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 condition 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.

• Homeostatic Control Mechanisms: Self-regulating processes that maintain homeostasis, typically involving three components: receptor, control center, and effector.

Feedback Mechanisms

• Negative Feedback: The output reduces or shuts off the original effect of the stimulus. Most homeostatic mechanisms function this way (e.g., body temperature regulation).

• Positive Feedback: The response enhances the original stimulus, accelerating the response (e.g., blood clotting, labor contractions).

Regulatory Systems

• Nervous System: Uses electrical impulses delivered by neurons; responses are quick and brief, typically causing muscle contraction or gland secretion.

• Endocrine System: Releases chemical messengers (hormones) into the blood; effects are slower but longer-lasting and can affect many 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 hormones into extracellular spaces, picked up by the circulatory system (e.g., adrenal, thyroid, pituitary).

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

Hormones

• Definition: Chemical messengers transported in the blood (or lymph) that alter the physiological activity of target cells.

• Classes of Hormones:

  • Amino Acid-Based Hormones: Usually water-soluble, cannot cross the plasma membrane; includes most hormones.

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

Mechanism of Hormonal Action

• Hormones affect only target cells with specific receptors (lock and key model).

• Hormones can produce different effects on different cells due to different receptors or varying receptor quantities.

Hormone Effects

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

• Stimulate synthesis of enzymes and other proteins.

• Activate/deactivate enzymes.

• Induce secretory activity.

• Stimulate mitosis.

Action of Hormones

• Water-Soluble Hormones: Act on plasma membrane receptors via G protein second messengers; cannot enter the cell.

• Lipid-Soluble Hormones: Act on intracellular receptors that directly activate genes; can enter the cell.

Second-Messenger Systems

• Amino Acid-Based Hormones (except thyroid hormone) use second-messenger systems such as cyclic AMP (cAMP).

Intracellular Receptors and Direct Gene Activation

• Lipid-Soluble Hormones (steroids and thyroid hormone) diffuse into target cells, bind to intracellular receptors, and influence gene transcription.

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

Endocrine Gland Stimuli

• Humoral Stimuli: Changing blood levels of ions/nutrients stimulate hormone secretion (e.g., low Ca2+ stimulates PTH release).

• Neural Stimuli: Nerve fibers stimulate hormone release (e.g., sympathetic stimulation of adrenal medulla).

• Hormonal Stimuli: Hormones stimulate other endocrine organs (e.g., hypothalamic hormones stimulate anterior pituitary).

Nervous System Modulation

• The nervous system can adjust or override endocrine controls (e.g., stress-induced increase in blood glucose).

Hormonal Activity: Half-life, Onset, and Duration

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

• Half-life: Time required for hormone level in blood to decrease by half.

• Removal: By degrading enzymes, kidneys, or liver.

• Onset: Some responses are immediate; others (especially steroids) may take hours to days.

• Duration: Ranges from seconds to hours; effects may persist even at low blood levels.

Hormone Interactions at Target Cells

• Permissiveness: One hormone requires another to exert its full effect (e.g., reproductive hormones need thyroid hormone).

• Synergism: More than one hormone produces the same effect, amplifying the response (e.g., glucagon and epinephrine).

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

Major Endocrine Glands and Their Hormones

The Hypothalamus and Pituitary Gland

• Hypothalamus: Connected to the pituitary gland via the infundibulum; regulates pituitary hormone secretion.

• Pituitary Gland: Has two lobes:

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

  • Anterior Pituitary: Glandular tissue; produces and releases six peptide hormones.

Posterior Pituitary Hormones

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

• Antidiuretic Hormone (ADH): Promotes water reabsorption in kidneys; release triggered by high solute concentration, pain, low blood pressure, and certain drugs; inhibited by alcohol and diuretics.

• Homeostatic Imbalances:

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

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

Anterior Pituitary Hormones

• Growth Hormone (GH): Stimulates growth, especially in bones and skeletal muscle; regulated by GHRH (stimulates) and GHIH (inhibits).

• 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).

• Gonadotropins (FSH and LH): FSH stimulates gamete production; LH stimulates gonadal hormone production (estrogen, progesterone, testosterone); regulated by gonadotropin-releasing hormone and feedback from gonadal hormones.

• Prolactin (PRL): Stimulates milk production; regulated by prolactin-inhibiting hormone (dopamine) and estrogen levels.

• Homeostatic Imbalances:

  • GH Hypersecretion: Gigantism in children, acromegaly in adults.

  • GH Hyposecretion: Dwarfism in children.

Thyroid Gland

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

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

• Calcitonin: Produced by parafollicular cells; lowers blood Ca2+ levels by inhibiting osteoclasts and stimulating Ca2+ uptake into bone.

• Homeostatic Imbalances:

  • Hyposecretion: Myxedema, goiter (if due to iodine deficiency).

  • Hypersecretion: Graves’ disease (autoimmune hyperthyroidism).

Parathyroid Glands

• Located on the posterior aspect of the thyroid; secrete parathyroid hormone (PTH).

• PTH: Most important hormone for Ca2+ homeostasis; increases blood Ca2+ by stimulating osteoclasts, enhancing kidney reabsorption, and activating vitamin D for increased intestinal absorption.

Adrenal Glands

• Located atop the kidneys; consist of the adrenal cortex (glandular) and adrenal medulla (nervous tissue).

Adrenal Cortex

Zone	Hormone Class	Main Hormones	Main Functions
Zona glomerulosa	Mineralocorticoids	Aldosterone	Regulates Na+ and K+ balance, blood pressure
Zona fasciculata	Glucocorticoids	Cortisol	Regulates metabolism, stress response, maintains blood glucose
Zona reticularis	Gonadocorticoids	Androgens	Sex drive, secondary sex characteristics

• Aldosterone: Stimulates Na+ reabsorption and K+ elimination by kidneys; increases blood volume and pressure.

• Cortisol: Promotes gluconeogenesis (formation of glucose from fats/proteins), helps resist stressors.

• Androgens: Weak male sex hormones; contribute to female sex drive and estrogen source post-menopause.

Adrenal Medulla

• Produces catecholamines: epinephrine (80%) and norepinephrine (20%).

• Effects: Vasoconstriction, increased heart rate, increased blood glucose, blood flow to brain/heart/muscles; regulated by the sympathetic nervous system (fight or flight response).

Pineal Gland

• Located in the roof of the third ventricle; secretes melatonin (derived from serotonin).

• Functions: Regulates day/night cycles, timing of puberty, body temperature, sleep, appetite, and antioxidant production.

Pancreas

• Located behind the stomach; has both exocrine (acinar cells) and endocrine (pancreatic islets) functions.

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

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

Hormone	Source	Main Effect
Glucagon	Alpha cells	Raises blood glucose by glycogenolysis and gluconeogenesis
Insulin	Beta cells	Lowers blood glucose by enhancing uptake and inhibiting glycogen breakdown

• Homeostatic Imbalances:

  • Diabetes Mellitus (DM): Due to hyposecretion (Type 1) or hypoactivity (Type 2) of insulin; characterized by polyuria, polydipsia, and polyphagia.

  • Untreated DM can lead to lipidemia, ketone production, ketoacidosis, and severe complications.

Gonads

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

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

Thymus Gland

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

• Secretes thymosins, essential for normal production and programming of T-cells (immune function).

Key Equations and Concepts

• Hormone Half-life:

  • Mathematically, hormone decay can be described as: $C_t = C_0 \times e^{-kt}$ Where $C_t$ is the concentration at time $t$, $C_0$ is the initial concentration, and $k$ is the rate constant.

• Gluconeogenesis (formation of glucose from non-carbohydrate sources): $2 \text{Pyruvate} + 4 \text{ATP} + 2 \text{GTP} + 2 \text{NADH} + 6 \text{H}_2\text{O} \rightarrow \text{Glucose} + 4 \text{ADP} + 2 \text{GDP} + 6 \text{P}_i + 2 \text{NAD}^+ + 2 \text{H}^+$

Summary Table: Major Endocrine Glands and Hormones

Gland	Main Hormones	Main Functions
Pituitary (anterior)	GH, TSH, ACTH, FSH, LH, PRL	Growth, metabolism, stress, reproduction, lactation
Pituitary (posterior)	Oxytocin, ADH	Uterine contraction, milk ejection, water balance
Thyroid	TH (T3, T4), Calcitonin	Metabolism, growth, 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/wake cycles
Pancreas	Insulin, Glucagon	Blood glucose regulation
Gonads	Estrogens, Progesterone, Testosterone	Reproduction, secondary sex characteristics
Thymus	Thymosins	T-cell development

Additional info: Some explanations and equations were expanded for clarity and completeness, and tables were inferred and formatted for study purposes.