Introduction to Anatomy & 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; 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).

• Stress: Anything that disturbs or alters the balance of the internal environment.

• Homeostatic Control Mechanisms: Self-regulating mechanisms that maintain the homeostatic steady state. These mechanisms involve at least three components: a receptor (sensor), a control center, and an 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., body temperature regulation, blood glucose control).

• Positive Feedback: The response enhances the original stimulus so that the response is accelerated. Rare in the body; examples include blood clotting, labor contractions, and orgasm.

Regulatory Systems

• The body uses neural (nervous system) and endocrine (hormonal) control systems to maintain homeostasis.

• Nervous System: Uses electrical impulses delivered by neurons to specific target cells; fast-acting, typically results in 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

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

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

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

Hormones

• Hormone: A secretion transported in the blood (or lymph) that alters the physiological activity of target cells; a chemical messenger produced by one type of cell that affects the metabolic activity of another type of cell.

• Two main classes of hormones:

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

  • Steroid Hormones: Derived from cholesterol; lipid-soluble and can cross the plasma membrane. Includes gonadal and some adrenal hormones.

Mechanism of Hormonal Action

• Hormones circulate in the bloodstream and affect only specific target tissues that have receptors for that hormone ("lock and key" analogy).

• Receptors are large proteins, mostly found on the cell surface for water-soluble hormones, or inside the cell for lipid-soluble hormones.

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

Hormone Effects

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

• Stimulates synthesis of enzymes and other proteins within the cell.

• Activates or deactivates enzymes.

• Induces secretory activity.

• Stimulates mitosis.

Action of Hormones

• Water-Soluble Hormones (all amino acid–based hormones except thyroid hormone): Act on plasma membrane receptors via G protein second messengers.

• Lipid-Soluble Hormones (steroid and thyroid hormones): Act on intracellular receptors that directly activate genes.

Second-Messenger Systems

• Most amino acid-based hormones exert effects through second-messenger systems (e.g., cAMP, PIP2-calcium).

Direct Gene Activation

• Lipid-soluble hormones diffuse into target cells, bind with intracellular receptors, and the receptor-hormone complex enters the nucleus to bind to specific DNA regions, initiating transcription and protein synthesis.

Endocrine Gland Stimuli

• Endocrine glands are stimulated to synthesize and release hormones in response to:

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

  • Neural Stimuli: Nerve fibers stimulate hormone release (e.g., sympathetic nervous system stimulates adrenal medulla to secrete catecholamines).

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

Nervous System Modulation

• The nervous system can override or adjust hormone levels as needed (e.g., during stress, the hypothalamus and sympathetic nervous system override insulin to increase blood glucose for "fight or flight").

Hormone Activity: Half-Life, Onset, and Duration

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

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

• Hormones are removed from blood by degrading enzymes, kidneys, or liver.

• Onset and duration of hormone activity vary; some responses are immediate, others take hours to days, and some hormones are inactive until they enter target cells.

Interaction of Hormones at Target Cells

• Permissiveness: One hormone cannot exert its full effects without another hormone being 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).

Endocrine Glands and Their Hormones

The Hypothalamus and Pituitary Gland

• The hypothalamus is connected to the pituitary gland (hypophysis) via the infundibulum.

• The pituitary gland has two major lobes:

  • Posterior Pituitary (Neurohypophysis): Composed of neural tissue; stores and releases neurohormones (oxytocin and antidiuretic hormone/ADH) produced by the hypothalamus.

  • Anterior Pituitary (Adenohypophysis): Composed of glandular tissue; connected to the hypothalamus via the hypophyseal portal system; secretes hormones in response to hypothalamic releasing and inhibiting hormones.

Posterior Pituitary Hormones

• Oxytocin: Stimulates uterine contractions during childbirth and milk ejection during breastfeeding; regulated by positive feedback mechanisms.

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

• Homeostatic Imbalances:

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

  • Syndrome of Inappropriate ADH Secretion (SIADH): Excessive ADH; causes fluid retention, headache, and disorientation.

Anterior Pituitary Hormones

• All are peptide hormones; all but two are tropic hormones (regulate secretion of other hormones).

• Growth Hormone (GH): Stimulates growth of bones and skeletal muscle; regulated by growth hormone–releasing hormone (GHRH) and growth hormone–inhibiting hormone (GHIH/somatostatin).

• Thyroid-Stimulating Hormone (TSH): Stimulates normal development and secretory activity of the thyroid gland; regulated by thyrotropin-releasing hormone (TRH) 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):

  • Follicle-Stimulating Hormone (FSH): Stimulates production of gametes (egg or sperm).

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

  • Regulated by gonadotropin-releasing hormone (GnRH) and negative feedback from gonadal hormones.

• Prolactin (PRL): Stimulates milk production in females; regulation primarily by prolactin-inhibiting hormone (PIH, dopamine); increased estrogen stimulates PRL.

Homeostatic Imbalances of GH

• Hypersecretion (usually due to pituitary tumor):

  • In children: Gigantism (abnormally tall stature).

  • In adults: Acromegaly (overgrowth of hands, feet, and face).

• Hyposecretion:

  • In children: Pituitary dwarfism (short stature).

  • In adults: Usually causes no problems.

Thyroid and Parathyroid Glands

Thyroid Gland

• Located in the anterior neck, just inferior to the larynx; consists of follicles (spheres of epithelial cells) that produce thyroglobulin, and parafollicular cells that produce calcitonin.

• Thyroid Hormone (TH): Exists as T4 (thyroxine) and T3 (triiodothyronine); affects virtually every cell in the body by regulating metabolism, growth, and development.

• TH is stored extracellularly in the follicle lumen and released upon stimulation by TSH.

Homeostatic Imbalances of TH

• Hyposecretion in adults: Myxedema; if due to iodine deficiency, a goiter may develop.

• Hypersecretion: Most commonly Graves’ disease (autoimmune disorder); symptoms include elevated metabolic rate, sweating, rapid heartbeat, and exophthalmos (bulging eyes).

Calcitonin

• Produced by parafollicular (C) cells in response to high blood Ca2+ levels; antagonistic to parathyroid hormone (PTH).

• At high doses, lowers blood calcium by inhibiting osteoclast activity and stimulating Ca2+ uptake into bone.

Parathyroid Glands

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

• PTH is the most important hormone in Ca2+ homeostasis; secreted in response to low blood Ca2+ and inhibited by high Ca2+.

• Target organs: skeleton, kidneys, and intestine.

• PTH increases blood Ca2+ by stimulating osteoclasts, enhancing reabsorption of Ca2+ by kidneys, and promoting activation of vitamin D (increases intestinal absorption of Ca2+).

Adrenal Glands

Structure and Function

• Paired, pyramid-shaped organs atop the kidneys; each consists of an outer cortex and inner medulla.

• Adrenal Cortex: Three layers of glandular tissue that produce corticosteroids.

• Adrenal Medulla: Nervous tissue; part of the sympathetic nervous system; produces catecholamines (epinephrine and norepinephrine).

Adrenal Cortex Hormones

Adrenal Medulla Hormones

• Chromaffin cells synthesize catecholamines: Epinephrine (80%) and Norepinephrine (20%).

• Effects: Increase heart rate, blood glucose, and blood flow to brain, heart, and skeletal muscle; regulated by the sympathetic nervous system.

Pineal Gland

• Small gland hanging from the roof of the third ventricle in the brain.

• Pinealocytes secrete melatonin, derived from serotonin.

• Melatonin helps regulate sleep-wake cycles, body temperature, and appetite; may also have antioxidant functions.

Pancreas

• Located partially behind the stomach; has both exocrine and endocrine functions.

• Acinar cells (exocrine): Produce enzyme-rich juice for digestion.

• Pancreatic islets (islets of Langerhans): Contain endocrine cells:

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

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

Glucagon

• Potent hyperglycemic agent; targets the liver to break down glycogen (glycogenolysis), synthesize glucose from noncarbohydrates (gluconeogenesis), and release glucose into the blood.

Insulin

• Lowers blood glucose by enhancing membrane transport of glucose into cells, inhibiting glycogen breakdown, and inhibiting conversion of amino acids or fats to glucose.

Homeostatic Imbalance: Diabetes Mellitus (DM)

• Can be due to hyposecretion (Type 1) or hypoactivity (Type 2) of insulin.

• High blood glucose leads to glycosuria (glucose in urine), polyuria (excessive urination), polydipsia (excessive thirst), and polyphagia (excessive hunger).

• Fat metabolism leads to ketone body formation, which can cause ketoacidosis (acidic blood), potentially leading to coma or death if untreated.

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, sperm production, and maintenance of reproductive organs.

Thymus Gland

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

• Essential for normal production and programming of T-cells (a type of white blood cell involved in immunity).

Summary Table: Major Endocrine Glands and Their Hormones

Additional info: Some details, such as the names of specific hormones, mechanisms, and clinical conditions, were inferred or expanded for completeness and clarity based on standard Anatomy & Physiology textbooks.