Interdependence of systems

No organ system works alone. Your heart pumps, but your kidneys set how much blood it has to pump, your nervous system sets its rate, and your lungs load the blood it carries with oxygen. A change in one system is sensed and answered by others.

This is why a single problem spreads. After heavy bleeding, the heart speeds up, the blood vessels tighten, the kidneys hold on to water and the thirst center pushes you to drink, all to protect blood pressure. When you study a system, ask what it depends on and what depends on it.