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Anatomy and Physiology of the 11 Human Body Systems

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1. Integumentary System

Anatomy

• Layers: Epidermis (outer, avascular, 4-5 sublayers: stratum basale, spinosum, granulosum, lucidum, corneum), Dermis (dense irregular connective tissue with blood vessels, nerves, hair follicles, sweat glands), Hypodermis/subcutis (adipose and loose connective tissue)
• Accessory structures: Hair follicles, sebaceous glands, eccrine and apocrine sweat glands, nails, sensory receptors (Meissner's, Pacinian, Merkel, Ruffini corpuscles)

Physiology

• Protection: Physical and chemical barrier against the external environment
• Thermoregulation: Sweat evaporation cools the body; cutaneous vasoconstriction/dilation adjusts heat loss
• Sensation: Contains receptors for pain, touch, pressure, vibration, and temperature
• Vitamin D synthesis: UV exposure converts 7-dehydrocholesterol to cholecalciferol (Vitamin D3) in the epidermis
• Immunity: Langerhans cells (dendritic cells) in the epidermis are antigen-presenting cells
• Water balance: Prevents excessive transepidermal water loss

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2. Skeletal System

Anatomy

• 206 bones in the adult skeleton, divided into axial skeleton (80 bones: skull, vertebral column, rib cage, sternum) and appendicular skeleton (126 bones: pectoral girdles, upper limbs, pelvic girdle, lower limbs)
• Bone tissue types: Compact (cortical) bone - dense, outer layer; Spongy (cancellous/trabecular) bone - inner honeycomb lattice
• Bone cells: Osteoblasts (bone formation), osteocytes (bone maintenance), osteoclasts (bone resorption), osteogenic/periosteal cells
• Joints: Fibrous (sutures, syndesmoses), cartilaginous (synchondroses, symphyses), synovial (freely movable - ball and socket, hinge, pivot, condyloid, saddle, gliding)
• Cartilage: Hyaline, fibrocartilage, elastic

Physiology

• Support and framework: Provides rigid structure maintaining body shape
• Protection: Skull protects brain; vertebral column protects spinal cord; rib cage protects heart and lungs
• Movement: Serves as levers for muscle action at joints
• Hematopoiesis: Red bone marrow (in flat bones and epiphyses of long bones) produces red blood cells, white blood cells, and platelets
• Mineral storage: Stores and releases calcium (99% of body calcium) and phosphate; regulated by PTH (parathyroid hormone), calcitonin, and calcitriol
• Fat storage: Yellow marrow stores lipids as energy reserve
• Bone remodeling: Continuous process balanced between osteoblast deposition and osteoclast resorption, regulated by mechanical stress (Wolff's Law)

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3. Muscular System

Anatomy

• Three muscle types:
  • Skeletal muscle: Striated, voluntary; arranged in fiber bundles (fascicles) wrapped by perimysium, surrounded by epimysium
  • Cardiac muscle: Striated, involuntary; cells (cardiomyocytes) connected by intercalated discs with gap junctions
  • Smooth muscle: Non-striated, involuntary; found in walls of hollow organs, blood vessels, airways
• Gross anatomy: ~600 named skeletal muscles with origins (fixed attachment) and insertions (moving attachment)
• Connective tissue: Endomysium (individual fibers), perimysium (fascicles), epimysium (whole muscle), tendons (muscle to bone), aponeuroses (flat tendons)
• Microstructure: Sarcomere is the functional unit - composed of thick (myosin) and thin (actin) filaments, bounded by Z-lines

Physiology

• Sliding filament mechanism: Contraction occurs when myosin heads bind actin, form cross-bridges, and pull actin toward the center of the sarcomere (power stroke), requiring ATP and Ca²⁺
• Neuromuscular junction: Motor neuron releases acetylcholine (ACh) into the synaptic cleft → ACh binds nicotinic receptors on muscle → action potential → SR releases Ca²⁺ → troponin-tropomyosin shift exposes actin binding sites
• Muscle fiber types: Type I (slow-twitch, aerobic, fatigue-resistant), Type IIa (fast-twitch, oxidative-glycolytic), Type IIx (fast-twitch, glycolytic, fatigable)
• Energy sources: Creatine phosphate (immediate), anaerobic glycolysis (short-term), aerobic oxidative phosphorylation (long-term)
• Cardiac autorhythmicity: SA node pacemaker initiates each beat; intercalated disc gap junctions allow electrical syncytium
• Smooth muscle: Regulated by autonomic nerves, hormones, and local factors; sustained tonic contractions possible due to latch-bridge mechanism

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4. Nervous System

Anatomy

• Central Nervous System (CNS): Brain (cerebrum, cerebellum, brainstem) + Spinal cord
  • Cerebrum: Two hemispheres with 4 lobes each (frontal, parietal, temporal, occipital); gray matter (cortex) + white matter
  • Cerebellum: Coordinates movement, balance, proprioception
  • Brainstem: Midbrain, pons, medulla oblongata - controls vital autonomic functions
  • Spinal cord: 31 segments; butterfly-shaped gray matter (dorsal sensory, ventral motor horns) surrounded by white matter tracts
• Peripheral Nervous System (PNS): 12 pairs cranial nerves + 31 pairs spinal nerves
  • Somatic: Voluntary sensory and motor
  • Autonomic: Sympathetic (thoracolumbar), Parasympathetic (craniosacral), Enteric (GI tract)
• Cells: Neurons (cell body/soma, axon, dendrites) and neuroglia (astrocytes, oligodendrocytes, microglia, ependymal cells in CNS; Schwann cells, satellite cells in PNS)
• Meninges: Dura mater, arachnoid mater, pia mater
• Blood-Brain Barrier (BBB): Tight junctions between cerebral endothelial cells create a highly regulated environment for CNS neurons - Medical Physiology

Physiology

• Resting membrane potential: ~-70 mV in neurons; maintained by Na⁺/K⁺ ATPase and differential permeability
• Action potential: Depolarization (Na⁺ influx) → repolarization (K⁺ efflux) → hyperpolarization; all-or-none, propagated along axon
• Synaptic transmission: Neurotransmitters (glutamate, GABA, dopamine, serotonin, acetylcholine, norepinephrine) released into synaptic cleft, bind postsynaptic receptors
• Reflex arcs: Sensory receptor → afferent neuron → interneuron → efferent neuron → effector
• Autonomic regulation: Sympathetic ("fight or flight") - epinephrine/norepinephrine; Parasympathetic ("rest and digest") - acetylcholine; Enteric - regulates GI motility
• Higher functions: Cerebral cortex mediates consciousness, cognition, language, emotion, planning, and voluntary movement

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5. Endocrine System

Anatomy

• Major glands: Hypothalamus, pituitary gland (anterior + posterior), thyroid, parathyroid (4 glands), adrenal glands (cortex + medulla), pancreas (islets of Langerhans), gonads (ovaries/testes), pineal gland, thymus
• Diffuse endocrine cells: GI tract (enteroendocrine cells), kidney (juxtaglomerular cells), heart (atrial natriuretic cells), adipose tissue (adipokine-secreting cells)
• Hypothalamo-pituitary axis: Hypothalamic nuclei secrete releasing/inhibiting hormones into portal circulation → anterior pituitary responds by secreting tropic hormones

Physiology

• Hormone classes: Peptides/proteins (insulin, GH), steroids (cortisol, estrogen, testosterone), amines (thyroid hormones, catecholamines)
• Mechanisms of action: Lipid-soluble hormones (steroids, thyroid) - cross membrane, bind nuclear receptors, alter gene expression; Water-soluble hormones (peptides, catecholamines) - bind cell surface receptors, activate second messengers (cAMP, IP3, DAG)
• Feedback loops: Negative feedback predominates (e.g., high cortisol suppresses CRH and ACTH)
• Key axes: HPA (stress), HPT (metabolism), HPG (reproduction), GH-IGF-1 (growth)
• Thyroid hormones: Act synergistically with growth hormone; increase BMR, heat production, and oxygen consumption; alter cardiovascular and respiratory systems - Costanzo Physiology
• Pancreatic regulation: Insulin (beta cells) - lowers blood glucose; Glucagon (alpha cells) - raises blood glucose
• Adrenal cortex: Mineralocorticoids (aldosterone - Na⁺ retention), Glucocorticoids (cortisol - stress, metabolism, immunity), Androgens (DHEA)
• Adrenal medulla: Epinephrine and norepinephrine - sympathetic stress response

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6. Cardiovascular System

Anatomy

• Heart: 4 chambers (right atrium, right ventricle, left atrium, left ventricle); 4 valves (tricuspid, pulmonary, mitral/bicuspid, aortic); layers: endocardium, myocardium, pericardium
• Conduction system: SA node (pacemaker, 60-100 bpm) → AV node → Bundle of His → Left/Right bundle branches → Purkinje fibers
• Blood vessels:
  • Arteries: Thick-walled, muscular/elastic, carry blood away from heart
  • Arterioles: Primary resistance vessels; regulate blood flow distribution
  • Capillaries: Single-cell-thick walls; site of gas/nutrient exchange
  • Venules and veins: Low-pressure, thin-walled, capacitance vessels; veins have valves
• Circulations: Pulmonary (right heart → lungs → left heart) and Systemic (left heart → body → right heart)
• Coronary arteries: RCA (right coronary), LAD (left anterior descending), LCx (left circumflex) supply myocardium

Physiology

• Cardiac cycle: Systole (ventricular contraction, ejection) and diastole (ventricular relaxation, filling); duration ~0.8 sec at rest
• Cardiac output (CO): CO = Heart Rate × Stroke Volume; normal ~5 L/min at rest
• Regulation: Frank-Starling law (increased preload → increased stroke volume); Contractility (inotropy); Afterload (resistance against which heart pumps)
• Blood pressure: Systolic (~120 mmHg) / Diastolic (~80 mmHg); Mean arterial pressure = diastolic + 1/3 pulse pressure
• Baroreceptor reflex: Carotid sinus and aortic arch baroreceptors detect BP changes → medullary cardiovascular center adjusts HR and vascular tone
• Capillary exchange: Starling forces govern fluid movement - net filtration pressure = (capillary oncotic P + interstitial hydrostatic P) - (capillary hydrostatic P + interstitial oncotic P)
• Hemostasis: Vascular spasm → platelet plug (primary hemostasis) → coagulation cascade (secondary hemostasis) → clot dissolution (fibrinolysis)

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7. Respiratory System

Anatomy

• Upper airway: Nasal cavity (turbinates, mucosal lining), nasopharynx, oropharynx, larynx (epiglottis, vocal cords, thyroid/cricoid cartilages)
• Lower airway: Trachea (C-shaped cartilage rings) → Primary bronchi → Secondary (lobar) bronchi → Tertiary (segmental) bronchi → Bronchioles → Terminal bronchioles → Respiratory bronchioles → Alveolar ducts → Alveoli
• Lungs: Right (3 lobes: upper, middle, lower) and Left (2 lobes: upper, lower with lingula); covered by visceral pleura; chest wall has parietal pleura
• Alveoli: ~300-500 million; lined by Type I pneumocytes (gas exchange, ~95% surface), Type II pneumocytes (surfactant production, ~5%), alveolar macrophages
• Respiratory muscles: Diaphragm (primary), external intercostals (inspiration); internal intercostals, abdominal muscles (forced expiration)

Physiology

• Mechanics of breathing: Inspiration - diaphragm contracts, thoracic volume increases, intrapleural pressure becomes more negative (-8 to -10 cmH₂O), alveolar pressure drops below atmospheric, air flows in; Expiration - passive elastic recoil
• Lung volumes: Tidal volume (TV, ~500 mL), Inspiratory Reserve Volume (IRV, ~3100 mL), Expiratory Reserve Volume (ERV, ~1200 mL), Residual Volume (RV, ~1200 mL); Total Lung Capacity = TV + IRV + ERV + RV (~6 L)
• Surfactant: Dipalmitoylphosphatidylcholine (DPPC) from Type II pneumocytes reduces alveolar surface tension, prevents collapse
• Gas exchange: Diffusion driven by partial pressure gradients: O₂ (alveoli P₀₂ ~100 mmHg → blood ~40 mmHg); CO₂ (blood ~45 mmHg → alveoli ~40 mmHg)
• Oxygen transport: 98.5% bound to hemoglobin (cooperative binding, sigmoid O₂-Hb dissociation curve); 1.5% dissolved; Bohr effect - acidosis/hypercapnia right-shifts curve (lower affinity, more O₂ release to tissues)
• CO₂ transport: 70% as bicarbonate (HCO₃⁻); 23% bound to hemoglobin (carbaminohemoglobin); 7% dissolved
• Ventilation-perfusion (V/Q) matching: Normal ratio ~0.8; mismatching is the most common cause of hypoxemia
• Control of breathing: Medullary respiratory center (pre-Bötzinger complex); central chemoreceptors (medulla, respond to CO₂/pH); peripheral chemoreceptors (carotid and aortic bodies, respond to O₂, CO₂, pH)

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8. Digestive System

Anatomy

• GI tract (mouth to anus, ~9 meters): Oral cavity (teeth, tongue, salivary glands) → Pharynx → Esophagus → Stomach → Small intestine (duodenum, jejunum, ileum) → Large intestine (cecum, ascending/transverse/descending/sigmoid colon, rectum) → Anal canal
• Wall structure (4 layers): Mucosa (epithelium, lamina propria, muscularis mucosae), Submucosa (Meissner's plexus), Muscularis externa (inner circular + outer longitudinal smooth muscle, Auerbach's/myenteric plexus), Serosa/adventitia
• Accessory organs: Liver (largest gland, 4 lobes, hepatic lobules), gallbladder, pancreas (exocrine acini + endocrine islets), salivary glands (parotid, submandibular, sublingual)
• Surface area enhancement: Plicae circulares (folds), villi, microvilli (brush border) multiply small intestinal surface area ~200-fold

Physiology

• Digestion: Mechanical (chewing, churning, peristalsis) and chemical (enzymatic hydrolysis)
• Salivary amylase: Begins starch digestion in mouth
• Stomach: HCl (parietal cells, activated by gastrin, ACh, histamine) maintains pH 1-2; pepsinogen → pepsin breaks proteins; intrinsic factor (vitamin B12 absorption)
• Pancreatic enzymes: Amylase (starch), lipase (fat), proteases (trypsinogen, chymotrypsinogen, elastase) - neutralized by bicarbonate; released by CCK and secretin
• Bile: Produced in liver, stored in gallbladder, released by CCK; emulsifies fat for lipase action; contains bile salts (for micelle formation), cholesterol, phospholipids, bilirubin
• Absorption: Carbohydrates (monosaccharides via SGLT1/GLUT5), proteins (amino acids via Na⁺-dependent cotransport), fats (micelles → free fatty acids + monoglycerides → re-esterified into triglycerides → chylomicrons → lacteals), vitamins and minerals
• Motility: Peristalsis (propulsion), segmentation (mixing); regulated by ENS, vagal, hormonal, and local inputs
• Liver functions: Metabolism of carbohydrates, lipids, proteins; detoxification; bile production; clotting factor synthesis; glycogen storage; albumin synthesis

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9. Urinary System

Anatomy

• Kidneys: Paired, retroperitoneal; outer cortex (glomeruli, proximal/distal tubules), inner medulla (loops of Henle, collecting ducts, renal pyramids), renal pelvis
• Nephron (functional unit, ~1 million per kidney): Glomerulus (fenestrated capillaries + Bowman's capsule) → Proximal convoluted tubule (PCT) → Loop of Henle (descending thin, ascending thin, ascending thick limbs) → Distal convoluted tubule (DCT) → Collecting duct
• Blood supply: Renal artery → interlobar → arcuate → interlobular arteries → afferent arterioles → glomerular capillaries → efferent arterioles → peritubular capillaries/vasa recta
• Juxtaglomerular apparatus: JG cells (renin-producing), macula densa (Na⁺-sensing), extraglomerular mesangial cells
• Urinary passages: Ureters (peristaltic, urothelium-lined) → Bladder (detrusor muscle, trigone, urothelium, capacity ~500 mL) → Urethra

Physiology

• Filtration (GFR ~125 mL/min, ~180 L/day): Driven by glomerular hydrostatic pressure; filtered fluid = plasma minus large proteins
• Tubular reabsorption: PCT reabsorbs ~67% of filtered Na⁺, water, glucose (entirely), amino acids, HCO₃⁻, phosphate; Loop of Henle - NKCC2 in thick ascending limb reabsorbs NaCl without water → creates medullary osmotic gradient (300-1200 mOsm/kg); DCT - fine-tuning Na⁺/K⁺/H⁺ balance
• Tubular secretion: K⁺, H⁺, organic acids/bases, drugs
• Concentration/dilution: ADH (vasopressin) from posterior pituitary inserts aquaporin-2 channels in collecting duct → water reabsorption; no ADH → dilute urine
• RAAS: Renin (JG cells) → Angiotensin I → ACE → Angiotensin II → aldosterone (adrenal cortex) → Na⁺ retention, K⁺ excretion, BP elevation
• Acid-base balance: Kidneys regulate HCO₃⁻ reabsorption and H⁺ excretion; compensate respiratory acid-base disorders over days
• Erythropoietin (EPO): Kidney peritubular cells secrete EPO in response to hypoxia → stimulates RBC production in bone marrow
• Micturition: Detrusor contraction (parasympathetic, S2-S4) + internal urethral sphincter relaxation (inhibition of sympathetic); external sphincter voluntary (somatic)

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10. Reproductive System

Anatomy

• Testes: In scrotum (3-5°C below body temperature for spermatogenesis); seminiferous tubules (Sertoli cells support spermatogenesis, Leydig cells produce testosterone), rete testis
• Ductal system: Epididymis (sperm maturation, ~20 days) → Vas deferens → Ejaculatory duct → Urethra
• Accessory glands: Seminal vesicles (60% of semen, fructose, prostaglandins), Prostate (20-30%, citric acid, PSA, zinc), Bulbourethral glands (pre-ejaculatory alkaline fluid)
• Penis: Erectile tissue (corpora cavernosa x2 + corpus spongiosum surrounding urethra), glans, prepuce

• Ovaries: Cortex (follicles at various stages), medulla; produce ova and sex hormones
• Uterine tubes (Fallopian): Fimbriae (catch ovum) → infundibulum → ampulla (fertilization site) → isthmus → cornua
• Uterus: Perimetrium, myometrium (thick smooth muscle), endometrium (stratum functionalis + basalis); cervix opens into vagina
• Vagina: Fibromuscular canal; stratified squamous epithelium
• External genitalia (vulva): Labia majora, labia minora, clitoris, vestibule, Bartholin's glands

Physiology

• Spermatogenesis: Spermatogonia (mitosis) → Primary spermatocytes (meiosis I) → Secondary spermatocytes (meiosis II) → Spermatids → Spermatozoa; ~74 days; FSH (tubular function) + LH (testosterone production) regulate
• Testosterone: Virilization, spermatogenesis, libido, anabolic effects; negative feedback on HPG axis
• Erection: Parasympathetic → NO release → cGMP → smooth muscle relaxation → arterial dilation → corpus cavernosum engorgement

• Oogenesis: Begins in fetal life; meiosis arrested at prophase I until ovulation (completes meiosis I) and again at metaphase II until fertilization
• Menstrual cycle (28 days): Follicular phase (days 1-14, FSH drives follicle growth, rising estrogen) → LH surge (day 14) → Ovulation → Luteal phase (days 14-28, corpus luteum produces progesterone + estrogen) → if no fertilization, corpus luteum degenerates, menstruation
• Pregnancy: hCG maintains corpus luteum; placenta takes over progesterone/estrogen production at ~10 weeks; HPL for fetal nutrition

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11. Lymphatic / Immune System

Anatomy

• Primary lymphoid organs: Thymus (T-cell maturation, involutes after puberty), Bone marrow (B-cell and all blood cell development)
• Secondary lymphoid organs: Lymph nodes (bean-shaped, distributed along lymphatic vessels; cortex - B-cells in follicles, paracortex - T-cells, medulla - plasma cells and macrophages), Spleen (white pulp - immune function; red pulp - RBC filtration and storage), Tonsils and adenoids (Waldeyer's ring), Peyer's patches (gut-associated lymphoid tissue, GALT), Appendix
• Lymphatic vessels: Lymph capillaries (interstitium) → collecting lymphatics → lymph trunks → thoracic duct (drains body below diaphragm + left upper body into left subclavian vein) + right lymphatic duct
• Lymph: Filtered interstitial fluid + chylomicrons from gut (chyle)
• Blood cells: Erythrocytes (RBCs, biconcave, ~4.5-5.5 million/µL), Leukocytes (WBCs - neutrophils, eosinophils, basophils, monocytes, lymphocytes), Thrombocytes (platelets)

Physiology

• Lymph drainage: Returns ~2-4 L of interstitial fluid/day to circulation, preventing edema; driven by lymphatic smooth muscle contractions and skeletal muscle pump
• Innate immunity (non-specific, immediate):
  • Physical barriers (skin, mucus, cilia)
  • Phagocytes (neutrophils, macrophages) engulf pathogens
  • Natural killer cells destroy virus-infected and tumor cells
  • Complement system (classical, lectin, alternative pathways) → opsonization, membrane attack complex
  • Pattern recognition receptors (Toll-like receptors) recognize PAMPs (pathogen-associated molecular patterns)
  • Inflammation: Vasodilation, increased permeability, leukocyte recruitment (chemotaxis) driven by IL-1, TNF, IL-6, prostaglandins, histamine
• Adaptive immunity (specific, delayed, with memory):
  • Humoral (B-cell mediated): Antigen presentation → B-cell activation + T-helper cell co-stimulation → plasma cells → antibodies (IgG, IgM, IgA, IgE, IgD); memory B-cells
  • Cell-mediated (T-cell mediated): CD4⁺ T-helper cells (coordinate immune response via cytokines); CD8⁺ cytotoxic T-cells (kill virus-infected/cancer cells via perforin/granzyme); T-regulatory cells (suppress autoimmunity); memory T-cells
  • MHC presentation: MHC class I (on all nucleated cells → presents intracellular antigens to CD8⁺); MHC class II (on APCs → presents extracellular antigens to CD4⁺)
  • Clonal selection and expansion: Selected lymphocytes proliferate; produce effector cells and long-lived memory cells (basis of immunological memory and vaccination)

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Summary Table

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Disease Examples for the 11 Human Body Systems

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1. Integumentary System

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2. Skeletal System

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3. Muscular System

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4. Nervous System

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5. Endocrine System

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6. Cardiovascular System

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7. Respiratory System

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8. Digestive System

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9. Urinary System

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10. Reproductive System

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11. Lymphatic / Immune System

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Master Summary Table

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Part 1: DM Type 1 vs DM Type 2 - Told as a Story

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The Town of Sugarville and Two Very Different Problems

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🔑 Story 1: Type 1 Diabetes - "The Key Factory Burns Down"

"In type 1 diabetes, a complex interplay of genetic, environmental, and autoimmune factors selectively targets insulin-producing pancreatic islet β cells and ultimately destroys them... β-cell destruction is largely mediated by CD8+ and CD4+ T cells, macrophages - causing apoptosis or cellular destruction." - Goldman-Cecil Medicine

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🏚️ Story 2: Type 2 Diabetes - "The Locks Get Rusty"

"Type 2 diabetes: a combination of insulin resistance and defective insulin secretion." - Henry's Clinical Diagnosis

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Head-to-Head Comparison Table

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Part 2: The Journey of Blood Around the Body - Oxygenated and Deoxygenated Exchange

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The Two Loops

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🔵 Loop 1: The Pulmonary Circulation (Right Heart → Lungs → Left Heart)

Body tissues
    ↓  (used up O₂, full of CO₂)
Superior Vena Cava  +  Inferior Vena Cava
    ↓
Right Atrium  →  [Tricuspid Valve]  →  Right Ventricle
    ↓  [Pulmonic Valve]
Pulmonary Trunk  →  Right + Left Pulmonary ARTERIES
    ↓  (the ONLY arteries carrying deoxygenated blood!)
Pulmonary Capillaries surrounding Alveoli
    ↓  GAS EXCHANGE HAPPENS HERE:
       O₂ diffuses IN (alveolar PO₂ ~100 mmHg → blood ~40 mmHg)
       CO₂ diffuses OUT (blood PCO₂ ~45 mmHg → alveoli ~40 mmHg)
    ↓  (now fresh, oxygenated, bright red blood)
Pulmonary VEINS (the ONLY veins carrying oxygenated blood!)
    ↓
Left Atrium

"Deoxygenated blood from the systemic circulation flows from the right atrium into the right ventricle and from there into the pulmonary circulation... the blood is enriched with oxygen and carbon dioxide is released into the airways. The oxygenated blood leaves the lungs by the pulmonary veins and flows to the left atrium." - Color Atlas of Human Anatomy Vol. 2

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🔴 Loop 2: The Systemic Circulation (Left Heart → Entire Body → Right Heart)

Left Atrium  →  [Mitral Valve]  →  Left Ventricle
    ↓  [Aortic Valve]
AORTA  (pumped at 120 mmHg systolic - high pressure)
    ↓  branches into organ-specific arteries
    ├── Cerebral arteries → Brain (15% of cardiac output)
    ├── Coronary arteries → Heart muscle (5%)
    ├── Renal arteries → Kidneys (25%)
    ├── Mesenteric/celiac arteries → GI tract (25%)
    ├── Skeletal muscle arteries → Muscles (25% at rest, up to 80% during exercise)
    └── Cutaneous arteries → Skin (5%)
         ↓  (at each organ: capillaries)
         GAS EXCHANGE:
           O₂ leaves blood → enters cells
           CO₂ leaves cells → enters blood
           Nutrients (glucose, amino acids) delivered
           Waste products (CO₂, urea, lactic acid) collected
         ↓  (now deoxygenated, dark red blood)
    ├── Veins from legs/abdomen → Inferior Vena Cava
    └── Veins from head/arms → Superior Vena Cava
         ↓
Right Atrium  (cycle restarts)

"Oxygenated blood from the lung flows from the left atrium of the heart into the left ventricle. From there it is pumped through the aorta into the systemic circulation, which consists of numerous separate circuits supplying individual organs... At the capillary plexus, exchange of gases and metabolic products occurs." - Color Atlas of Human Anatomy Vol. 2

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🌿 The Portal Circulation - A Special Detour

GI capillaries (nutrient-rich after eating)
    ↓
Portal Vein
    ↓
Liver sinusoids (hepatocytes process glucose, amino acids, toxins)
    ↓
Hepatic Veins
    ↓
Inferior Vena Cava → Right Atrium

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Where Exactly Does Gas Exchange Happen?

In the Lungs (Pulmonary)

In the Tissues (Systemic Capillaries)

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The Complete Journey - One Breath's Worth of Oxygen

1. You breathe in → O₂ enters alveoli
2. O₂ diffuses across the thin alveolar-capillary membrane (0.5 µm thick) into blood
3. O₂ binds hemoglobin in red blood cells (98.5%) → blood turns bright red
4. Pulmonary veins carry it to the left atrium
5. Left ventricle pumps it into the aorta at high pressure
6. Blood distributes to every organ through branches of the aorta
7. At tissue capillaries: O₂ unloads into cells; CO₂ loads onto blood
8. Dark, deoxygenated blood travels back through veins → vena cava → right atrium
9. Right ventricle pumps it through the pulmonary artery back to the lungs
10. CO₂ is exhaled → you breathe out
11. The cycle repeats ~70 times per minute, every minute of your life

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