The Endocrine System: Structure, Function, and Major Glands

Objective 9: The Endocrine System

Overview

The endocrine system is a major regulatory system of the human body, working alongside the nervous system to maintain homeostasis. It consists of glands that secrete hormones directly into the bloodstream, influencing growth, metabolism, development, and other vital functions.

9.1 Define Key Terms Related to the Endocrine System

Nervous System vs. Endocrine System

• Nervous System: Acts through electrical impulses and neurotransmitters; effects are localized and short-term.

• Endocrine System: Acts through chemical messengers called hormones; effects are generalized and long-term.

Nervous System	Endocrine System
Initiates responses rapidly	Initiates responses slowly
Short-duration responses	Long-duration responses
Acts via action potentials and neurotransmitters	Acts via hormones released into the blood
Acts at specific locations (axon pathways)	Acts at diffuse locations (targets anywhere blood reaches)
Neurotransmitters act over very short distances	Hormones act over long distances

• Endocrinology: The study of endocrine organs and hormones.

• Hormones: Chemical messengers secreted into the blood, affecting metabolism and activity of target cells.

9.2 Discuss the Primary Functions of the Endocrine System

• Reproduction

• Growth and development

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

• Regulation of cellular metabolism and energy balance

• Mobilization of body defenses

9.3 Differentiate Between Endocrine and Exocrine Glands

• Exocrine glands: Have ducts; secrete nonhormonal products to a surface or cavity (e.g., sweat, sebaceous, mammary glands).

• Endocrine glands: Ductless; secrete hormones into blood or lymph, which are carried to target tissues throughout the body.

• Some organs have both endocrine and exocrine functions: pancreas, gonads, placenta.

• Other hormone-producing tissues: adipose cells, thymus, stomach, small intestine, kidneys, heart.

9.4 Discuss the General Functions of Hormones

• Hormones: Secretory products of endocrine glands; chemical messengers that travel in blood and lymph to influence target cells.

• Endocrine glands have a rich blood supply for efficient hormone distribution.

Classes of Hormones

• Protein hormones: Most body hormones; made from amino acids; inactivated by stomach acid; must be injected.

• Steroid hormones: Sex hormones and adrenal cortex hormones; made from lipids (cholesterol); may be taken orally.

9.5 Discuss How the Secretion of Endocrine Hormones is Regulated

• Hormones circulate systemically, but only cells with specific receptors (target cells) are affected.

• Hormone actions on target cells include:

  • Altering plasma membrane permeability or potential (opening/closing ion channels)

  • Stimulating synthesis of enzymes or proteins

  • Activating/deactivating enzymes

  • Inducing secretory activity

  • Stimulating mitosis

• Regulation is mainly by negative feedback to maintain hormone levels within a narrow range.

Stimuli for Hormone Release

1. Humoral: Direct response to changing blood levels of ions/nutrients (e.g., Ca2+ stimulates PTH release).

2. Neural: Nerve fibers stimulate hormone release (e.g., sympathetic fibers stimulate adrenal medulla).

3. Hormonal: Hormones stimulate other endocrine organs (e.g., hypothalamic hormones stimulate pituitary).

• Nervous system can override endocrine controls during stress ("fight or flight").

• Target cell activation depends on:

  • Blood levels of hormone

  • Number of receptors on target cell

  • Affinity (strength) of binding

• Up-regulation: Target cells form more receptors in response to low hormone levels.

• Down-regulation: Target cells lose receptors in response to high hormone levels.

Hormone Receptor Sites and Target Tissue

• Hormones must match the receptor site on a cell (lock and key response).

• All cells with receptor sites for a hormone make up its target tissue.

• Target tissue can be localized or scattered throughout the body.

• Receptor sites may be on the plasma membrane or inside the cell.

9.6 Identify the Main Glands of the Endocrine System and Their Location

• Neuro-Endocrine: Hypothalamus

• Sole Endocrine: Pituitary (anterior & posterior), Thyroid, Parathyroid, Adrenal (cortex & medulla), Pineal

• Other Organs with Endocrine Function: Pancreas, Gonads (testes & ovaries)

9.7 Identify the Hormones Secreted by the Main Endocrine Glands and Their Functions

Gland	Hormone	Function
Anterior Pituitary	GH, TSH, ACTH, FSH, LH, PRL	GH (Growth Hormone): Stimulates growth, metabolism; excess causes gigantism/acromegaly, deficiency causes dwarfism. TSH (Thyroid-Stimulating Hormone): Stimulates thyroid hormone release. ACTH (Adrenocorticotropic Hormone): Stimulates adrenal cortex to release corticosteroids. FSH (Follicle-Stimulating Hormone): Stimulates gamete production. LH (Luteinizing Hormone): Stimulates production of gonadal hormones. PRL (Prolactin): Stimulates milk production.
Posterior Pituitary	Oxytocin, ADH	Oxytocin: Stimulates uterine contractions, milk ejection. ADH (Antidiuretic Hormone): Promotes water reabsorption in kidneys; high levels cause vasoconstriction.
Thyroid	TH (T3, T4), Calcitonin	TH (Thyroid Hormone): Regulates metabolism. Calcitonin: Lowers blood Ca2+ levels.
Parathyroid	PTH	PTH (Parathyroid Hormone): Raises blood Ca2+ levels; stimulates osteoclasts, enhances Ca2+ absorption.
Adrenal Cortex	Mineralocorticoids, Glucocorticoids, Gonadocorticoids	Aldosterone: Regulates blood volume and pressure. Cortisol: Regulates metabolism, stress response. Sex hormones: Minor role in adults.
Adrenal Medulla	Epinephrine, Norepinephrine	Epinephrine/Norepinephrine: Fight or flight response; increase heart rate, blood glucose.
Pancreas	Insulin, Glucagon	Insulin: Lowers blood glucose. Glucagon: Raises blood glucose.
Ovaries	Estrogen, Progesterone	Estrogen/Progesterone: Regulate female reproductive system.
Testes	Testosterone	Testosterone: Regulates male reproductive system.
Pineal	Melatonin	Melatonin: Regulates sleep-wake cycles.

Thyroid Hormone Conditions

• Goiter: Iodine deficiency leads to thyroid enlargement.

• Cretinism: Infant deficiency of thyroid hormone; causes mental and skeletal abnormalities.

• Myxedema: Adult hypothyroidism; lethargy, weight gain, low metabolic rate.

• Grave's Disease: Autoimmune hyperthyroidism; protruding eyes, high metabolic rate.

Parathyroid Hormones

• Hypoparathyroidism: Increased nerve excitability, tetany, respiratory paralysis.

• Hyperparathyroidism: Excess osteoclast activity, bone softening, kidney stones.

Adrenal Gland Hormones and Stress Response

• Adrenal Medulla: Short-term stress response (epinephrine/norepinephrine).

• Adrenal Cortex: Long-term stress response (cortisol, aldosterone).

• Addison's Disease: Hyposecretion of glucocorticoids; can be fatal if untreated.

• Cushing's Syndrome: Hypersecretion; elevated blood glucose, edema, masculinization in females.

9.8 Hormones Secreted by Other Endocrine Glands

• Thymus: Secretes thymosin; stimulates lymphocyte development for immune function.

• Stomach lining: Secretes gastrin; stimulates gastric acid production.

• Small intestine mucosa: Secretes secretin and cholecystokinin; regulate pancreatic and gallbladder function.

• Atria of the heart: Secretes atrial natriuretic peptide (ANP); decreases blood volume and pressure.

• Kidneys: Secrete renin; initiates renin-angiotensin-aldosterone mechanism.

• Placenta: Secretes human chorionic gonadotropin (hCG), estrogens, and progesterone; maintains pregnancy.

Key Equations and Concepts

• Negative Feedback Regulation:

$\text{Increased hormone level} \rightarrow \text{Effect on target organ} \rightarrow \text{Inhibition of further hormone release}$

• Calcium Homeostasis:

$\text{Low } Ca^{2+} \rightarrow \text{PTH release} \rightarrow \text{Increase } Ca^{2+} \text{ in blood}$

• Blood Glucose Regulation:

$\text{High blood glucose} \rightarrow \text{Insulin release} \rightarrow \text{Lower blood glucose}$ $\text{Low blood glucose} \rightarrow \text{Glucagon release} \rightarrow \text{Raise blood glucose}$

Summary Table: Major Endocrine Glands and Hormones

Gland	Main Hormones	Primary Functions
Pituitary (anterior)	GH, TSH, ACTH, FSH, LH, PRL	Growth, metabolism, reproduction, milk production
Pituitary (posterior)	Oxytocin, ADH	Uterine contraction, water balance
Thyroid	TH, Calcitonin	Metabolism, calcium regulation
Parathyroid	PTH	Calcium homeostasis
Adrenal Cortex	Aldosterone, Cortisol	Blood pressure, stress response
Adrenal Medulla	Epinephrine, Norepinephrine	Fight or flight response
Pancreas	Insulin, Glucagon	Blood glucose regulation
Ovaries	Estrogen, Progesterone	Female reproductive system
Testes	Testosterone	Male reproductive system
Pineal	Melatonin	Sleep-wake cycles

Additional info: These notes expand on the provided slides and images, offering definitions, examples, and academic context for each major concept in the endocrine system. All tables have been recreated in HTML format for clarity and study purposes.