Final answer:
The darker and reddish-brown portion of the kidneys that contains the medullary pyramids is called the renal medulla, characterized by cone-shaped renal pyramids and containing renal columns with blood vessels and connective tissue.
Step-by-step explanation:
The portion of the kidneys that is deep to the cortex, darker and reddish-brown, containing the medullary pyramids, is known as the renal medulla. This area is found inside the kidney and is characterized by a series of cone-shaped tissue masses called the renal pyramids. The renal medulla and the pyramids within it play an essential role in the concentration and transportation of urine.
Between these pyramids are structures known as renal columns, which are extensions of the renal cortex that contain blood vessels and connective tissue. The renal pyramids consist of collecting ducts and loops of Henle from juxtamedullary nephrons, which are important structures in the kidney's function. It is located deep to the renal cortex and has a darker and reddish brown appearance compared to the cortex. The renal medulla consists of multiple pyramidal tissue masses, called the renal pyramids, which contain collecting ducts and loops of Henle.