If we subtract the atomic number from the weight, we get the number of neutrons in the particle. This is because protons and neutrons each have a weight of 1, while electrons are 0. And since the atomic number is also the number of protons in the atom, subtracting it from total weight gives us the number iif neutrons.
The immune system, and the excretory system
(Honestly, all of them work to keep us 'healthy', but I believe these are the ones you're looking for)
Zygote,mitosis,meiosis,sex cells,and fertilization
