• The Nervous System — neurons, brain, spinal cord, peripheral nerves, autonomic regulation
• The Cardiovascular System — heart, blood vessels, circulation, oxygen delivery
• The Respiratory System — lungs, airways, gas exchange, breathing mechanics
• The Musculoskeletal System — bones, muscles, joints, fascia, movement
• The Digestive System — gut, liver, absorption, metabolism
• The Immune System — immune cells, defense, inflammation, autoimmunity