Chapter 9: The Endocrine System – Anatomy & Physiology Study Notes

Chapter 9: The Endocrine System

Overview of the Endocrine System

The endocrine system is a major regulatory system of the body, composed of glands that secrete hormones directly into the bloodstream. These hormones coordinate and regulate various physiological processes, including growth, metabolism, and reproduction.

• Endocrine glands are ductless and release hormones into blood or lymph.

• Some glands have purely endocrine functions (e.g., anterior pituitary, thyroid, adrenals, parathyroids).

• Other glands are mixed, with both endocrine and exocrine functions (e.g., pancreas, gonads).

Major Endocrine Organs

• Hypothalamus

• Pituitary gland

• Pineal gland

• Thyroid gland

• Parathyroid glands

• Thymus

• Adrenal glands

• Pancreas

• Gonads (testes and ovaries)

Hormone Action Mechanisms

Direct Gene Activation

This mechanism is used by steroid hormones and thyroid hormone. Hormones pass through the cell membrane and bind to intracellular receptors, influencing gene expression.

• Step 1: Steroid hormone enters the target cell.

• Step 2: Hormone binds to a receptor protein in the cytoplasm or nucleus.

• Step 3: Hormone-receptor complex binds to DNA.

• Step 4: Specific genes are activated, leading to mRNA synthesis.

• Step 5: mRNA directs synthesis of new proteins.

Second-Messenger System

Used by protein and peptide hormones, which cannot cross the cell membrane. These hormones bind to membrane receptors, triggering a cascade via second messengers (e.g., cAMP).

• Step 1: Hormone (first messenger) binds to receptor on cell membrane.

• Step 2: Receptor activates an enzyme.

• Step 3: Enzyme converts ATP to cAMP (second messenger).

• Step 4: cAMP triggers cellular responses (e.g., glycogen breakdown).

Control of Hormone Release

Feedback Mechanisms

Hormone levels are regulated primarily by negative feedback, maintaining homeostasis.

• Negative feedback: Low hormone levels stimulate release; once optimal levels are reached, release stops.

Types of Stimuli

• Hormonal stimulus: Hormones from one gland stimulate another gland to release hormones.

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

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

Pituitary Gland and Hypothalamus

Pituitary Gland

The pituitary gland is a pea-sized structure hanging from the hypothalamus, protected by the sella turcica of the sphenoid bone. It consists of two lobes:

• Anterior pituitary: Glandular tissue; produces and releases hormones.

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

Often called the "master endocrine gland" due to its regulatory role.

Hypothalamus

• Produces releasing and inhibiting hormones that regulate anterior pituitary.

• Makes oxytocin and antidiuretic hormone (ADH), stored in posterior pituitary.

Posterior Pituitary Hormones

• Oxytocin: Stimulates uterine contractions and milk ejection.

• Antidiuretic hormone (ADH): Promotes water reabsorption in kidneys.

Anterior Pituitary Hormones and Targets

Hormone	Main Target(s)
Growth hormone (GH)	Bones and muscles
Prolactin (PRL)	Mammary glands
Follicle-stimulating hormone (FSH)	Testes or ovaries
Luteinizing hormone (LH)	Testes or ovaries
Thyrotropic hormone (TH)	Thyroid
Adrenocorticotropic hormone (ACTH)	Adrenal cortex

Pineal Gland

The pineal gland hangs from the roof of the third ventricle of the brain and secretes melatonin.

• Regulates the body's sleep/wake cycle.

• May coordinate reproductive hormones and inhibit reproductive system until maturity.

Thyroid Gland

Located at the base of the throat, the thyroid gland consists of two lobes connected by an isthmus. Follicles store colloidal material.

• Thyroid hormone: Major metabolic hormone, controls rate of glucose oxidation, tissue growth, and development.

• Composed of thyroxine (T4) and triiodothyronine (T3).

• Calcitonin: Decreases blood calcium by promoting calcium deposition in bone; antagonistic to parathyroid hormone.

Parathyroid Glands

Tiny masses on the posterior of the thyroid, secrete parathyroid hormone (PTH).

• Regulates calcium ion (Ca2+) homeostasis in blood.

• Stimulates osteoclasts to release calcium from bone.

• Increases blood calcium levels (hypercalcemic effect).

• Stimulates kidneys and intestines to absorb more calcium.

Calcium Homeostasis

Hormone	Effect
Calcitonin	Decreases blood Ca2+ by stimulating bone deposition
PTH	Increases blood Ca2+ by stimulating bone resorption

Normal blood calcium: 9–11 mg/100 mL

Thymus

Located in the upper thorax, posterior to the sternum. Largest in infants and children, decreases in size with age.

• Produces thymosin, which matures certain white blood cells.

• Important for immune system development.

Adrenal Glands

Located atop the kidneys, each adrenal gland has two regions:

• Adrenal cortex: Outer glandular region, produces corticosteroids.

• Adrenal medulla: Inner neural tissue region, produces catecholamines.

Adrenal Cortex Hormones

• Mineralocorticoids (e.g., aldosterone): Regulate sodium and potassium balance.

• Glucocorticoids (e.g., cortisone, cortisol): Help resist long-term stress by increasing blood glucose.

• Sex hormones: Mainly androgens (male), some estrogens (female).

Aldosterone release is stimulated by humoral factors (low sodium/high potassium) and inhibited by atrial natriuretic peptide (ANP).

Adrenal Medulla Hormones

• Epinephrine (adrenaline) and norepinephrine (noradrenaline) prepare the body for short-term stress ("fight or flight").

• Effects: Increased heart rate, blood pressure, blood glucose; bronchiole dilation.

Stress Response

Short-Term Stress	Long-Term Stress
Catecholamines (adrenal medulla)	Corticosteroids (adrenal cortex)
Increased heart rate, blood pressure, glucose	Retention of sodium/water, increased blood volume/pressure, immune suppression

Pancreatic Islets

The pancreas is a mixed gland located near the stomach. The islets of Langerhans produce hormones that regulate blood sugar.

• Insulin: Produced by beta cells; lowers blood glucose by promoting uptake into cells.

• Glucagon: Produced by alpha cells; raises blood glucose by stimulating glycogen breakdown in liver.

• Insulin and glucagon are antagonists, maintaining blood sugar homeostasis.

Normal blood glucose: ~90 mg/100 mL

Gonads

Ovaries

• Produce eggs and steroid hormones: estrogens and progesterone.

Testes

• Produce sperm and androgens, mainly testosterone.

Additional Hormone-Producing Tissues and Organs

• Stomach

• Small intestine

• Kidneys

• Heart

• Placenta: Maintains pregnancy, produces hCG, estrogen, progesterone, hPL, and relaxin.

Key Terms and Definitions

• Hormone: Chemical messenger secreted by endocrine glands, affecting target cells.

• Endocrine gland: Ductless gland that releases hormones into blood or lymph.

• Exocrine gland: Gland that releases secretions via ducts to body surfaces or cavities.

• Negative feedback: Regulatory mechanism that maintains homeostasis by inhibiting further hormone release when optimal levels are reached.

Important Equations

• Blood Calcium Homeostasis:

• $9-11\ \text{mg}/100\ \text{mL}$ (normal blood calcium concentration)

• Blood Glucose Homeostasis:

• $\sim 90\ \text{mg}/100\ \text{mL}$ (normal blood glucose concentration)