Five complete body systems units built around interactive simulations and scaffolded reading. ⭐ Circulatory System (4 lessons) — Heart rate, blood flow, training & performance, blocked arteries ⭐ Digestive System (4 lessons) — Stomach acid, enzymes, nutrient absorption, bezoars ⭐ Immune System (4 lessons) — White blood cells vs. pathogens, antibiotic resistance, vaccines ⭐ Nervous System (4 lessons) — Brain wiring, daily choices, long-term changes, brain extremes ⭐ Respiratory System (5 lessons