ELI5: physiology

In the context of substance abuse, physiology means how drugs or alcohol change the way the inside of your body and brain physically work.

Think of your body as a busy, organized school. Your brain is the principal, and your organs—like your heart, lungs, and stomach—are the teachers and students doing their daily jobs. Everyone talks to each other using quiet, polite messages to keep the school running smoothly.

When someone uses drugs or alcohol, it is like a loud, wild marching band suddenly bursts into the school:

• Hijacking the brain: Drugs trick the brain by mimicking its natural chemicals, flooding it with unnatural feel-good signals.
• Changing the body's rules: Over time, the body gets used to the loud music. It changes its normal settings, which is called tolerance. The person now needs more of the substance just to feel normal.
• Physical dependence: The body forgets how to work without the substance. If the marching band suddenly stops playing, the school falls into chaos. This causes withdrawal, making the person feel physically sick with shaking, sweating, or pain.
• Organ damage: Just like loud music can damage the school's speakers, substances wear down vital organs. The liver has to work too hard to clean out the poison, the heart beats too fast or too slow, and breathing can become dangerously weak.