I'm not sure whether you want one particular system or all of them, so I'll tell you all the ones I know.
Digestive system :-
Mouth chews food, food goes through oesophagus into stomach, stomach contents (chyme) are mixed with bile from the liver and enzymes from the pancreas, everything travels through large intestine which absorbs nutrients and then the large intestine which absorbs water, I'm sure you know what happens after that :)
Nervous system:-
Made up of many different nerves which are made up of neurones, nerves link muscles to the brain through the spinal cord or into the back of the brain (called the central nervous system), these nerves carry electrical impulses which are processed by the brain and then sent back to the muscles through nerves to either tell it to relax or contract.
Cardiovascular system:-
Heart pumps blood around the body, blood enters heart through the vena cava, blood is pumped through the first 2 chambers (right atrium & right ventricle), blood then goes through pulmonary artery towards the lungs to collect oxygen and remove carbon dioxide, blood goes through pulmonary vein back to the heart, blood goes through left atrium and left ventricle before leaving through the aorta. Heart supplies oxygen and energy to muscles around the body.
Renal system:-
Kidneys draw water, glucose, urea and salts out of blood, all substances that need to be reabsorbed (glucose and some water) will be reabsorbed back into the bloodstream, waste goes through ureter to be excreted.
(I'm not sure which systems you meant, so if this helped then that's good, otherwise tell me which ones specifically and I can properly help you with your answer).