Body systems working together
Each body system has one main job, and the systems depend on each other: lungs take in oxygen, blood delivers it.
A worked example
Counting a pulse for 15 seconds gives a quarter of a minute, so 24 beats in 15 seconds is 24 x 4 = 96 beats per minute.
The mistake people actually make
The lungs do not carry oxygen round the body and the heart does not collect it. Breathing and delivering are separate jobs done by two systems that meet in the lungs.