Endocrine System: Anatomy & Physiology Study Notes

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Endocrine System

Introduction

The endocrine system is a complex network of glands that synthesize and secrete chemical messengers known as hormones. These hormones regulate a wide range of physiological processes to maintain homeostasis and coordinate body functions.

Comparison of Endocrine and Nervous System Functions

Nervous System: Utilizes neurons and neurotransmitters to produce immediate but short-lived effects; uses stimulation as its main mode of action.
Endocrine System: Secretes hormones into the bloodstream, resulting in slower onset (seconds to days) but longer-lasting effects.

Endocrine System Basics

Endocrine glands: Secrete hormones directly into body fluids or blood.
Paracrine signaling: Chemicals affect nearby but different cell types in the extracellular space.
Autocrine signaling: Chemicals affect the same cell or cell type that secreted them, acting via the interstitial fluid.

Endocrine Physiology (Functions)

Regulates metabolic processes by controlling chemical reaction rates.
Aids in transporting substances through membranes.
Helps regulate water balance, electrolyte balance, and blood pressure.
Plays a role in reproduction, development, and growth.

Major Endocrine Glands

Pituitary gland
Hypothalamus
Pineal gland
Thyroid gland
Parathyroid glands
Thymus
Adrenal glands
Pancreas

Actions of Hormones

Definition and Target Cells

Definition: Substances secreted by cells that affect the metabolic activity of other cells or tissues (target cells).
Target Cells: Cells with specific receptors for a given hormone.
Characteristics: Hormones have specific molecular structures and are very potent.
Groups of Steroid Hormones:
- Steroid hormones: Derived from cholesterol.
- Nonsteroid hormones: Derived from amino acids.

Steroid Hormone Action

Steroid hormones diffuse through the cell membrane and enter the nucleus.
They combine with a protein receptor to form a hormone-receptor complex.
This complex binds to DNA, activating specific genes to produce mRNA.
mRNA moves to the cytoplasm and directs protein synthesis.

Nonsteroid Hormone Action

Nonsteroid hormone acts as the first messenger and travels in the blood to a target cell.
It binds to a receptor on the target cell membrane, activating the enzyme adenylate cyclase.
Adenylate cyclase converts ATP to cyclic AMP (cAMP), the second messenger.
cAMP activates enzymes that produce changes in cellular metabolic processes, such as altering cell permeability, promoting movement, or activating enzymes.

Prostaglandins

Paracrine chemicals that affect only neighboring cells; sometimes called "local hormones".
Produced in small quantities but are very potent.
Functions include regulating cellular responses to hormones, activating or inhibiting adenylate cyclase, relaxing or contracting smooth muscle, stimulating hormone secretion, inhibiting stomach acid, regulating blood pressure, affecting reproductive physiology, and promoting inflammation.

Hormone Secretion Regulation

Hormonal: Releasing hormones from the hypothalamus stimulate the anterior pituitary gland.
Humoral: Glands respond directly to changes in the internal environment (e.g., blood glucose levels regulate insulin and glucagon secretion from the pancreas).

Negative Feedback in Hormone Regulation

Most hormone levels are regulated by negative feedback loops, where increased hormone levels inhibit further secretion.

Hormone Concentration at Target Cells

Determined by rate of production, rate of delivery, and rate of degradation/elimination.
Hormones have a limited lifespan (half-life) and are often secreted in pulses.

Pituitary Gland

Location and Structure

Located at the base of the brain, attached to the hypothalamus by the infundibulum, and sits in the sella turcica of the sphenoid bone.
Has two lobes:
- Anterior lobe: Made of epithelial tissue; controlled by releasing hormones from the hypothalamus.
- Posterior lobe: Made of neuroglial cells; controlled by nerve impulses from the hypothalamus.

Anterior Pituitary Hormones

Growth Hormone (GH): Stimulates body cell growth, cartilage growth, amino acid movement, and increases fat/protein metabolism. Stimulated by growth hormone-releasing hormone (GHRH) from the hypothalamus.
- Disorders: Dwarfism (deficiency in childhood), gigantism (excess in childhood), acromegaly (excess in adulthood).
Prolactin (PRL): Promotes breast development and milk production; decreases effect of luteinizing hormone in males. Stimulated by prolactin-releasing hormone (PRH).
Thyroid-stimulating hormone (TSH): Controls secretion of thyroid hormones. Stimulated by thyrotropin-releasing hormone (TRH).
Adrenocorticotropic hormone (ACTH): Controls secretion of adrenal cortex hormones. Stimulated by corticotropin-releasing hormone (CRH).
Follicle-stimulating hormone (FSH) and Luteinizing Hormone (LH): Gonadotropins that influence reproductive organs in both sexes. Stimulated by gonadotropin-releasing hormone (GnRH).

Posterior Pituitary Hormones

Antidiuretic hormone (ADH): Reduces water excretion by kidneys; deficiency causes diabetes insipidus.
Oxytocin (OT): Stimulates uterine contractions and milk ejection in females; may stimulate movement in the male reproductive tract.

Thyroid Gland

Location and Structure

Located below the larynx, anterior to the trachea; composed of follicles with follicular cells that secrete hormones and store them in colloid.
Removes iodine from the blood to produce hormones.

Thyroid Hormones

Thyroxine (T4) and Triiodothyronine (T3): Regulate metabolism, increase protein synthesis, stimulate lipid and carbohydrate breakdown, and promote nervous system development. T3 is about five times more potent than T4. Release is stimulated by TSH.
Calcitonin: Lowers blood calcium and phosphate by inhibiting bone resorption. Increases deposition in bone, and increases kidney excretion. Release is stimulated by elevated blood calcium.

Thyroid Disorders

Hypothyroidism: Undersecretion of thyroid hormones.
- Congenital hypothyroidism (cretinism): Stunted growth, abnormal bone formation, mental retardation, sluggishness in infants.
- Myxedema: Low metabolic rate, sluggishness, poor appetite, swollen tissues, mental dullness in adults.
- Simple (endemic) goiter: Iodine deficiency causes thyroid enlargement but reduced function.
Hyperthyroidism: Oversecretion of thyroid hormones (e.g., Graves' disease). Symptoms include nervousness, restlessness, hyperactivity, weight loss, and goiter.

Parathyroid Glands

Location and Function

Located on the posterior surface of the thyroid gland.
Secrete parathyroid hormone (PTH), which increases blood calcium and decreases blood phosphate.
Functions:
- Stimulates release of calcium from bone.
- Stimulates absorption of calcium and excretion of phosphate by kidneys.
- Stimulates absorption of calcium by intestines.
Regulated by negative feedback: increased blood calcium inhibits PTH secretion.

Parathyroid Disorders

Hypercalcemia: Too much calcium in the blood, often due to overactive parathyroid (hyperparathyroidism).
Hypocalcemia: Too little calcium in the blood, often due to underactive parathyroid (hypoparathyroidism).
Calcitonin may be used as treatment when PTH is excessive.

Adrenal Glands

Location and Structure

Located on top of each kidney; consist of an inner medulla and outer cortex.

Adrenal Medulla

Secretes epinephrine (adrenaline) and norepinephrine.
Effects mimic sympathetic nervous system but last longer ("fight or flight" response): increased heart rate, blood pressure, breathing rate, and decreased digestive activity.
Secretion controlled by nerve impulses from the hypothalamus.

Adrenal Cortex

Secretes many hormones, including:
- Aldosterone: Conserves sodium, excretes potassium, reduces urine output, maintains blood volume and pressure.
- Cortisol: Influences metabolism of carbohydrates, proteins, and fats; stimulates glucose synthesis; maintains blood glucose between meals. Controlled by negative feedback and ACTH.
- Adrenal sex hormones: Androgens and estrogens supplement sex hormones from the gonads.

Adrenal Cortex Disorders

Addison's disease: Decreased secretion of adrenal cortex hormones, often due to autoimmunity or infection. Symptoms: low sodium, glucose, blood pressure; high potassium; skin pigmentation; salt craving.
Cushing's syndrome: Increased cortisol secretion due to tumor or overproduction. Symptoms: increased tissue protein, bone tissue loss, edema, glucose and sodium retention, fat deposits on face (moon face) and back (buffalo hump).

Pancreas

Location and Structure

Located posterior to the stomach, connected to the duodenum.
Contains groups of cells called pancreatic islets (Islets of Langerhans).
Functions as both an exocrine (digestive) and endocrine gland.

Pancreatic Hormones

Glucagon: Raises blood glucose by stimulating the liver to break down glycogen and convert noncarbohydrates into glucose.
Insulin: Lowers blood glucose by stimulating the liver to store glucose as glycogen and enhancing protein and fat synthesis.
Both hormones are regulated by negative feedback from blood glucose levels.

Diabetes Mellitus

Type 1 (Insulin-dependent): Juvenile onset, autoimmune destruction of beta cells, requires insulin therapy.
Type 2 (Non-insulin-dependent): Adult onset, associated with heredity and obesity, often managed with diet and exercise.

Other Endocrine Glands and Hormone-Secreting Tissues

Pineal gland: Attached to the thalamus, deep in the brain. Secretes melatonin, which regulates circadian rhythms and promotes sleep. Darkness stimulates melatonin production; light exposure is linked to seasonal affective disorder (SAD).
Thymus: Located behind the sternum, large in children, shrinks with age. Secretes thymosins, which affect lymphocyte (WBC) production.
Reproductive glands: Ovaries (estrogen, progesterone), testes (testosterone), placenta (estrogen, progesterone).
Other hormone-producing organs: Stomach, small intestine, kidneys, heart, adipose tissue (leptin, which induces satiety via the hypothalamus).

Stress and the Endocrine System

Stress: The condition produced when the body responds to stressors that threaten homeostasis.
Stressors: Factors capable of stimulating a stress response (physical or psychological).
Physical stress: Environmental factors harmful to tissues (e.g., cold, injury, prolonged exercise).
Psychological stress: Emotional responses to real or imagined threats (e.g., fear, anxiety).
Stress responses: Increased sympathetic nervous system activity and adrenal hormone secretion.
General Adaptation Syndrome: Controlled by the hypothalamus to maintain homeostasis; involves CRH release, sympathetic impulses, and increased secretion of hormones like epinephrine, norepinephrine, and cortisol.

Summary Table: Major Endocrine Glands and Their Hormones

Gland	Hormones	Main Functions
Pituitary (anterior)	GH, PRL, TSH, ACTH, FSH, LH	Growth, milk production, thyroid/adrenal/gonadal regulation
Pituitary (posterior)	ADH, OT	Water balance, uterine contraction, milk ejection
Thyroid	T3, T4, Calcitonin	Metabolism, calcium regulation
Parathyroid	PTH	Calcium and phosphate regulation
Adrenal medulla	Epinephrine, Norepinephrine	Fight or flight response
Adrenal cortex	Aldosterone, Cortisol, Androgens	Electrolyte balance, metabolism, sex hormones
Pancreas	Insulin, Glucagon	Blood glucose regulation
Pineal	Melatonin	Sleep/wake cycle
Thymus	Thymosin	Lymphocyte production
Ovaries/Testes	Estrogen, Progesterone, Testosterone	Reproductive function

Major Control Systems of the Body

The body maintains homeostasis and coordinates functions through two major control systems: the nervous system and the endocrine system. Each system uses distinct methods to communicate and regulate physiological processes.

Nervous System: Utilizes electrical impulses and neurotransmitters for rapid, short-term responses. Acts at discrete locations via axon pathways and regulates muscles and glands.
Endocrine System: Uses hormones as chemical messengers, which travel through the blood to distant targets. Effects are typically slower but longer-lasting, regulating processes such as growth, metabolism, and reproduction.

Example: The nervous system controls muscle contraction, while the endocrine system regulates growth and metabolism.

16.2 Chemical Structure of Hormones

Classification of Hormones

Hormones are classified based on their chemical structure, which determines their solubility, transport, and mechanism of action.

Hormone Characteristic	Amino Acid-Based Hormones	Steroid Hormones
Synthesized from cholesterol		X
Transported in blood bound to carrier protein		X
Freely dissolved in plasma	X
Acts on cell surface receptors	X
Acts on intracellular receptors		X

16.3 Hormone Action: Second Messengers and Gene Activation

Mechanisms of Hormone Action

Second Messenger: Water-soluble hormones (amino acid-based) bind to cell surface receptors, activating intracellular signaling cascades (e.g., cAMP).
Gene Activation: Lipid-soluble hormones (steroids) enter cells and bind to intracellular receptors, directly affecting gene transcription.

Example: Epinephrine uses cAMP as a second messenger to trigger cellular responses, while steroid hormones like estrogen bind to nuclear receptors to regulate gene expression.

16.4 Regulation of Hormone Release

Types of Stimuli

Humoral: Changes in blood levels of ions or nutrients stimulate hormone release.
Neural: Nerve fibers stimulate hormone release.
Hormonal: Hormones stimulate other endocrine glands to release hormones.

Negative feedback mechanisms help maintain stable hormone levels.

16.5 Target Cell Activation and Hormone Receptors

Factors Influencing Target Cell Action

Concentration of hormone in blood.
Number of receptors on/in target cell.
Affinity of receptor for hormone.

Up-regulation: Target cells increase receptor number in response to low hormone levels. Down-regulation: Target cells decrease receptor number in response to high hormone levels.

16.6 Endocrine Organs and Their Locations

Pituitary gland: base of brain
Thyroid gland: anterior neck
Parathyroid glands: posterior thyroid
Adrenal glands: atop kidneys
Pancreas: posterior to stomach
Pineal gland: deep in brain
Thymus: behind sternum
Ovaries/testes: pelvic cavity/scrotum

16.7 Types of Chemical Signals: Hormones, Paracrines, and Autocrines

Definitions and Differences

Hormones: Secreted into blood, act on distant targets.
Paracrines: Act locally, affect neighboring cells.
Autocrines: Affect the same cell that secreted them.

Example: Somatostatin acts locally to inhibit growth hormone release in the pituitary.

16.8 Summary Table: Key Endocrine Concepts

Concept	Key Point
Control System	Nervous: electrical impulses; Endocrine: chemical messengers
Hormone Class	Amino acid-based (water-soluble), steroid (lipid-soluble)
Regulation	Negative feedback, humoral/neural/hormonal stimuli
Target Cell Activation	Depends on hormone concentration, receptor number, affinity
Example Disorders	Diabetes mellitus, hypothyroidism, Cushing's syndrome

Key Equations and Feedback Mechanisms

Negative Feedback Example: Cold Regulation

Exposure to cold stimulates hypothalamus to release TRH → pituitary releases TSH → thyroid releases T3/T4 → increased metabolism and heat production.

Blood Glucose Regulation

High blood glucose stimulates insulin release; low blood glucose stimulates glucagon release.

Histological Identification

Pituitary gland: Has distinct cell types, including acidophils, basophils, and chromophobes.
Thyroid gland: Follicular cells surround colloid; parafollicular cells secrete calcitonin.
Adrenal gland: Cortex has three zones (glomerulosa, fasciculata, reticularis); medulla contains chromaffin cells.
Pancreas: Islets of Langerhans contain alpha (glucagon) and beta (insulin) cells.

Conclusion

The endocrine system is essential for maintaining homeostasis, growth, metabolism, and reproduction. Understanding the anatomy, physiology, and regulation of endocrine glands and hormones is crucial for students of anatomy and physiology.

Additional info: The cartoon at the beginning humorously illustrates the effect of excessive growth hormone, as in conditions like gigantism or acromegaly, where abnormal hormone levels lead to accelerated growth.