What happens when you run?
Rest, run, then slow down: watch your heart, lungs and leg muscles speed up together.
Resting
You are sitting still. Your heart beats about 70 times a minute and you breathe about 15 times a minute. Your leg muscles are barely working, so they only need a small share of the blood. Everything is calm. Go to the next step to start running. (The numbers are typical ones for a grown-up, not yours.)
A resting adult heart beats 60 to 100 times a minute and a resting adult breathes 12 to 20 times a minute. Cardiac output, the blood the heart pumps each minute, is about 5 litres, and skeletal muscle at rest gets roughly a fifth of it. The rates here (70 beats, 15 breaths) are typical round numbers and the sound is the heartbeat. They are illustrative: age, fitness, mood and even caffeine change them. This is a model, not a way to measure your own body.
Running
Now you run. Your leg muscles squeeze again and again, and they burn fuel with oxygen to do it. They need much more blood, fast. So your heart beats faster, about 150 times a minute, to push more blood down the leg pipes. Your lungs pull in more air, about 40 breaths a minute, to hand over oxygen and get rid of the waste gas the muscles make. Heart, lungs and legs speed up as a team. These numbers are typical, not yours: hard running can be faster or slower for you.
Working muscle uses oxygen and makes carbon dioxide faster. Sensors in the brain and arteries notice the changes, and the nervous system raises heart rate and the strength of each beat, so cardiac output rises from about 5 litres a minute to 20 or more. Blood vessels in working muscle widen and those to the gut narrow, so most of the output goes to the muscles. Breathing rate goes up to 30 to 60 breaths a minute in hard exercise, and each breath is deeper. Here: 150 beats, 40 breaths, 160 steps a minute (each leg 80), all illustrative round numbers for a young adult; a hard effort can reach 180 to 200 beats. The leg squeeze and the breathing are exaggerated so you can see them.
Slowing down
You stop. Your legs rest, but your body is not done: the muscles still need extra oxygen to refill their fuel, and the heat has to go. So your heart and breathing stay fast for a while, then ease down. Here they are at about 100 beats and 24 breaths a minute, on the way back. A few minutes later, you are at the first step again: resting.
After exercise, heart rate falls fast in the first minute, as the nerves that slow the heart take over from the ones that sped it up, then more slowly over several minutes. Breathing stays raised because the body repays an oxygen debt (excess post-exercise oxygen consumption): restoring muscle fuel stores, clearing lactate and cooling down. How quickly you slow down depends on fitness, heat and how hard you ran. The numbers here (100 beats, 24 breaths, legs still) are a snapshot of a typical recovery, not a timeline. Go back to the first step to see rest again.