A fist-sized muscle that beats about 100,000 times a day. Its rhythm starts with an electrical spark that depends on sodium, potassium and calcium.
How it works, step by step
The sinoatrial node fires an electrical impulse.
The upper chambers (atria) contract.
The signal passes the AV node, then spreads through the ventricles.
Ventricles contract, pushing blood to the lungs and body.
Electrolytes
Potassium levels that are too high or too low can disrupt heart rhythm — one reason kidneys guard them so closely.
Practical
Regular physical activity, not smoking, and managing blood pressure are among the best-supported ways to protect the heart.
When to get help
Chest pain, pressure or shortness of breath — especially with sweating or nausea — needs emergency care.
Evidence & sources
Adults should do at least 150 minutes of moderate-intensity aerobic activity a week. Evidence: strong. Known: Based on the Physical Activity Guidelines Advisory Committee's systematic review. Unknown: Benefits begin below this level; more brings additional benefit. Source: US Department of Health and Human Services, Physical Activity Guidelines for Americans, 2nd edition (2018).