The cardiovascular, respiratory and muscular systems don't wait to respond — they react in the first seconds of movement, ramping up delivery and clearance before you've even noticed you're working. Slide the intensity slider to see how each system responds.
When you start moving, the heart immediately increases its rate and the volume of blood pumped per beat (stroke volume). Heart rate can climb from around 70 bpm at rest to 180+ bpm during vigorous exercise. Stroke volume rises as the heart contracts more forcefully. Together, these two changes drive cardiac output up — sometimes to five times resting levels — flooding working muscles with oxygen-rich blood and clearing carbon dioxide and metabolic waste faster.
Acute responses — heart rate, breathing rate, muscle temperature — happen within seconds of starting exercise and return to baseline soon after you stop.