The function of the renal medulla is to hold masses of tissues called renal pyramids in each kidney, according to InnerBody. These pyramids face towards the convex or inner part of the kidney, while the bases face the outside part of the organ. The renal medulla is the innermost part of the kidney protected by the renal cortex on the outside.
Renal pyramids consist of blood vessels and tube-like structures that collect filtrate. These pyramids contain nephrons, the basic unit that filters blood in the kidneys, according to Healthline. Millions of nephrons extend from the cortex to inner regions of the kidney. These structures regulate water concentration, extract salts from blood and reabsorb needed nutrients back into the body. Waste material is then excreted through urine. This process helps regulate blood pressure and blood volume throughout the body.
Medulla regions are darker than the outer cortex. Healthline states that most humans have seven pyramid structures inside the renal medulla. The pyramids appear striped because nephrons align parallel to each other throughout the inner kidney.
Kidneys help maintain homeostasis in the body by regulating water, ions and acid-base levels in the blood. The bean-shaped organs secrete hormones that regulate blood pressure, bone marrow levels and vitamin D absorption, according to About.com.