Chapter 16 The Endocrine System Study Guide

Endocrine System Overview

Definition and Function of Hormones

Hormones are chemical messengers produced by endocrine glands. They are secreted into the bloodstream and travel to target organs or tissues, where they trigger specific physiological responses. Hormones regulate essential body processes such as growth, metabolism, reproduction, and homeostasis.

• Key Point: Hormones act at distant sites from their origin, unlike local messengers (paracrines or autocrines).

• Example: Insulin, produced by the pancreas, regulates blood glucose levels throughout the body.

Endocrine vs. Exocrine Glands

Comparison and Examples

Endocrine glands secrete hormones directly into the bloodstream, while exocrine glands release substances through ducts to a surface or cavity.

• Endocrine gland example: Pituitary gland (secretes growth hormone, ACTH, etc.)

• Exocrine gland example: Salivary glands (secrete saliva into the mouth)

Hormone Structure and Cell Membrane Permeability

Amino Acid-Based vs. Steroid Hormones

Hormones can be classified by their structure, which affects their ability to cross cell membranes and their mechanism of action.

• Amino acid-based hormones: Cannot cross the plasma membrane; bind to receptors on the cell surface.

• Steroid hormones: Can cross the plasma membrane; bind to intracellular receptors. They are lipid-soluble.

Key Point: The location of hormone receptors (cell surface vs. intracellular) determines the hormone's mechanism of action.

Hormone-Receptor Interactions

Factors Affecting Target Cell Response

The response of a target cell to a hormone depends on several factors:

• Blood level of the hormone

• Number of receptors on the target cell

• Strength of binding (affinity) between hormone and receptor

Example: Upregulation increases receptor number, making cells more sensitive to a hormone.

Regulation of Hormone Release

Nervous System and Feedback Loops

The nervous system can influence hormone release in several ways:

• Direct neural stimulation (e.g., hypothalamus to pituitary gland)

• Feedback loops (negative feedback is most common, maintaining homeostasis)

Example: The hypothalamus monitors internal conditions and signals the pituitary to adjust hormone output.

Pituitary Gland Structure and Function

Anterior vs. Posterior Pituitary

The pituitary gland consists of two structurally and functionally distinct parts:

• Anterior pituitary: Glandular tissue; connected to the hypothalamus by blood vessels (hypophyseal portal system).

• Posterior pituitary: Nervous tissue; connected by nerve fibers; stores and releases hormones made by the hypothalamus.

Hypothalamic Control of the Pituitary

The hypothalamus synthesizes hormones and sends them to the posterior pituitary for storage and release. It also releases regulatory hormones into the portal system to stimulate or inhibit the anterior pituitary.

Pituitary Hormones: Functions and Targets

Table: Pituitary Hormones, Source, Targets, and Effects

Hormone	Released by	Target cell(s)	Effects
Oxytocin	Posterior Pituitary	Uterus, smooth muscle, mammary glands	Stimulates uterine contractions and milk ejection during labor and lactation
Antidiuretic hormone (ADH, vasopressin)	Posterior Pituitary	Kidneys	Promotes water reabsorption, reduces urine output, conserves body water
Growth hormone (GH)	Anterior Pituitary	Most body cells (bones, muscles, liver)	Stimulates growth, protein synthesis, fat breakdown, and glucose metabolism
Thyroid-stimulating hormone (TSH)	Anterior Pituitary	Thyroid gland	Stimulates release of thyroid hormones
Adrenocorticotropic hormone (ACTH)	Anterior Pituitary	Adrenal cortex	Stimulates release of glucocorticoids (cortisol)
Follicle-stimulating hormone (FSH)	Anterior Pituitary	Ovaries and testes	Stimulates gamete production and maturation
Luteinizing hormone (LH)	Anterior Pituitary	Ovaries and testes	Triggers ovulation and sex hormone production
Prolactin (PRL)	Anterior Pituitary	Mammary glands	Stimulates milk production

Other Endocrine Hormones

Table: Major Non-Pituitary Hormones

Hormone	Endocrine gland	Target cell(s)	Effects
Thyroid hormone (TH, T3/T4)	Thyroid gland	Most body cells	Increases metabolism, growth, and development
Calcitonin	Thyroid gland	Bone (osteoclasts), kidneys	Lowers blood calcium by inhibiting bone resorption
Parathyroid hormone (PTH)	Parathyroid glands	Bone, kidneys, intestines	Raises blood calcium by stimulating bone breakdown and calcium reabsorption
Aldosterone	Adrenal cortex	Kidney tubules	Promotes sodium retention and potassium excretion
Cortisol	Adrenal cortex	Most body cells	Increases blood glucose, helps body cope with stress, anti-inflammatory
Epinephrine (adrenaline)	Adrenal medulla	Heart, lungs, liver, muscles	Increases heart rate, blood pressure, and metabolic rate (fight or flight)
Norepinephrine (noradrenaline)	Adrenal medulla	Blood vessels, heart	Constricts blood vessels, increases blood pressure (fight or flight)
Melatonin	Pineal gland	Brain (hypothalamus)	Regulates sleep-wake cycles
Glucagon	Pancreas (alpha cells)	Liver, adipose tissue	Raises blood glucose by stimulating glycogen breakdown
Insulin	Pancreas (beta cells)	Most body cells	Lowers blood glucose by promoting uptake and storage
Estrogen	Ovaries	Uterus, breasts, brain, bones	Promotes development of female sex characteristics and reproductive system
Testosterone	Testes	Muscles, bones, male reproductive organs	Promotes development of male sex characteristics and sperm production
Thymosin	Thymus	T lymphocytes (T cells)	Stimulates T cell development and immune response

Endocrine Glands and Lifespan Changes

Examples of Glands That Change Over Time

• Thymus: Large and active in childhood, shrinks after puberty, becomes mostly fatty tissue in adults. Produces thymosin for T cell development.

• Pineal gland: Most active in childhood/adolescence, produces melatonin for sleep-wake cycles. Melatonin declines with age.

• Ovaries and Testes: Ovaries decrease estrogen/progesterone production with age (menopause). Testes show slow decline in testosterone with age.

Key Point: Changes in endocrine glands affect immunity, sleep, reproduction, bone health, and metabolism throughout life.

Vocabulary and Key Terms

Selected Vocabulary with Definitions and Examples

• Hormone: Chemical messenger produced by endocrine glands (e.g., insulin, growth hormone).

• Endocrine gland: Ductless gland that secretes hormones into the bloodstream (e.g., thyroid, pituitary).

• Parathyroid hormone (PTH): Increases blood calcium by stimulating bone breakdown.

• Prolactin: Stimulates milk production in mammary glands after childbirth.

• Growth hormone (GH): Stimulates body growth and cell reproduction.

• Oxytocin: Stimulates uterine contractions during labor and milk ejection during breastfeeding.

• Pineal gland: Produces melatonin, regulating sleep-wake cycles.

• Corticosteroids: Steroid hormones from the adrenal cortex; reduce inflammation and help the body respond to stress.

Example: "After giving birth, prolactin levels rise to stimulate milk production in the mother."

Key Equations and Concepts

• Hormone-Receptor Binding: The effect of a hormone depends on the concentration of the hormone, the number of receptors, and the affinity between hormone and receptor.

• Negative Feedback: Most hormone release is regulated by negative feedback loops to maintain homeostasis.

Equation (example for hormone-receptor interaction):

$\text{Response} \propto [\text{Hormone}] \times [\text{Receptors}] \times \text{Affinity}$

Additional info: This equation is a conceptual representation; actual cellular responses are more complex and may involve second messengers and signal amplification.