Final answer:
The external outermost layer of the kidneys is the renal fascia, while the internal outermost layer is the renal cortex, which contains the nephrons.
Step-by-step explanation:
The outermost layer of the kidneys is the renal cortex. The kidneys are surrounded by three layers externally: the outermost being the renal fascia, followed by the perirenal fat capsule, and the innermost layer called the renal capsule. Internally, the kidney consists of an outer cortex, a medulla in the middle, and the renal pelvis. The renal cortex contains numerous nephrons, which are the functional unit of the kidney, whereas the renal medulla houses the renal pyramids. The renal pelvis serves as a funnel for urine flowing to the ureter.