What is the darker reddish brown internal area of the kidney?

The darker reddish-brown internal area of the kidney is known as the renal medulla. It consists of renal pyramids, which contain the nephrons, the functional units of the kidney responsible for filtering blood and producing urine. The medulla contrasts with the lighter cortex, which surrounds it and contains additional nephron structures. This differentiation in color and structure is essential for the kidney's filtration and concentration processes.