Reabsorbtion. Occuring more commonly within the nephron system than filtration, this is a form of active transport which takes the the useful substances for the body from the tubules and places them back into the blood filled capillaries.
Oleanders are shrubs, but they are _big_ shrubs. Most of the varieties in cultivation top out at 8 to 12 feet tall and about the same width. The wild form, and some of the older cultivars used for windbreaks top out at 20 x 20 feet.
Nannyberry (Viburnum lentago) is a native that grows to 18 feet in filtered shade. If you want slightly taller ornamental trees, look at speckled alder (Alnus rugosa), Juneberry (Amelanchier arborea) or Allegheny serviceberry (Amelachier laevis), all of which grow to between 15 and 25 feet tall.