The Endocrine System: Structure, Function, and Hormones

The Endocrine System

Overview of the Endocrine System

The endocrine system is a complex network of glands and organs that use hormones to control and coordinate the body's metabolism, energy level, reproduction, growth, and development, as well as responses to injury, stress, and mood. Unlike the nervous system, which uses electrical impulses for rapid communication, the endocrine system relies on chemical messengers that act over longer periods.

• Endocrine glands secrete hormones directly into the bloodstream.

• Exocrine glands release substances through ducts to the outside of the body or into the digestive tract.

• Major endocrine organs include the pituitary, thyroid, parathyroid, adrenal glands, pancreas, gonads, and pineal gland.

Comparison: Endocrine vs. Nervous System

• Structure: The nervous system consists of neurons and supporting cells; the endocrine system is made up of glands and hormone-secreting cells.

• Function: The nervous system provides rapid, short-term responses; the endocrine system regulates slower, long-term processes.

• Communication: The nervous system uses neurotransmitters; the endocrine system uses hormones.

General Functions of the Endocrine System

• Regulation of metabolism and energy balance

• Control of growth and development

• Regulation of tissue function

• Control of reproductive processes

• Response to stress and injury

Endocrine vs. Exocrine Glands

• Endocrine glands: Ductless; secrete hormones into the blood (e.g., thyroid gland).

• Exocrine glands: Have ducts; secrete substances onto epithelial surfaces (e.g., sweat glands, salivary glands).

Example: The pancreas is both an endocrine (insulin, glucagon) and exocrine (digestive enzymes) organ.

Classification of Endocrine Glands

• Pure endocrine organs: Only endocrine function (e.g., pituitary, thyroid, parathyroid, adrenal, pineal glands).

• Organs with both endocrine and other functions: Pancreas, gonads, placenta.

• Neuroendocrine organs: Hypothalamus.

• Other tissues with endocrine cells: Heart, kidneys, adipose tissue, etc.

Hormones: Structure and Classification

Categories of Hormones

• Amino acid-based hormones: Includes most hormones; water-soluble (e.g., insulin, epinephrine).

• Steroid hormones: Derived from cholesterol; lipid-soluble (e.g., cortisol, estrogen, testosterone).

Characteristics of Hormone Categories

• Water-soluble hormones: Cannot cross the plasma membrane; act on receptors on the cell surface.

• Lipid-soluble hormones: Can cross the plasma membrane; act on intracellular receptors.

Hormone Actions and Target Cells

Hormone Specificity

Only cells with specific receptors for a hormone (target cells) will respond to that hormone. The presence or absence of receptors determines cellular response.

Target Cell Responses

• Alteration of plasma membrane permeability or membrane potential

• Stimulation of protein synthesis or regulatory molecules

• Activation or deactivation of enzymes

• Induction of secretory activity

• Stimulation of mitosis

Water-Soluble Hormone Signaling

• Bind to cell surface receptors (often G protein-coupled receptors)

• Trigger second messenger systems (e.g., cAMP pathway)

cAMP Second Messenger System Steps

1. Hormone (first messenger) binds to receptor.

2. Receptor activates G protein (GDP replaced by GTP).

3. G protein activates adenylate cyclase.

4. Adenylate cyclase converts ATP to cAMP (second messenger).

5. cAMP activates protein kinases, which phosphorylate proteins and trigger cellular responses.

Equation:

$ATP \rightarrow cAMP + PP_i$

Lipid-Soluble Hormone Signaling

• Diffuse through plasma membrane

• Bind to intracellular receptors (cytoplasmic or nuclear)

• Hormone-receptor complex binds DNA, altering gene expression

Endocrine Gland Stimuli

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

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

• Hormonal stimulus: Hormones stimulate other endocrine glands (e.g., hypothalamic hormones stimulate pituitary).

Nervous System Modulation

The nervous system can override or modulate endocrine controls, especially during stress or emergencies.

Endocrine Organs and Their Hormones

Hypothalamus and Pituitary Gland

• Structural relationship: The hypothalamus is connected to the pituitary gland by the infundibulum.

• Anterior pituitary: Connected via the hypothalamic-hypophyseal portal system (blood vessels).

• Posterior pituitary: Connected via the hypothalamic-hypophyseal tract (neurons).

Functional Relationships

• Anterior pituitary: Hypothalamus releases hormones into portal system to regulate anterior pituitary hormone secretion.

• Posterior pituitary: Stores and releases hormones made by the hypothalamus (oxytocin, ADH).

Hormones of the Hypothalamus and Pituitary

Hormone	Produced By	Target Tissue	Action
Oxytocin	Hypothalamus (stored in posterior pituitary)	Uterus, mammary glands	Stimulates uterine contractions, milk ejection
ADH (Vasopressin)	Hypothalamus (stored in posterior pituitary)	Kidneys	Promotes water reabsorption
GH (Growth Hormone)	Anterior pituitary	Most tissues	Stimulates growth, protein synthesis
TSH (Thyroid-Stimulating Hormone)	Anterior pituitary	Thyroid gland	Stimulates thyroid hormone release
ACTH (Adrenocorticotropic Hormone)	Anterior pituitary	Adrenal cortex	Stimulates cortisol release
FSH (Follicle-Stimulating Hormone)	Anterior pituitary	Gonads	Stimulates gamete production
LH (Luteinizing Hormone)	Anterior pituitary	Gonads	Stimulates sex hormone production
PRL (Prolactin)	Anterior pituitary	Mammary glands	Stimulates milk production

Thyroid Gland

• Hormones: Thyroid hormone (T3, T4), calcitonin

• Cells producing T3/T4: Follicular cells

• Cells producing calcitonin: Parafollicular (C) cells

Structure of MIT, DIT, T3, T4:

• MIT: Monoiodotyrosine (one iodine atom)

• DIT: Diiodotyrosine (two iodine atoms)

• T3: Triiodothyronine (three iodine atoms)

• T4: Thyroxine (four iodine atoms)

Formation of T3 and T4: Iodination and coupling of tyrosine residues in thyroglobulin within thyroid follicles.

Parathyroid Glands

• Hormone: Parathyroid hormone (PTH)

• Action: Increases blood calcium levels by stimulating osteoclasts, increasing intestinal absorption, and promoting kidney reabsorption of calcium.

Adrenal Glands

• Structure: Outer cortex (glandular tissue), inner medulla (nervous tissue)

• Cortex hormones: Mineralocorticoids (aldosterone), glucocorticoids (cortisol), gonadocorticoids (androgens)

• Medulla hormones: Catecholamines (epinephrine, norepinephrine)

Regulation of Aldosterone:

• Renin-angiotensin-aldosterone system (RAAS): Low blood pressure stimulates renin release, leading to angiotensin II formation, which stimulates aldosterone secretion.

• Atrial natriuretic peptide (ANP): Released by the heart; inhibits aldosterone secretion to lower blood pressure.

Pineal Gland

• Hormone: Melatonin

• Action: Regulates circadian rhythms (sleep-wake cycles)

Pancreas

• Exocrine tissue: Acinar cells (digestive enzymes)

• Endocrine tissue: Pancreatic islets (islets of Langerhans)

• Alpha cells: Secrete glucagon (raises blood glucose)

• Beta cells: Secrete insulin (lowers blood glucose)

Gonads

• Ovaries: Produce estrogen and progesterone (regulate female reproductive system)

• Testes: Produce testosterone (regulates male reproductive system)

Types of Hormones

• Tropic hormones: Stimulate other endocrine glands (e.g., TSH, ACTH, FSH, LH)

• Neurohormones: Hormones produced by neurons (e.g., oxytocin, ADH)

• Non-tropic hormones: Act directly on target tissues (e.g., GH, PRL)

Additional info: This guide covers the main concepts, structures, and functions of the endocrine system, as well as the hormones produced by major endocrine organs. For each hormone, understanding its source, target tissue, and physiological effect is essential for mastering endocrine physiology.