Endocrine System Overview

Major Control Systems of the Body

The endocrine system is one of the body's two major control systems, working alongside the nervous system to coordinate and integrate the activity of body cells. It regulates processes such as reproduction, growth, development, metabolism, and homeostasis.

• Endocrinology: The study of hormones and endocrine organs.

• Hormones: Chemical messengers secreted into the blood or lymph, affecting distant target organs.

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

• Neuroendocrine organs: Organs with both neural and endocrine functions (e.g., hypothalamus).

• Other hormone-producing tissues: Pancreas, gonads, placenta, adipose tissue, thymus, small intestine, stomach, kidneys, heart.

Hormone Structure and Classification

Chemical Classes of Hormones

The chemical structure of a hormone determines its mechanism of action and how it is transported in the blood.

• Amino acid-based hormones: Includes derivatives, peptides, and proteins; generally water-soluble.

• Steroid hormones: Synthesized from cholesterol; lipid-soluble.

Types of chemical signaling:

• Autocrine: Acts on the same cell that secreted it.

• Paracrine: Acts on neighboring cells.

• Endocrine: Acts on distant target cells via the bloodstream.

Mechanisms of Hormone Action

How Hormones Act

Hormones affect target cell activity by altering membrane permeability, stimulating gene transcription, activating or deactivating enzymes, inducing secretory activity, or stimulating mitosis.

• Water-soluble hormones (amino acid-based, except thyroid hormone): Cannot enter cells; act on plasma membrane receptors and use second messengers (e.g., cAMP, Ca2+, cGMP).

• Lipid-soluble hormones (steroids and thyroid hormone): Can enter cells; act on intracellular receptors to directly activate genes.

Second Messenger Systems

• cAMP Mechanism: Hormone binds to receptor → activates G protein → stimulates adenylate cyclase → converts ATP to cAMP → cAMP activates protein kinases → cellular response.

• cAMP is degraded by phosphodiesterase (turns signal off).

• Other second messengers: DAG, IP3, Ca2+.

Gene Activation by Lipid-Soluble Hormones:

• Hormone diffuses through plasma membrane.

• Binds to intracellular receptor.

• Receptor-hormone complex enters nucleus and binds to specific DNA region.

• Initiates transcription and directs protein synthesis.

Hormone Release and Regulation

Stimuli Causing Hormone Release

Three main types of stimuli regulate hormone secretion:

• Humoral stimuli: Changes in blood levels of ions or nutrients.

• Neural stimuli: Nerve fibers stimulate hormone release.

• Hormonal stimuli: Hormones stimulate other endocrine glands to release hormones.

Hormone levels are primarily controlled by negative feedback mechanisms, maintaining homeostasis.

Target Cell Specificity and Response

Determining Cell Responses to Hormones

Cells respond to a hormone only if they have specific receptors for that hormone.

• ACTH receptors: Found only on certain cells of the adrenal cortex.

• Thyroxin receptors: Found on nearly all body cells.

Target cell activation depends on:

• Blood hormone levels

• Number of receptors on/in target cell

• Affinity (strength) of receptor for hormone

Regulation of receptor number:

• Up-regulation: More receptors in response to low hormone levels.

• Down-regulation: Fewer receptors in response to high hormone levels; prevents overstimulation.

Hormone Transport, Half-Life, and Duration

Hormone Circulation and Activity

Hormones circulate in the blood either free or bound to plasma proteins (steroids and thyroid hormone are usually bound).

• Concentration reflects: Rate of release and speed of inactivation/removal (by enzymes, kidneys, or liver).

• Half-life: Time required for hormone blood level to decrease by half (ranges from <1 min to a week).

• Onset and duration: Immediate or delayed (steroids may take hours to days); effects may persist even at low blood levels.

Comparison: Lipid-Soluble vs. Water-Soluble Hormones

Hypothalamus and Pituitary Gland

Structure and Function

The hypothalamus is a neuroendocrine organ that controls the release of hormones from the pituitary gland (hypophysis) via the infundibulum. The pituitary gland has two lobes:

• Posterior pituitary (neurohypophysis): Neural tissue; releases neurohormones (oxytocin, antidiuretic hormone/ADH).

• Anterior pituitary (adenohypophysis): Glandular tissue; produces and secretes peptide hormones.

Hypophyseal Portal System

Hypothalamic neurons synthesize releasing and inhibiting hormones, which are transported via the hypophyseal portal system to the anterior pituitary, regulating hormone secretion into systemic circulation.

• Releasing hormones: GHRH, TRH, CRH, GnRH

• Inhibiting hormones: Somatostatin (GHIH), PIH

Anterior Pituitary Hormones

• Growth hormone (GH)

• Prolactin (PRL)

• Thyroid-stimulating hormone (TSH)

• Adrenocorticotropic hormone (ACTH)

• Follicle-stimulating hormone (FSH)

• Luteinizing hormone (LH)

Tropic hormones regulate the secretion of other hormones.

Posterior Pituitary Hormones

• Oxytocin

• Antidiuretic hormone (ADH)

These hormones are synthesized in the hypothalamus and released from the posterior pituitary in response to neural signals.

Summary of Homeostatic Mechanisms

The endocrine system works with the autonomic nervous system to maintain homeostasis through hormone release and regulation. Hormones travel via the bloodstream to target organs, with slower but longer-lasting effects compared to neural responses.

Key Equations

• cAMP formation:

• Hormone half-life:

Additional info: These notes are based on lecture slides and textbook references (Marieb & Hoehn, 11th Ed., Ch. 16), suitable for ANP 1105A Anatomy & Physiology I students.