It depends if you are running or jumping ?
The nervous system and cardiovascular system
Answer:
kidney
Explanation:
Urea is carried in the bloodstream to the kidneys. The kidneys are bean-shaped organs about the size of your fists. They are near the middle of the back, just below the rib cage. The kidneys remove urea from the blood through tiny filtering units called nephrons.
Well he could be teething, if hes 18 months, but it could be an abnormality in his muscles
