Respiratory system pathway: Air enters through Nostrils → Pharynx (throat) → Larynx (voice box) → Trachea (windpipe) → Bronchi (two branches to lungs) → Bronchioles (smaller tubes) → Alveoli (tiny air sacs where gas exchange occurs). Alveoli have extremely thin walls (one cell thick) surrounded by capillaries. O₂ diffuses from alveoli into blood; CO₂ diffuses from blood into alveoli for exhalation. Diaphragm is the primary muscle of breathing — contracts during inhalation (moves down), relaxes during exhalation (moves up).

Body organization hierarchy: Cells → Tissues → Organs → Organ Systems → Organism. Four types of tissues: Epithelial (covering/lining — skin, intestine lining), Connective (supporting/connecting — blood, bone, cartilage, fat), Muscular (movement — skeletal, smooth, cardiac), Nervous (signal transmission — neurons). Cell Theory: Schleiden (1838, plants) + Schwann (1839, animals) = all living things made of cells. Virchow (1855): 'Omnis cellula e cellula' — all cells arise from pre-existing cells.

Nervous system: Central NS (brain + spinal cord) and Peripheral NS (cranial + spinal nerves). Reflex actions are automatic, involuntary responses to stimuli. Reflex arc: Receptor (sense organ) → Sensory neuron (afferent) → Spinal cord (processing center) → Motor neuron (efferent) → Effector (muscle/gland). Examples: knee-jerk reflex, pupil constriction in bright light, pulling hand from hot object. Reflexes are controlled by spinal cord, NOT brain (faster response).

Digestive system: Mouth (teeth+saliva, salivary amylase breaks starch) → Esophagus (peristalsis) → Stomach (HCl + pepsin, protein digestion) → Small intestine (bile from liver + pancreatic juice, main absorption site — villi increase surface area) → Large intestine (water absorption) → Rectum → Anus. Liver is the largest gland in the body. Bile emulsifies fats (breaks into smaller droplets).

REET Exam Tips: 4 questions — focus on respiratory pathway in exact order, body organization hierarchy, Cell Theory contributors (Schleiden=plants, Schwann=animals, Virchow=cells from cells), and reflex arc components in sequence.