• Hormones can influence the number and affinity of their own or other hormone receptors.

Hormone Interactions at Target Cells

• Permissiveness: One hormone cannot exert its effects without another hormone being present (e.g., thyroid hormone for reproductive hormones).

• Synergism: More than one hormone produces the same effects, amplifying the response (e.g., glucagon and epinephrine).

• Antagonism: One hormone opposes the action of another (e.g., insulin and glucagon).

Major Endocrine Glands and Their Hormones

• Hypothalamus and Pituitary Gland: The hypothalamus controls the pituitary gland, which is divided into anterior and posterior lobes.

  • Posterior pituitary: Stores and releases ADH (antidiuretic hormone) and oxytocin produced by the hypothalamus.

  • Anterior pituitary: Synthesizes and releases six hormones: GH, TSH, ACTH, FSH, LH, PRL.

• Thyroid Gland: Produces thyroid hormones (T3 and T4) that regulate metabolism, and calcitonin, which lowers blood calcium.

• Parathyroid Glands: Secrete parathyroid hormone (PTH), the main regulator of blood calcium levels.

• Adrenal Glands: Consist of the adrenal cortex (produces corticosteroids: mineralocorticoids, glucocorticoids, and androgens) and adrenal medulla (produces catecholamines: epinephrine and norepinephrine).

• Pineal Gland: Secretes melatonin, involved in circadian rhythms.

• Pancreas: Has both exocrine and endocrine functions; endocrine portion (islets of Langerhans) secretes insulin (lowers blood glucose) and glucagon (raises blood glucose).

• Gonads: Ovaries produce estrogens and progesterone; testes produce testosterone.

• Other Organs: Heart (atrial natriuretic peptide), kidneys (erythropoietin, renin), GI tract (various hormones), adipose tissue (leptin), bone (osteocalcin), thymus (thymosins).

Summary Table: Major Hormones and Their Functions

Gland/Source	Hormone	Main Function
Thyroid	Thyroxine (T4), Triiodothyronine (T3)	Regulate metabolism, heart rate, nervous activity
Thyroid	Calcitonin	Decreases blood Ca2+, promotes bone formation
Parathyroid	PTH	Increases blood Ca2+, stimulates bone resorption
Adrenal Cortex	Aldosterone (mineralocorticoid)	Regulates Na+ and K+ balance
Adrenal Cortex	Cortisol (glucocorticoid)	Controls carbohydrate, protein, and fat metabolism
Adrenal Medulla	Epinephrine, Norepinephrine	Stimulate sympathetic responses, increase blood glucose
Pancreatic Islets	Glucagon	Raises blood glucose by promoting glycogenolysis and gluconeogenesis
Pancreatic Islets	Insulin	Lowers blood glucose by promoting glucose uptake and glycogenesis
Ovaries	Estrogen, Progesterone	Regulate reproductive activity and secondary sex characteristics
Testes	Testosterone	Regulates male reproductive activity and secondary sex characteristics
Pineal	Melatonin	Regulates circadian rhythms
Kidney	Erythropoietin	Stimulates red blood cell production
Heart	Atrial natriuretic peptide	Decreases Na+ in extracellular fluid, lowers blood pressure

Key Equations and Concepts

• Half-life of a hormone: The time required for a hormone's blood level to decrease by half.

• Permissiveness, Synergism, Antagonism: Types of hormone interactions at target cells.

• Second Messenger Example:

$\text{ATP} \xrightarrow{\text{Adenylate Cyclase}} \text{cAMP} + \text{PPi}$

$\text{Hormone} + \text{Receptor} \rightarrow \text{G Protein Activation} \rightarrow \text{Effector Enzyme Activation} \rightarrow \text{Second Messenger Production} \rightarrow \text{Cellular Response}$

Additional info:

• Some hormones (e.g., eicosanoids) act locally and are not always classified as true hormones by all endocrinologists.

• Hormone release is often regulated by negative feedback mechanisms to maintain homeostasis.