The kidneys are at the back of the abdominal cavity, with one sitting on each side of the spine.The right kidney is generally slightly smaller and lower than the left, to make space for the liver.Each kidney weighs 125–170 grams (g) in males and 115–155 g in females.A tough, fibrous renal capsule surrounds each kidney. Beyond that, two layers of fat serve as protection. The adrenal glands lay on top of the kidneys.Inside the kidneys are a number of pyramid-shaped lobes. Each consists of an outer renal cortex and an inner renal medulla. Nephrons flow between these sections. These are the urine-producing structures of the kidneys.Blood enters the kidneys through the renal arteries and leaves through the renal veins. The kidneys are relatively small organs but receive\xa020–25 percent\xa0of the heart’s output.Each kidney excretes urine through a tube called the ureter that leads to the bladder.The main role of the kidneys is maintaining homeostasis. This means they manage fluid levels, electrolyte balance, and other factors that keep the internal environment of the body consistent and comfortable.They serve a wide range of functions.Waste excretionThe kidneys remove a number of waste products and get rid of them in the urine. Two major compounds that the kidneys remove are:\turea, which results from the breakdown of proteins\turic acid from the breakdown of nucleic acidsReabsorption of nutrientsThe kidneys reabsorb nutrients from the blood and transport them to where they would best support health.They also reabsorb other products to help maintain homeostasis.