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.
Two culprits. Oleander leaf scorch is the reason the leaves on these long-lived shrubs are drooping, turning brown and dying. Individual branches die back; then, as more branches are affected, the entire plant dies. As summer's heat peaks and Santa Anas roar, the demise of infected oleanders will usually accelerate.