Due to their similar atomic radii, rubidium and caesium in the body mimic potassium and are taken up similarly.
dˈuː tə ðɛɹ sˈɪmɪlɚɹ ɐtˈɑːmɪk ɹˈeɪdɪˌaɪ, ɹuːbˈɪdiəm ænd sˈiːsiəm ɪnðə bˈɑːdi mˈɪmɪk pətˈæsiəm ænd ɑːɹ tˈeɪkən ˌʌp sˈɪmɪlɚli.