<b>Cells</b> (basic unit) → <b>Tissues</b> (groups of similar cells): <b>Epithelial</b> (lining surfaces), <b>Connective</b> (support, collagen), <b>Nervous</b> (neurons + glia), <b>Muscle</b> (movement) → <b>Organs</b> (multiple tissues) → <b>Organ Systems</b> (related organs working together).

<b>Homeostasis</b>: Maintaining relatively constant internal conditions (temperature, pH, glucose, water) despite external changes. Essential for survival.

<b>Negative Feedback</b>: Response blocks further action (e.g., thermostat turns heat off when warm). Body uses this to maintain homeostasis. <b>Positive Feedback</b>: Amplifies response (e.g., oxytocin during labor).

<b>Hypothalamus</b>: Body's thermostat. When cold: speed up cell activity (generates heat), shivering. When hot: sweat (evaporation cools), slow cell activity. <b>Ectotherms</b> (rely on environment) vs. <b>Endotherms</b> (internal heat).

<b>Detoxification</b>: Breaks down toxins (ammonia → urea). <b>Glucose Regulation</b>: Stores excess glucose as glycogen, releases when blood sugar drops. Prevents dangerous glucose fluctuations.

<b>Calories (kcal)</b>: Energy measurement (1 Calorie = 1000 calories). Food provides energy for ATP production + raw materials for growth/repair. Body needs at least 45 substances it can't make.

Most important nutrient. Required for chemical reactions, blood, extracellular fluid. Lost via urine, sweat, breathing. Need ~1L/day minimum. <b>Dehydration</b>: Can be fatal.

<b>Simple</b> (monosaccharides like glucose, disaccharides like sucrose). <b>Complex</b> (polysaccharides: starch, glycogen). Excess stored as glycogen (liver/muscles) or fat. <b>Fiber</b> (cellulose): Can't digest but needed for bulk/movement.

<b>Saturated</b> (all single bonds, solid at room temp). <b>Unsaturated</b> (double bonds, liquid). <b>Trans fats</b> (hydrogenated, linked to heart disease). Functions: cell membranes, hormone production, vitamin absorption, energy storage, insulation.

Made of <b>amino acids</b> (20 types; 8 essential from food). Functions: enzymes, structure (collagen), transport, regulation, sometimes energy. Animal sources provide all essential amino acids.

<b>Fat-soluble vitamins</b> (A, D, E, K): Stored in body fat. <b>Water-soluble</b> (B, C): Can't be stored, needed daily. <b>Minerals</b>: Inorganic nutrients (calcium, iron, sodium, etc.) for various functions.

<b>Path</b>: <b>Mouth</b> (ingestion) → <b>Esophagus</b> (peristalsis) → <b>Stomach</b> (acidic, churning) → <b>Small Intestine</b> (nutrient absorption) → <b>Large Intestine/Colon</b> (water reabsorption) → <b>Rectum</b> → <b>Anus</b> (elimination). <b>Accessory</b>: Salivary glands, liver, gallbladder, pancreas.

<b>Mechanical</b>: Chewing, churning (breaks into smaller pieces). <b>Chemical</b>: Enzymes break bonds. <b>Enzymes</b>: <b>Amylase</b> (saliva/pancreas, breaks starch), <b>Pepsin</b> (stomach, breaks proteins), <b>Bile</b> (liver/gallbladder, emulsifies fats). <b>Absorption</b>: Villi/microvilli in small intestine increase surface area. <b>Peristalsis</b>: Wave-like muscle contractions move food.

Removes metabolic wastes (urea, CO₂), maintains <b>osmoregulation</b> (water/solute balance), regulates blood pH, blood pressure. Organs: skin, lungs, liver, kidneys.

<b>Nephron</b>: Functional unit of kidney. Parts: <b>Glomerulus</b> (capillary cluster), <b>Bowman's Capsule</b> (surrounds glomerulus), <b>Proximal tubule</b>, <b>Loop of Henle</b>, <b>Distal tubule</b>, <b>Collecting Duct</b>.

<b>Filtration</b>: Blood pressure forces water/small molecules from glomerulus into Bowman's capsule. <b>Reabsorption</b>: Useful substances (glucose, water, ions) reclaimed into blood. <b>Secretion</b>: Additional wastes secreted into tubule. Result: concentrated urine.

<b>Kidneys</b> (filter blood, produce urine) → <b>Ureters</b> (transport urine) → <b>Bladder</b> (stores urine) → <b>Urethra</b> (eliminates urine). Also includes: <b>Renal artery</b> (brings blood), <b>Renal vein</b> (removes filtered blood).

<b>Central Nervous System (CNS)</b>: Brain + spinal cord (protected by meninges, cerebrospinal fluid). <b>Peripheral Nervous System (PNS)</b>: Somatic (voluntary) + Autonomic (involuntary: sympathetic + parasympathetic).

<b>Cell body</b> (nucleus, organelles) → <b>Dendrites</b> (receive signals) → <b>Axon</b> (transmits signal, long) → <b>Myelin Sheath</b> (glia cells, insulates, speeds transmission) → <b>Synaptic terminals</b> (release neurotransmitters).

<b>Resting potential</b>: -70mV (inside negative). <b>Depolarization</b>: Na⁺ channels open, inside becomes positive. <b>Repolarization</b>: K⁺ channels open, returns to negative. <b>All-or-none</b>: Either fires completely or not at all.

<b>Synapse</b>: Gap between neurons. <b>Synaptic cleft</b>: Space. Signal crosses via <b>neurotransmitters</b> (chemicals): Acetylcholine (muscle stimulation), Dopamine (reward/movement), Serotonin (mood), GABA (inhibitory), Glutamate (excitatory).

<b>Cerebrum</b>: Largest part, <b>cerebral cortex</b> (gray matter, higher functions), divided into lobes. <b>Cerebellum</b>: Coordination, balance, motor learning. <b>Brainstem</b>: Controls vital functions (breathing, heart rate, swallowing). <b>Limbic system</b>: Emotion, memory (hippocampus, amygdala).

<b>Frontal lobe</b>: Motor cortex, planning, decision-making, language (Broca's area). <b>Parietal lobe</b>: Touch, temperature, spatial awareness. <b>Temporal lobe</b>: Hearing, memory (hippocampus), language (Wernicke's area). <b>Occipital lobe</b>: Visual processing.

Connects brain to peripheral nerves. <b>Gray matter</b> (inside, neuron cell bodies). <b>White matter</b> (outside, myelinated axons). <b>Reflex arc</b>: Sensory neuron → interneuron → motor neuron (bypasses brain for speed).

<b>Sympathetic</b>: 'Fight-or-flight' (accelerates heart, dilates pupils, inhibits digestion, releases epinephrine). <b>Parasympathetic</b>: 'Rest-and-digest' (slows heart, stimulates digestion, constricts pupils). Usually oppose each other.

<b>Sensory neurons</b>: Transmit info from skin, muscles, sensory organs to CNS. <b>Motor neurons</b>: Transmit commands from CNS to muscles. Voluntary control (conscious movement). Usually one synapse.

<b>Hippocampus</b>: Converts short-term to long-term memory. <b>Amygdala</b>: Processes fear/emotions. <b>Synaptic plasticity</b>: Connections strengthen with use (learning). <b>Corpus callosum</b>: Connects brain hemispheres.

<b>Support</b> (framework), <b>Protection</b> (skull protects brain, ribs protect organs), <b>Movement</b> (attachment for muscles), <b>Mineral storage</b> (calcium, phosphorus), <b>Blood cell production</b> (bone marrow).

<b>Compact bone</b>: Dense, outer layer, contains <b>osteons</b> (concentric rings). <b>Spongy bone</b>: Inner, porous, lightweight. <b>Bone marrow</b>: Red (produces blood cells), Yellow (fat storage).

<b>Skull</b> (cranium + facial bones), <b>Hyoid bone</b> (throat), <b>Vertebral column</b> (spine: cervical, thoracic, lumbar, sacrum, coccyx), <b>Ribcage</b> (ribs + sternum). Forms central axis, protects CNS and organs.

<b>Upper</b>: <b>Pectoral girdle</b> (clavicle, scapula), arms (humerus, radius, ulna), hands. <b>Lower</b>: <b>Pelvic girdle</b> (hip bones), legs (femur, tibia, fibula, patella), feet. For movement and manipulation.

<b>Fibrous</b>: Immovable (skull sutures). <b>Cartilaginous</b>: Slightly movable (vertebrae). <b>Synovial</b>: Freely movable, have synovial fluid for lubrication (shoulder, knee, elbow). Types: hinge, ball-and-socket, pivot.

<b>Osteoblasts</b>: Build bone. <b>Osteoclasts</b>: Break down bone. Continuous remodeling. <b>Calcium regulation</b>: Parathyroid hormone (PTH) increases blood Ca²⁺, calcitonin decreases it.

<b>Skeletal</b>: Voluntary, striated, multinucleated, attached to bones. <b>Smooth</b>: Involuntary, non-striated, walls of organs/blood vessels. <b>Cardiac</b>: Involuntary, striated, branched, intercalated discs, only in heart.

<b>Sarcomere</b>: Functional unit. Contains <b>myofilaments</b>: <b>Actin</b> (thin) + <b>Myosin</b> (thick, has heads). <b>Sliding filament theory</b>: Myosin heads pull actin, sarcomere shortens. Requires ATP + Ca²⁺.

Synapse between motor neuron and muscle fiber. Neuron releases <b>acetylcholine</b> → binds to receptors on <b>sarcolemma</b> (muscle membrane) → triggers muscle contraction.

<b>Epidermis</b>: Outer layer, constantly replaced, no blood vessels. <b>Dermis</b>: Inner layer, blood vessels, nerves, glands. <b>Hair</b>, <b>Nails</b>. <b>Sweat glands</b> (cooling), <b>Sebaceous glands</b> (oil).

<b>Protection</b>: Barrier against pathogens, UV, injury. <b>Temperature regulation</b>: Sweat evaporation cools, blood flow adjusts heat loss. <b>Sensation</b>: Touch, pain, temperature receptors. <b>Vitamin D synthesis</b>: UV exposure converts precursors.

Four chambers: <b>Right atrium</b> (receives deoxygenated blood) → <b>Right ventricle</b> (pumps to lungs) → <b>Left atrium</b> (receives oxygenated blood) → <b>Left ventricle</b> (pumps to body). <b>Valves</b>: <b>Tricuspid</b> (right), <b>Bicuspid/Mitral</b> (left), <b>Semilunar</b> (exits).

<b>Systole</b>: Contraction (ventricles pump blood). <b>Diastole</b>: Relaxation (chambers fill). <b>Pacemaker (SA node)</b>: Sets rhythm. <b>Electrocardiogram (ECG)</b>: Records electrical activity.

<b>Arteries</b>: Carry blood away from heart, thick walls, high pressure. <b>Veins</b>: Carry blood to heart, thin walls, low pressure, valves prevent backflow. <b>Capillaries</b>: Tiny, one-cell thick, site of gas/nutrient exchange.

<b>Plasma</b>: Liquid (water, proteins, ions, nutrients). <b>Red blood cells (RBCs)</b>: No nucleus, contain hemoglobin (carries O₂). <b>White blood cells (WBCs)</b>: Immune defense. <b>Platelets</b>: Blood clotting.

<b>Pulmonary</b>: Right ventricle → pulmonary artery → lungs (gas exchange) → pulmonary vein → left atrium. <b>Systemic</b>: Left ventricle → aorta → body tissues → vena cava (superior/inferior) → right atrium. <b>Closed system</b>: Blood stays in vessels.

<b>Systolic</b>: Pressure during ventricular contraction (higher, ~120 mmHg). <b>Diastolic</b>: Pressure during relaxation (lower, ~80 mmHg). Regulated by heart rate, blood volume, vessel diameter.

<b>Nasal cavity</b> (filters, warms air) → <b>Pharynx</b> (throat) → <b>Larynx</b> (voice box) → <b>Trachea</b> (windpipe) → <b>Bronchi</b> (left/right) → <b>Bronchioles</b> (smaller branches) → <b>Alveoli</b> (air sacs, gas exchange).

<b>Alveoli</b>: Tiny sacs with huge surface area, surrounded by capillaries. <b>Diffusion</b>: O₂ moves from alveoli → blood (high to low concentration), CO₂ moves blood → alveoli. Thin walls facilitate exchange.

<b>Inspiration</b>: Diaphragm contracts (flattens), intercostal muscles lift ribs, chest cavity expands, air rushes in. <b>Expiration</b>: Diaphragm relaxes (rises), ribs lower, chest cavity shrinks, air pushed out. Usually passive.

<b>Chemical messengers</b>: Travel in blood, affect distant target cells. <b>Endocrine glands</b>: Secrete into blood (no ducts). <b>Exocrine glands</b>: Secrete through ducts (e.g., sweat, digestive enzymes).

<b>Receptors</b>: <b>Cell membrane</b> (protein hormones, trigger cascade) vs. <b>Intracellular</b> (steroid hormones, enter cell, affect transcription). <b>Up-regulation</b>: More receptors when hormone low. <b>Down-regulation</b>: Fewer receptors when hormone high.

<b>Hypothalamus</b>: Controls hormone release, homeostasis, connects nervous + endocrine. <b>Anterior pituitary</b>: Releases FSH, LH, TSH, ACTH, GH, prolactin. <b>Posterior pituitary</b>: Stores/releases ADH (water retention), oxytocin (labor, bonding). 'Master gland.'

<b>Thyroid</b>: <b>Thyroxine (T4)</b> increases metabolism, growth. Requires iodine. <b>Calcitonin</b> lowers blood Ca²⁺. <b>Parathyroid glands</b>: <b>PTH</b> raises blood Ca²⁺ (stimulates bone breakdown, kidney reabsorption).

<b>Cortex</b> (outer): <b>Cortisol</b> (stress response, raises blood sugar), <b>Aldosterone</b> (Na⁺/K⁺ balance, blood pressure). <b>Medulla</b> (inner): <b>Epinephrine/Norepinephrine</b> (adrenaline, 'fight-or-flight').

<b>Insulin</b>: Lowers blood glucose (promotes uptake, storage as glycogen). <b>Glucagon</b>: Raises blood glucose (breaks down glycogen). <b>Diabetes</b>: Type I (no insulin production), Type II (insulin resistance).

<b>Testosterone</b>: Testes produce (males), stimulates sperm production, secondary sex characteristics (deep voice, body hair, muscle). <b>Estrogen</b>: Ovaries produce (females), menstrual cycle, secondary sex characteristics (breast development, wider hips). <b>Progesterone</b>: Maintains uterine lining.

<b>Testes</b> (produce sperm + testosterone), <b>Scrotum</b> (holds testes outside body, cooler temp), <b>Seminiferous tubules</b> (sperm production), <b>Epididymis</b> (sperm maturation), <b>Vas deferens</b> (sperm transport), <b>Urethra</b> (sperm exit), <b>Penis</b> (copulation). <b>Accessory glands</b>: Seminal vesicles, prostate, bulbourethral.

Occurs in <b>seminiferous tubules</b>. <b>Sertoli cells</b>: Nourish developing sperm. <b>Spermatogonium</b> (diploid) → meiosis I → <b>secondary spermatocyte</b> (haploid) → meiosis II → 4 <b>spermatids</b> → mature to <b>sperm</b> (flagellum, neck with mitochondria, head with DNA, acrosome).

Hypothalamus releases <b>GnRH</b> → anterior pituitary releases <b>FSH</b> (stimulates Sertoli cells, sperm production) + <b>LH</b> (stimulates interstitial cells of Leydig to produce testosterone). <b>Negative feedback</b>: High testosterone inhibits GnRH/FSH/LH. <b>Inhibin</b> (from Sertoli): Inhibits FSH when sperm count high.

<b>Ovaries</b> (produce eggs + hormones), <b>Oviducts/Fallopian tubes</b> (fimbriae capture egg, cilia move it, fertilization site), <b>Uterus</b> (muscular, endometrium lining), <b>Cervix</b> (opening to uterus), <b>Vagina</b> (birth canal, copulation). <b>External</b>: Clitoris, labia majora/minora.

<b>Oogonium</b> → <b>primary oocyte</b> (meiosis I arrested in prophase I before birth). At puberty: <b>FSH</b> triggers development. Meiosis I completes → <b>secondary oocyte</b> (most cytoplasm) + <b>polar body</b> (dies). Arrested at metaphase II. Ovulation releases secondary oocyte. If fertilized, meiosis II completes → egg + 2nd polar body.

<b>GnRH</b> → <b>FSH</b> + <b>LH</b> from pituitary. <b>Estrogen</b>: Follicle growth, endometrium thickening, secondary sex characteristics. <b>Progesterone</b>: Maintains endometrium. Feedback: Low estrogen inhibits GnRH, high estrogen stimulates LH surge.

<b>Follicular phase</b> (days 1-14): FSH stimulates follicle growth, rising estrogen thickens endometrium. <b>Ovulation</b> (day 14): LH surge triggers egg release. <b>Luteal phase</b> (days 15-28): <b>Corpus luteum</b> secretes progesterone + estrogen, maintains endometrium. If no fertilization: corpus luteum degenerates, hormone drop, menstruation (shedding).

<b>Fertilization</b>: Sperm + egg fuse (usually in oviduct). <b>Zygote</b> → <b>cleavage</b> (rapid mitosis) → <b>blastula</b>. In mammals: <b>blastocyst</b> (inner cell mass + trophoblast). <b>Gastrulation</b>: Forms 3 <b>germ layers</b>: <b>Ectoderm</b> (skin, nervous), <b>Mesoderm</b> (muscle, connective, circulatory), <b>Endoderm</b> (digestive, respiratory lining).

<b>Trimesters</b>: 1st (organ formation), 2nd (growth, movement), 3rd (rapid growth). <b>Placenta</b>: Nutrient/waste exchange, produces hCG (maintains corpus luteum), estrogen, progesterone. <b>Labor</b>: <b>Oxytocin</b> (posterior pituitary) causes uterine contractions. 3 stages: cervix dilation, baby delivery, placenta expulsion. <b>Lactation</b>: Prolactin (milk production), oxytocin (milk release).

<b>First line</b>: Physical barriers (skin, mucus, stomach acid). <b>Second line</b>: <b>Inflammatory response</b> (redness, swelling, heat, pain from histamine + increased blood flow), <b>Fever</b> (inhibits pathogens), <b>Phagocytes</b> (WBCs engulf pathogens). Non-specific, immediate.

Specific to pathogen, has <b>memory</b> (faster response on re-exposure). <b>B cells</b>: Antibody-mediated (humoral). <b>T cells</b>: Cell-mediated. Takes days to develop but long-lasting protection.

Y-shaped proteins produced by B cells. <b>Antigen-binding sites</b>: Recognize specific antigens (pathogens). <b>Classes</b>: <b>IgG</b> (most common, crosses placenta), <b>IgM</b> (first response), <b>IgA</b> (secretions: saliva, tears), <b>IgE</b> (allergies, parasites), <b>IgD</b> (B cell activation).

Antigen binds to B cell → <b>clonal selection</b> (rapid division). Differentiate into: <b>Plasma cells</b> (produce antibodies, short-lived) + <b>Memory B cells</b> (long-lived, rapid response on re-exposure).

<b>Helper T cells (CD4+)</b>: Activate B cells + cytotoxic T cells, release cytokines. <b>Cytotoxic T cells (CD8+)</b>: Kill infected cells. Require <b>MHC presentation</b>: Cells display antigens on MHC molecules for T cell recognition. Mature in thymus.

<b>Lymph</b>: Fluid similar to plasma. <b>Lymph vessels</b>: Return fluid to circulatory system, one-way flow. <b>Lymph nodes</b>: Filter lymph, house lymphocytes. <b>Spleen</b>: Filters blood, removes old RBCs. <b>Thymus</b>: T cell maturation.

<b>Allergies</b>: Overreaction to harmless antigens (IgE, histamine release). <b>Autoimmune diseases</b>: Immune system attacks own cells (Type I diabetes, lupus, MS). <b>Immunodeficiency</b>: Weakened immune system (HIV/AIDS attacks helper T cells).