Chapter 7: Introduction to the Endocrine System – Human Physiology

Introduction to the Endocrine System

Overview

The endocrine system is a major regulatory system in the human body, responsible for long-term, ongoing functions such as metabolism, growth, development, reproduction, and regulation of the internal environment. Endocrinology is the study of hormones, which are chemical messengers secreted by cells and transported via the blood to distant targets, where they exert effects at very low concentrations.

7.1 Hormones

Definition and Functions

• Hormones are chemical signals secreted by cells or groups of cells into the blood.

• They regulate metabolism, growth, reproduction, and development.

• Hormones act in three basic ways:

  1. Modulating rates of enzymatic reactions

  2. Controlling transport of ions or molecules across cell membranes

  3. Regulating gene expression and protein synthesis

Example: Insulin regulates glucose uptake and metabolism in cells.

Historical Perspective

• Diseases of the endocrine system have been documented since ancient times.

• Classic steps to identify an endocrine gland and its hormone:

  1. Remove the suspected gland

  2. Replace the hormone

  3. Create hormone excess

Characteristics of Hormones

• Hormones are chemical signals secreted into the blood.

• Transported by blood to distant targets.

• Exert effects at very low concentrations.

• Bind to specific receptors on or in target cells to initiate cellular responses.

• Hormone action must be terminated; half-life indicates duration of activity.

7.2 The Classification of Hormones

Types of Hormones

• Peptide/Protein Hormones: Most common type; synthesized as inactive precursors (preprohormones), processed to prohormones, and stored in vesicles until release.

• Steroid Hormones: Derived from cholesterol; synthesized on demand in adrenal cortex and gonads; not stored; transported bound to carrier proteins; longer half-life.

• Amino Acid-Derived Hormones: Derived from tryptophan (e.g., melatonin) or tyrosine (e.g., catecholamines, thyroid hormones).

Table 7.1: Comparison of Peptide, Steroid, and Amino Acid-Derived Hormones

Feature	Peptide Hormones	Steroid Hormones	Amine Hormones (Catecholamines)	Amine Hormones (Thyroid Hormones)
Synthesis & Storage	Made in advance; stored in vesicles	Made on demand; not stored	Made in advance; stored in vesicles	Made in advance; stored in vesicles
Release from Parent Cell	Exocytosis	Simple diffusion	Exocytosis	Simple diffusion
Transport in Blood	Dissolved in plasma	Bound to carrier proteins	Dissolved in plasma	Bound to carrier proteins
Half-Life	Short	Long	Short	Long
Location of Receptor	Cell membrane	Cytoplasm or nucleus	Cell membrane	Nucleus
Response to Receptor-Ligand Binding	Activation of second messenger systems; may activate genes	Activation of genes for transcription and translation; may have nongenomic actions	Activation of second messenger systems	Activation of genes for transcription and translation
Examples	Insulin, parathyroid hormone	Estrogen, cortisol	Epinephrine, norepinephrine, dopamine	Thyroxine (T4)

Peptide Hormone Synthesis and Processing

• Preprohormone: Large, inactive precursor.

• Prohormone: Processed to smaller, still inactive form.

• Active hormone: Stored in vesicle, released upon signal.

• Bind to surface membrane receptors; initiate signal transduction pathways.

Steroid Hormones

• Derived from cholesterol; synthesized in adrenal cortex and gonads.

• Released by simple diffusion; not stored.

• Transported in blood bound to carrier proteins; longer half-life.

• Bind to cytoplasmic or nuclear receptors for genomic effects; can also bind to membrane receptors for nongenomic effects.

Amino Acid-Derived Hormones

• Tryptophan-derived: Melatonin (pineal gland).

• Tyrosine-derived:

  • Catecholamines (epinephrine, norepinephrine, dopamine) – behave like peptide hormones.

  • Thyroid hormones – behave like steroid hormones.

7.3 Control of Hormone Release

Reflex Pathways

• Components: Stimulus, sensor, input signal, integration, output signal, target(s), response.

• In simple endocrine reflexes, the endocrine cell is the sensor (e.g., parathyroid hormone).

• Many endocrine reflexes involve the nervous system.

• Neurohormones are secreted into the blood by neurons:

  1. Catecholamines

  2. Hypothalamic nuclei

  3. Hypothalamic neurohormones to anterior pituitary

The Pituitary Gland

• Composed of two fused glands:

  • Posterior pituitary: Stores and releases two neurohormones – antidiuretic hormone (ADH) and oxytocin.

  • Anterior pituitary: Secretes six hormones – prolactin (PRL), thyrotropin (TSH), adrenocorticotropin (ACTH), growth hormone (GH), follicle-stimulating hormone (FSH), luteinizing hormone (LH).

• Regulated by hypothalamic hormones (releasing and inhibiting).

Portal System

• Connects hypothalamus and anterior pituitary via two sets of capillaries in series by a vein.

• Ensures small amounts of concentrated hormone reach their target efficiently.

Table: Hormones of the Hypothalamic-Anterior Pituitary Pathway

Anterior Pituitary Hormone	Hypothalamic Releasing Hormone	Hypothalamic Inhibiting Hormone
Prolactin (PRL)	None	Dopamine (PIH)
Thyrotropin (TSH)	Thyrotropin-releasing hormone (TRH)	None
Adrenocorticotropin (ACTH)	Corticotropin-releasing hormone (CRH)	None
Growth hormone (GH)	GHRH (somatotropin)	Somatostatin (GHIH)
Follicle-stimulating hormone (FSH)	Gonadotropin-releasing hormone (GnRH)	None
Luteinizing hormone (LH)	Gonadotropin-releasing hormone (GnRH)	None

22.6 Homeostatic Control of Metabolism

Pancreatic Hormones

• The pancreas secretes insulin and glucagon from the islets of Langerhans:

  • Beta cells: Insulin

  • Alpha cells: Glucagon

  • D cells: Somatostatin

  • PP (F) cells: Pancreatic polypeptide

• The insulin-to-glucagon ratio regulates metabolism:

  • Fed state: Insulin dominates

  • Fasting state: Glucagon dominates

Insulin Function

• Binds to tyrosine kinase receptor; activates insulin-receptor substrates (IRS).

• Lowers plasma glucose by:

  1. Increasing glucose transport into most insulin-sensitive cells

  2. Enhancing utilization and storage of glucose

  3. Enhancing utilization of amino acids

  4. Promoting fat synthesis

Example: After a meal, insulin facilitates glucose uptake in muscle and adipose tissue.

Glucagon Function

• Antagonist to insulin; prevents hypoglycemia.

• Primary target is the liver; stimulates glycogenolysis and gluconeogenesis.

• Release stimulated by low blood glucose and plasma amino acids.

7.4 Hormone Interactions

Types of Hormone Interactions

• Permissiveness: One hormone allows another to exert its full effect.

• Antagonism: Two hormones have opposing effects (e.g., insulin vs. glucagon).

• Synergism: Combined effect of hormones is greater than the sum of their individual effects.

7.5 Endocrine Pathologies

Types of Pathologies

• Hypersecretion: Excess hormone; often caused by tumors or exogenous treatment; may lead to atrophy of gland due to negative feedback.

• Hyposecretion: Deficient hormone; caused by decreased synthesis or atrophy; absence of negative feedback leads to overproduction of trophic hormones.

• Abnormal Tissue Responsiveness: Down-regulation (decreased receptor number), receptor/signal transduction abnormalities.

Diagnosis of Endocrine Pathologies

• Primary pathology: Last endocrine gland in pathway (e.g., adrenal cortex).

• Secondary pathology: Pituitary gland.

• Tertiary pathology: Hypothalamus.

Feedback Loops

• Long-loop negative feedback: Hormone suppresses upstream trophic hormone production.

• Short-loop negative feedback: Pituitary hormone suppresses hypothalamic hormone production.

• Ultra-short-loop negative feedback: Autocrine/paracrine signals regulate secretion within hypothalamus or pituitary.

Additional info: These notes are based on textbook slides and include expanded academic context for clarity and completeness.