17.1 Overview of the Endocrine System

Functions of the Endocrine System

The endocrine system is a network of glands that synthesize and secrete hormones, which regulate various physiological processes throughout the body.

• Ductless glands produce and release hormones directly into the bloodstream.

• Hormones travel through the bloodstream to target cells with specific receptors.

• The endocrine system works alongside the nervous system, but its effects are slower and longer-lasting.

Comparison to Nervous System

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

• Endocrine effects are widespread and long-lasting, while nervous system effects are rapid and localized.

17.2 Major Endocrine Glands & Organs

Primary Endocrine Organs

Primary endocrine organs are specialized for hormone secretion.

• Pituitary, Pineal, Thyroid, Parathyroid, and Adrenal glands

• These glands function exclusively in hormone secretion.

Secondary Endocrine Tissues

Secondary endocrine tissues are found in organs with additional functions beyond hormone secretion.

• Examples: hypothalamus, skin, thymus, heart, liver, stomach, pancreas, small intestine, adipose tissue, kidneys, and gonads (testes/ovaries).

Hormone Release Mechanisms

• Hormonal Stimulation: Hormones stimulate other glands to release hormones (e.g., pituitary hormones trigger thyroid hormone release).

• Humoral Stimulation: Changes in blood levels of ions/nutrients trigger hormone release (e.g., glucose levels affect insulin release).

• Nervous System Stimulation: Direct neural stimulation causes hormone release (e.g., adrenal medulla releases epinephrine).

17.3 Hormone Categories

Circulating Hormones

• Steroids: Lipid-soluble; derived from cholesterol (e.g., estrogen, cortisol).

• Biogenic Amines: Modified amino acids (e.g., epinephrine, thyroid hormone).

• Proteins: Water-soluble chains of amino acids (e.g., insulin, glucagon).

Local Hormones

• Eicosanoids: Act locally (e.g., prostaglandins, leukotrienes).

• Autocrine signaling: Affects the cell that releases it.

• Paracrine signaling: Affects neighboring cells.

17.4 Hormone Transport & Half-Life

Hormones are transported in the blood and have varying half-lives depending on their solubility.

• Lipid-soluble hormones: Require carrier proteins for transport.

• Water-soluble hormones: Travel freely in blood.

• Half-life: The time required for hormone levels to decrease by half.

Formula:

• , where is the rate constant for hormone degradation.

• Water-soluble hormones have a short half-life.

17.5 Hormone Action Mechanisms

Lipid-Soluble Hormones

• Diffuse through membranes and bind to intracellular receptors.

• Directly influence gene expression.

Water-Soluble Hormones

• Bind to membrane receptors and activate second messenger systems (e.g., cAMP, DAG, IP3).

17.6 Target Cells & Hormone Interactions

Receptor Regulation

• Up-regulation: Increased receptor numbers (increased sensitivity).

• Down-regulation: Decreased receptor numbers (reduced sensitivity).

Hormone Interactions

• Synergistic: Hormones work together (e.g., estrogen & progesterone).

• Permissive: One hormone requires another (e.g., oxytocin needs prolactin).

• Antagonistic: Opposing effects (e.g., insulin vs. glucagon).

17.7 Hypothalamus & Pituitary Gland

The hypothalamus and pituitary gland are central regulators of the endocrine system.

• Hypothalamus: Stores and releases ADH (water balance) and Oxytocin (labor & bonding).

• Anterior Pituitary (Adenohypophysis): Releases hormones such as TSH, ACTH, GH, PRL, FSH, and LH.

17.8 Thyroid Gland & Hormones

• Produces Thyroid Hormones (T3, T4): Regulate metabolism.

• Regulated by TRH (hypothalamus) → TSH (pituitary) → TH (thyroid).

• Calcitonin: Lowers blood calcium levels.

17.9 Adrenal Glands & Stress Response

• Adrenal Cortex: Releases corticosteroids (e.g., cortisol for metabolism, aldosterone for Na+ retention).

• Adrenal Medulla: Releases epinephrine & norepinephrine (fight-or-flight response).

17.10 Pancreatic Hormones & Glucose Regulation

• Insulin: Lowers blood glucose.

• Glucagon: Raises blood glucose.

• Diabetes Mellitus: Chronic high blood glucose (Type 1, Type 2, Gestational).

17.11 Other Endocrine Organs

• Pineal Gland: Produces melatonin (circadian rhythms).

• Heart: Releases ANP (reduces blood pressure).

17.12 Aging & Endocrine System

• Hormone production decreases with age.

• GH, sex hormones, and TH decline.