Introduction to Anatomy, Physiology, and the Endocrine System

Introduction to Anatomy and Physiology

Definitions and Concepts

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

• Physiology concerns the function of the body, explaining how body parts work and carry out life-sustaining activities.

• Homeostasis is the maintenance of a stable internal environment (e.g., temperature, blood pressure, glucose levels).

• Homeostatic imbalance refers to any disturbance or alteration of the body's internal balance, which can lead to disease.

Homeostatic Control Mechanisms

• Homeostatic mechanisms are generally self-regulating and maintain the body's steady state.

• All mechanisms involve three main components:

  • Receptor: Detects changes (stimuli) in the environment.

  • Control Center: Processes information and determines the response.

  • Effector: Carries out the response to restore balance.

Feedback Mechanisms

• Negative Feedback:

  • The output reduces or shuts off the original stimulus.

  • Most homeostatic mechanisms operate this way (e.g., body temperature regulation).

• Positive Feedback:

  • The response enhances the original stimulus, accelerating the process.

  • Examples: blood clotting, labor contractions, orgasm.

Regulatory Systems

• The body uses two main regulatory systems for feedback mechanisms:

  • Nervous System: Uses electrical impulses delivered by neurons for rapid, specific responses (e.g., muscle contraction, gland secretion).

  • Endocrine System: Releases chemical messengers (hormones) into the blood for widespread, slower, and longer-lasting effects.

The Endocrine System

Overview and Functions

• Acts with the nervous system to coordinate and integrate body cell activity.

• Influences metabolic activities via hormones transported in the blood.

• Major functions include:

  • Reproduction

  • Growth and development

  • Maintenance of electrolyte, water, and nutrient balance

  • Regulation of cellular metabolism and energy balance

  • Mobilization of body defenses

Types of Glands

• Endocrine glands: Ductless glands that secrete hormones into extracellular spaces, picked up by the circulatory system (e.g., adrenal, thyroid, pituitary).

• Exocrine glands: Secrete non-hormonal substances via ducts onto surfaces (e.g., salivary, sweat, digestive glands).

Hormones

• Hormones are chemical messengers transported in blood or lymph, altering the physiological activity of target cells.

• Two main classes:

  • Amino acid-based hormones: Usually water-soluble, cannot cross the plasma membrane (majority of hormones).

  • Steroid hormones: Lipid-soluble, can cross the plasma membrane (e.g., gonadal and some adrenal hormones).

Mechanism of Hormonal Action

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

• Binding triggers a cascade of events, resulting in physiological changes.

• A single hormone can have different effects on different cells due to receptor type or quantity.

Typical Hormone Effects

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

• Stimulate synthesis of enzymes and proteins.

• Activate or 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 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) use second-messenger systems:

• Main systems:

  • Cyclic AMP (cAMP)

  • PIP2-calcium

Direct Gene Activation

• Lipid-soluble hormones diffuse into target cells, bind intracellular receptors, and the complex binds DNA to initiate transcription and protein synthesis.

Endocrine Gland Stimuli

• Endocrine glands are stimulated by:

  • Humoral stimuli: Changing blood levels of ions/nutrients (e.g., low Ca2+ triggers PTH release).

  • Neural stimuli: Nerve fibers stimulate hormone release (e.g., sympathetic nerves stimulate 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 response increases blood glucose).

Hormone Activity: Half-life, Onset, and Duration

• Hormones circulate free or bound (steroids and thyroid hormone are protein-bound).

• Concentration depends on rate of release and removal (by enzymes, kidneys, or liver).

• Half-life: Time for hormone level to decrease by half (varies from minutes to a week).

• Onset and duration of effects vary by hormone type.

Hormone Interactions at Target Cells

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

• Synergism: Multiple hormones amplify the same effect (e.g., glucagon and epinephrine on glucose release).

• Antagonism: One hormone opposes another (e.g., insulin vs. glucagon).

Major Endocrine Glands and Hormones

The Hypothalamus and Pituitary Gland

• Hypothalamus connects to the pituitary gland via the infundibulum.

• Pituitary gland (hypophysis) has two lobes:

  • Posterior pituitary (neurohypophysis): Neural tissue, stores and releases neurohormones (oxytocin, ADH).

  • Anterior pituitary (adenohypophysis): Glandular tissue, produces and releases hormones under hypothalamic control.

Posterior Pituitary Hormones

• Oxytocin: Stimulates uterine contractions (childbirth) and milk ejection (positive feedback).

• Antidiuretic hormone (ADH): Promotes water reabsorption in kidneys; released in response to high solute concentration, pain, low blood pressure, or drugs; inhibited by alcohol and diuretics.

Homeostatic Imbalances

• Diabetes insipidus: ADH deficiency, causes excessive urination and thirst.

• SIADH: Excess ADH, leads to fluid retention, headache, disorientation.

Anterior Pituitary Hormones

• All are peptide hormones; most are tropic (regulate other endocrine glands).

Hormone	Main Function	Regulation
Growth hormone (GH)	Stimulates growth (bones, muscles), metabolism	GHRH (stimulates), GHIH (inhibits)
Thyroid-stimulating hormone (TSH)	Stimulates thyroid gland	TRH (stimulates), thyroid hormones (inhibit)
Adrenocorticotropic hormone (ACTH)	Stimulates adrenal cortex	CRH (stimulates), feedback inhibition
Follicle-stimulating hormone (FSH)	Stimulates gamete production	GnRH (stimulates), gonadal hormones (inhibit)
Luteinizing hormone (LH)	Stimulates gonadal hormone production	GnRH (stimulates), gonadal hormones (inhibit)
Prolactin (PRL)	Stimulates milk production	PIH/dopamine (inhibits), estrogen (stimulates)

Homeostatic Imbalances of GH

• Hypersecretion (children: gigantism; adults: acromegaly).

• Hyposecretion (children: pituitary dwarfism; adults: usually no effect).

Thyroid Gland

• Located in the anterior neck, inferior to the larynx.

• Composed of follicles (produce thyroglobulin), colloid (stores hormone precursor), and parafollicular cells (produce calcitonin).

Thyroid Hormone (TH)

• Major metabolic hormone (T3 and T4 forms, both iodine-containing).

• Increases basal metabolic rate, heat production, regulates growth, development, and blood pressure.

• Stored extracellularly and released upon TSH stimulation.

Homeostatic Imbalances

• Hyposecretion: Myxedema in adults; goiter if due to iodine deficiency.

• Hypersecretion: Graves' disease (autoimmune, antibodies mimic TSH).

Calcitonin

• Produced by parafollicular cells in response to high Ca2+ levels.

• Inhibits osteoclast activity, stimulates Ca2+ uptake into bone (antagonist to PTH).

Parathyroid Glands

• Four to eight small glands on the posterior thyroid.

• Secrete parathyroid hormone (PTH), the main regulator of blood Ca2+.

• PTH increases blood Ca2+ by:

  • Stimulating osteoclasts to release Ca2+ from bone.

  • Enhancing kidney reabsorption of Ca2+ and excretion of phosphate.

  • Activating vitamin D to increase intestinal Ca2+ absorption.

Adrenal Glands

• Paired, pyramid-shaped organs atop the kidneys (suprarenal glands).

• Two regions:

  • Adrenal cortex: Three layers producing corticosteroids.

  • Adrenal medulla: Nervous tissue, part of the sympathetic nervous system.

Adrenal Cortex Layer	Hormone Type	Main Function
Zona glomerulosa	Mineralocorticoids (e.g., aldosterone)	Regulate Na+/K+ balance, blood pressure
Zona fasciculata	Glucocorticoids (e.g., cortisol)	Regulate metabolism, stress response, blood glucose
Zona reticularis	Gonadocorticoids (sex hormones)	Contribute to puberty, sex drive, secondary sex characteristics

• Adrenal medulla produces catecholamines (epinephrine and norepinephrine):

  • Increase heart rate, blood glucose, vasoconstriction, and blood flow to muscles.

  • Regulated by the sympathetic nervous system.

Pineal Gland

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

• Melatonin regulates sleep-wake cycles, puberty timing, and other rhythmic processes.

Pancreas

• Located behind the stomach; both exocrine (digestive enzymes) and endocrine (hormones) functions.

• Pancreatic islets contain:

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

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

Hormone	Main Action	Trigger
Glucagon	Raises blood glucose (glycogenolysis, gluconeogenesis)	Low blood glucose, high amino acids, sympathetic activity
Insulin	Lowers blood glucose (increases uptake, inhibits glycogen breakdown and gluconeogenesis)	High blood glucose

Homeostatic Imbalance: Diabetes Mellitus (DM)

• Type 1 DM: Hyposecretion of insulin.

• Type 2 DM: Hypoactivity of insulin.

• Symptoms: Polyuria (excess urine), polydipsia (thirst), polyphagia (hunger).

• Untreated DM leads to lipidemia, ketone production, and potentially ketoacidosis (dangerous acidification of blood).

Gonads

• Ovaries produce estrogens and progesterone:

  • Estrogen: Maturation of reproductive organs, secondary sexual characteristics.

  • Progesterone: With estrogen, regulates uterine cycle and breast development.

• Testes produce testosterone:

  • Initiates and maintains male reproductive organs, secondary sexual characteristics, and sperm production.

Thymus Gland

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

• Produces thymosins, essential for T-cell development (immunity).

Summary Table: Major Endocrine Glands and Their Hormones

Gland	Hormone(s)	Main Function(s)
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, calcium regulation
Parathyroid	PTH	Calcium homeostasis
Adrenal cortex	Aldosterone, Cortisol, Androgens	Electrolyte balance, stress response, sex traits
Adrenal medulla	Epinephrine, Norepinephrine	Fight-or-flight response
Pineal	Melatonin	Sleep-wake cycles
Pancreas	Insulin, Glucagon	Blood glucose regulation
Ovaries	Estrogen, Progesterone	Female sex traits, reproduction
Testes	Testosterone	Male sex traits, sperm production
Thymus	Thymosins	T-cell development

Additional info: Where the original notes were incomplete or fragmented, standard academic context and definitions were added for clarity and completeness.