Introduction to Anatomy, Physiology, and the Endocrine System

Introduction to Anatomy and Physiology

Definitions and Concepts

• Anatomy: The study of the structure of body parts and their relationships to one another.

• Physiology: The study of the function of the body, focusing on how body parts work and carry out life-sustaining activities.

• Homeostasis: The process of maintaining the body's internal environment in a relatively constant state (e.g., steady temperature, blood pressure, glucose levels).

• Homeostatic Imbalance: Any disturbance or alteration of the internal environment's balance, requiring corrective activities to restore equilibrium.

Homeostatic Control Mechanisms

• Self-regulating processes that maintain the homeostatic steady state.

• All mechanisms involve at least three components: receptor, control center, and effector.

Feedback Mechanisms

• Negative Feedback: The output shuts off the original effect of the stimulus or reduces its intensity. Most homeostatic control mechanisms function this way (e.g., thermostat analogy).

• Positive Feedback: The response enhances the original stimulus, accelerating change in the same direction. Rare in life; examples include blood clotting, labor contractions, and orgasm.

Regulatory Systems

• Messages between body parts are conducted electrically or chemically.

• Nervous System: Uses electrical impulses delivered by neurons to specific target cells. Neural control is quick and brief, resulting in muscle contraction or gland secretion.

• Endocrine System: Releases chemical messengers (hormones) directly into the blood, affecting varied target cells throughout the body.

The Endocrine System

Overview and Functions

The endocrine system works with the nervous system to coordinate and integrate the activity of body cells. It influences metabolic activities via hormones transported in the blood.

• Controls and integrates reproduction, growth and development, maintenance of electrolyte, water, and nutrient balance, regulation of cellular metabolism and energy balance, and mobilization of body defenses.

Types of Glands

• Endocrine Glands: Secrete products into extracellular spaces, picked up and transported by the circulatory system. Examples: adrenal, thyroid, pituitary.

• Exocrine Glands: Produce non-hormonal substances and secrete products via ducts onto surfaces. Examples: salivary, sweat, digestive glands.

Hormones

• Definition: Chemical messengers produced by one type of cell, transported in blood or lymph, and affecting the metabolic activity of target cells.

• Two main classes:

  • Amino Acid-Based Hormones: Range from small amino acids to large proteins; usually water-soluble and cannot cross the plasma membrane. Includes the majority of hormones.

  • Steroid Hormones: Lipid-soluble, can cross the plasma membrane; includes only gonadal and some adrenal hormones.

Mechanism of Hormonal Action

• Hormones circulate in the bloodstream, exposing all tissues, but only affect cells with specific receptors.

• Receptors are large proteins, mostly on the cell surface, binding specific hormones (lock and key analogy).

• A single hormone can produce different effects on different cells due to different receptors or varying quantities of receptors.

Hormone Effects

• Alter plasma membrane permeability or membrane potential by opening/closing ion channels.

• Stimulate synthesis of enzymes and other proteins (start/stop protein synthesis).

• Activate/deactivate enzymes.

• Induce secretory activity.

• Stimulate mitosis.

Action of Hormones

• Water-Soluble Hormones (all amino acid–based except thyroid hormone):

  • Act on plasma membrane receptors.

  • Act via G protein second messengers.

  • Cannot enter the cell.

• Lipid-Soluble Hormones (steroid and thyroid hormones):

  • Act on intracellular receptors that directly activate genes.

  • Can enter the cell.

Second-Messenger Systems

• Amino acid-based hormones (except thyroid hormone) exert effects through second-messenger systems.

• Two main second-messenger systems: cyclic AMP and others (Additional info: e.g., phospholipase C pathway).

Intracellular Receptors and Direct Gene Activation

• Lipid-soluble steroid hormones and thyroid hormone diffuse into target cells and bind with intracellular receptors.

• Receptor-hormone complex enters nucleus and binds to specific DNA regions, triggering transcription of metabolic genes.

• mRNA is translated into specific proteins with various functions (metabolic, structural, or exported).

Endocrine Gland Stimuli

Types of Stimuli

• Humoral Stimuli: Changing blood levels of ions/nutrients stimulate hormone secretion. Example: Declining blood Ca2+ stimulates parathyroid glands to secrete PTH.

• Neural Stimuli: Nerve fibers stimulate hormone release. Example: Sympathetic nervous system stimulates adrenal medulla to secrete catecholamines.

• Hormonal Stimuli: Hormones stimulate other endocrine organs to release their hormones. Example: Hypothalamic hormones stimulate anterior pituitary; anterior pituitary hormones stimulate target organs.

Nervous System Modulation

• Nervous system can adjust hormone levels, modify stimulation/inhibition, and override normal endocrine controls (e.g., fight or flight response).

Hormonal Activity: Half-life, Onset, and Duration

Hormone Circulation and Removal

• Hormones circulate either free or bound (steroids and thyroid hormone attached to plasma proteins).

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

• Half-life: Time required for hormone level in blood to decrease by half; varies from seconds to a week.

Onset and Duration

• Response times vary: some immediate, some (especially steroids) take hours to days, some inactive until entering target cells.

• Duration ranges from seconds to hours; effects may disappear rapidly or persist at low blood levels.

Interaction of Hormones at Target Cells

Types of Interaction

• Permissiveness: One hormone cannot exert its full effects without another hormone present (e.g., reproductive hormones need thyroid hormone).

• Synergism: More than one hormone produces the same effects, and their combined effects are amplified (e.g., glucagon and epinephrine both cause liver to release glucose).

• Antagonism: One hormone opposes the action of another (e.g., insulin and glucagon).

Major Endocrine Glands and Their Hormones

Hypothalamus and Pituitary Gland

• Hypothalamus connected to pituitary via infundibulum.

• Pituitary has two lobes:

  • Posterior Pituitary (Neurohypophysis): Neural tissue; stores and releases oxytocin and ADH.

  • Anterior Pituitary: Glandular tissue; secretes six major peptide hormones, four of which are tropic (regulate other glands).

Posterior Pituitary Hormones

• Oxytocin: Stimulates uterine contractions during birth and triggers milk ejection; both are positive feedback mechanisms.

• Antidiuretic Hormone (ADH): Targets kidney tubules to reabsorb more water, inhibiting/preventing urine formation; release triggered by high solute concentration, pain, low blood pressure, drugs; inhibited by alcohol and diuretics; high concentrations cause vasoconstriction.

Homeostatic Imbalances

• Diabetes Insipidus: ADH deficiency; causes excess urination and thirst.

• SIADH: Excess ADH; causes fluid retention, headache, disorientation.

Anterior Pituitary Hormones

• Growth Hormone (GH): Stimulates cell growth and division, especially in bone and skeletal muscle; regulated by GHRH and GHIH (somatostatin).

• Thyroid-Stimulating Hormone (TSH): Stimulates thyroid gland; regulated by thyrotropin-releasing hormone and negative feedback from thyroid hormones.

• Adrenocorticotropic Hormone (ACTH): Stimulates adrenal cortex to release corticosteroids; regulated by corticotropin-releasing hormone (CRH).

• Follicle-Stimulating Hormone (FSH): Stimulates gamete production.

• Luteinizing Hormone (LH): Promotes production of gonadal hormones; triggers ovulation and hormone release in females, testosterone production in males.

• Prolactin (PRL): Stimulates milk production; regulated by prolactin-inhibiting hormone (PIH, dopamine).

Anterior Pituitary Hormone Imbalances

• GH Hypersecretion: Causes gigantism in children, acromegaly in adults.

• GH Hyposecretion: Causes dwarfism in children.

Thyroid Gland

• Located anterior neck, inferior to larynx; consists of follicles (follicular cells produce thyroglobulin), colloid (thyroglobulin + iodine), and parafollicular cells (produce calcitonin).

• Thyroid Hormone (TH): Major metabolic hormone; T3 and T4 forms; increases basal metabolic rate, regulates growth and development, maintains blood pressure.

• Calcitonin: Produced by parafollicular cells in response to high Ca2+; antagonistic to parathyroid hormone; inhibits osteoclast activity and stimulates Ca2+ uptake into bone at high doses.

Thyroid Hormone Imbalances

• Hyposecretion: Causes myxedema in adults; lack of iodine leads to goiter.

• Hypersecretion: Graves' disease (autoimmune); antibodies mimic TSH, stimulating TH release.

Parathyroid Gland

• Four to eight glands embedded in posterior thyroid; secrete parathyroid hormone (PTH).

• PTH: Most important hormone in Ca2+ homeostasis; stimulates osteoclasts, enhances Ca2+ reabsorption and phosphate secretion by kidneys, promotes vitamin D activation for increased Ca2+ absorption in intestines.

Adrenal Gland

• Paired, pyramid-shaped organs atop kidneys; two parts: adrenal cortex (glandular tissue) and adrenal medulla (nervous tissue).

Adrenal Cortex

• Produces corticosteroids; three layers:

  • Zona Glomerulosa: Mineralocorticoids (e.g., aldosterone) regulate Na+ and K+ balance.

  • Zona Fasciculata: Glucocorticoids (e.g., cortisol) influence metabolism, resist stressors, maintain blood glucose and pressure.

  • Zona Reticularis: Gonadocorticoids (weak androgens) contribute to sex drive and secondary sex characteristics.

Adrenal Medulla

• Chromaffin cells synthesize catecholamines: epinephrine (80%) and norepinephrine (20%).

• Effects: vasoconstriction, increased heart rate, increased blood glucose, blood diverted to brain, heart, skeletal muscle; regulated neurally by sympathetic nervous system.

Pineal Gland

• Small gland in the brain; pinealocytes secrete melatonin (derived from serotonin).

• Melatonin affects timing of puberty, day/night cycles, body temperature, sleep, appetite, and production of antioxidants.

Pancreas

• Triangular gland behind stomach; both exocrine (acinar cells produce digestive enzymes) and endocrine (pancreatic islets).

• Alpha Cells: Produce glucagon (raises blood glucose).

• Beta Cells: Produce insulin (lowers blood glucose).

Glucagon

• Triggered by low blood glucose, rising amino acids, or sympathetic stimulation.

• Raises blood glucose by targeting liver for glycogenolysis and gluconeogenesis.

Insulin

• Secreted when blood glucose increases; lowers blood glucose by enhancing transport into cells, inhibiting glycogen breakdown, and inhibiting conversion of amino acids/fats to glucose.

Homeostatic Imbalances

• Diabetes Mellitus (DM): Due to hyposecretion (Type 1) or hypoactivity (Type 2) of insulin; leads to high blood glucose, glycosuria, polyuria, polydipsia, excessive hunger, lipidemia, ketone production, and ketoacidosis.

Gonads

• Ovaries: Produce estrogens and progesterone; responsible for maturation of reproductive organs, secondary sexual characteristics, breast development, and uterine changes.

• Testes: Produce testosterone; initiates maturation of male reproductive organs, secondary sexual characteristics, sex drive, sperm production, and maintenance of reproductive organs.

Thymus Gland

• Located in front of the heart; large in infancy, diminishes with age.

• Produces thymosins, essential for normal production and programming of T-cells (white blood cells involved in immunity).

Summary Table: Major Endocrine Glands and Their Hormones

Gland	Main Hormones	Primary Functions
Pituitary (Anterior)	GH, TSH, ACTH, FSH, LH, PRL	Growth, thyroid regulation, adrenal regulation, gamete production, gonadal hormone production, milk production
Pituitary (Posterior)	Oxytocin, ADH	Uterine contraction, milk ejection, water balance
Thyroid	T3, T4, Calcitonin	Metabolism, growth, development, calcium regulation
Parathyroid	PTH	Calcium homeostasis
Adrenal Cortex	Aldosterone, Cortisol, Androgens	Electrolyte balance, stress response, sex characteristics
Adrenal Medulla	Epinephrine, Norepinephrine	Fight or flight response
Pineal	Melatonin	Sleep, circadian rhythms
Pancreas	Insulin, Glucagon	Blood glucose regulation
Ovaries	Estrogen, Progesterone	Female reproductive function
Testes	Testosterone	Male reproductive function
Thymus	Thymosins	T-cell development

Key Equations and Concepts

• Hormone Half-life: $t_{1/2} = \frac{\ln 2}{k}$ (where k is the rate constant for hormone removal)

• Blood Glucose Regulation: $\text{Glycogenolysis}: \text{Glycogen} \rightarrow \text{Glucose}$

• $\text{Gluconeogenesis}: \text{Non-carbohydrate sources} \rightarrow \text{Glucose}$

Additional info:

• Second-messenger systems include cyclic AMP and phospholipase C pathways.

• Hormonal feedback loops often involve negative feedback to maintain homeostasis.

• Some hormones (e.g., thyroid hormone) are stored extracellularly and released upon stimulation.