Endocrine System: Hormones, Glands, and Regulation – Study Notes

Endocrine System Overview

The endocrine system is a network of ductless glands that secrete hormones directly into the bloodstream to regulate various physiological processes. These hormones control growth, metabolism, reproduction, and homeostasis.

Key Terms and Definitions

• Endocrine glands: Ductless glands that release hormones into the blood (e.g., pituitary, thyroid, adrenal).

• Target cells: Cells with specific receptors for a hormone.

• Second messengers: Intracellular signaling molecules (e.g., cAMP, Ca2+) used by amino acid-based hormones to exert effects.

• Negative feedback mechanism: A process where rising hormone levels inhibit further release, maintaining homeostasis.

Hormone Signaling and Regulation

Mechanisms of Hormone Action

• Amino acid-based hormones: Bind to cell surface receptors, often using second messengers like cAMP to activate protein kinases and cause cellular responses.

• Steroid hormones: Lipid-soluble, pass through cell membranes, bind to intracellular receptors, and directly influence gene expression.

Types of Stimuli for Hormone Release

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

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

• Hormonal: Hormones stimulate release of other hormones (e.g., hypothalamic hormones stimulate anterior pituitary).

Pituitary Gland and Hypothalamus

The pituitary gland, located at the base of the brain, is divided into anterior and posterior lobes. It is closely linked to the hypothalamus, which regulates its activity.

Posterior Pituitary

• Releases neurohormones made in the hypothalamus.

• Oxytocin: Stimulates uterine contractions during childbirth and milk ejection.

• ADH (antidiuretic hormone): Promotes water retention by kidneys, regulates water balance.

Anterior Pituitary

• Produces and releases its own hormones, regulated by hypothalamic releasing/inhibiting hormones via the hypophyseal portal system.

• Growth hormone (GH): Stimulates body growth, protein synthesis, fat breakdown.

• Thyroid-stimulating hormone (TSH): Stimulates thyroid gland to release thyroid hormones.

• Adrenocorticotropic hormone (ACTH): Stimulates adrenal cortex to release corticosteroids (especially cortisol).

• Follicle-stimulating hormone (FSH) and Luteinizing hormone (LH): Control gamete production and sex hormone release.

• Prolactin (PRL): Promotes milk production; mainly controlled by inhibition (dopamine/PIH).

Growth Hormone Regulation

• Stimulated by GHRH; inhibited by GHIH (somatostatin).

• Promotes growth, especially in bones and muscles; mobilizes fats, increases blood glucose.

• Insulin-like growth factors (IGFs): Mediate many GH effects.

• Acromegaly: Excess GH in adults; enlarged hands, feet, face.

Thyroid Gland

The thyroid gland is located in the neck, below the larynx, and consists of two lobes connected by an isthmus. It produces hormones that regulate metabolism and development.

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

• Goiter: Enlarged thyroid, often due to iodine deficiency.

• Grave's disease: Autoimmune hyperthyroidism; symptoms include elevated metabolic rate, sweating, rapid heartbeat, weight loss, exophthalmos (bulging eyes).

Parathyroid Glands

Small glands located on the posterior aspect of the thyroid gland. They regulate calcium homeostasis.

• Parathyroid hormone (PTH): Increases blood calcium by stimulating osteoclasts, increasing intestinal absorption, and kidney reabsorption of calcium.

Adrenal Glands

Located on top of each kidney, the adrenal glands consist of the cortex and medulla, each producing different hormones.

Adrenal Cortex

• Zona glomerulosa: Produces mineralocorticoids (e.g., aldosterone) – regulate Na+ and K+ balance, blood pressure.

• Zona fasciculata: Produces glucocorticoids (e.g., cortisol) – regulate metabolism, stress response, increase blood glucose, suppress inflammation.

• Zona reticularis: Produces gonadocorticoids (androgens).

• Addison's disease: Hyposecretion of adrenal cortex hormones; symptoms include fatigue, weight loss, hypotension.

• Cushing's syndrome: Hypersecretion of cortisol; symptoms include hyperglycemia, loss of muscle/bone, fat redistribution (moon face, buffalo hump).

Adrenal Medulla

• Produces catecholamines: epinephrine (80%) and norepinephrine (20%).

• Part of the sympathetic nervous system; increases heart rate, blood pressure, blood glucose, and bronchodilation.

Pineal Gland

• Produces melatonin, which regulates sleep-wake cycles and circadian rhythms.

Pancreas and Other Endocrine Glands

The pancreas has both endocrine and exocrine functions. Its endocrine portion (islets of Langerhans) regulates blood glucose.

• Alpha cells: Secrete glucagon – raises blood glucose by promoting glycogen breakdown and gluconeogenesis.

• Beta cells: Secrete insulin – lowers blood glucose by promoting glucose uptake and glycogen synthesis.

• Diabetes mellitus: Chronic hyperglycemia due to insulin deficiency (Type 1) or resistance (Type 2).

• Signs: Polyuria (excess urination), polydipsia (excess thirst), polyphagia (excess hunger).

Other Hormone-Producing Organs

• Leptin: Produced by adipose tissue; regulates appetite and energy balance.

• Renin: Produced by kidneys; regulates blood pressure.

• Cholecalciferol: Inactive vitamin D3, converted to calcitriol in kidneys.

• Calcitriol: Active vitamin D; enhances calcium absorption in intestines.

• Thymus: Produces thymulin, thymopoietins, thymosins – involved in T-cell development and immune response.

Hormone Classification Table

The following table summarizes the main classes of hormones and their characteristics:

Hormone Type	Examples	Solubility	Mechanism of Action
Amino acid-based	Peptides, proteins, amines (e.g., insulin, epinephrine)	Water-soluble	Bind to cell surface receptors; use second messengers (e.g., cAMP)
Steroid-based	Cortisol, aldosterone, estrogen, testosterone	Lipid-soluble	Bind to intracellular receptors; directly affect gene transcription

Key Hormones and Their Functions

Hormone	Source	Main Function
GH (Growth Hormone)	Anterior pituitary	Stimulates growth, protein synthesis, fat breakdown
TSH (Thyroid-stimulating hormone)	Anterior pituitary	Stimulates thyroid hormone release
ACTH (Adrenocorticotropic hormone)	Anterior pituitary	Stimulates adrenal cortex to release corticosteroids
FSH, LH	Anterior pituitary	Regulate gamete production and sex hormone release
Insulin	Pancreas (beta cells)	Lowers blood glucose
Glucagon	Pancreas (alpha cells)	Raises blood glucose
Melatonin	Pineal gland	Regulates circadian rhythms
PTH (Parathyroid hormone)	Parathyroid glands	Increases blood calcium
Cortisol	Adrenal cortex	Increases blood glucose, suppresses inflammation
Aldosterone	Adrenal cortex	Regulates Na+ and K+ balance
Epinephrine, Norepinephrine	Adrenal medulla	Fight-or-flight response

Summary of Hormone Functions

• Regulation of metabolism: Thyroid hormones, cortisol, insulin, glucagon.

• Growth and development: GH, IGFs, thyroid hormones, sex hormones.

• Electrolyte and water balance: Aldosterone, ADH, PTH.

• Reproduction: FSH, LH, estrogen, testosterone, prolactin.

• Stress response: Cortisol, epinephrine, norepinephrine.

Equations and Formulas

• Second messenger (cAMP) pathway:

$\text{Hormone binds receptor} \rightarrow \text{activates G protein} \rightarrow \text{activates adenylate cyclase} \rightarrow \text{converts ATP to cAMP} \rightarrow \text{activates protein kinases} \rightarrow \text{cellular response}$

• Blood glucose regulation:

$\text{Insulin:} \quad \text{Glucose uptake} \uparrow, \quad \text{Blood glucose} \downarrow$

$\text{Glucagon:} \quad \text{Glycogenolysis, gluconeogenesis} \uparrow, \quad \text{Blood glucose} \uparrow$

Additional info:

• Some details, such as the exact figures referenced (e.g., Figure 16.4, 16.14, 16.19), are based on standard textbook content and may not correspond to a specific textbook edition.

• Tables and hormone lists are reconstructed for clarity and completeness.