The kidneys are paired, bean-shaped organs with medial concavity and lateral convexity that produce urine. They are positioned below the diaphragm (Read more!) on either side of the lumbar spine near the posterior wall of the abdomen.

The kidneys are somewhat reddish brown retroperitoneal organs. They are protected by the lower rib cage, surrounding structures, and renal coverings. Both kidneys are similar in size and shape, spanning from T11/T12 to L2/L3 levels. However, the left kidney is longer and more slender than the right and lies closer to the midline. The right kidney lies a bit lower than the left due to its relation to the liver. Each kidney is about the size of a fist.

• The left kidney extends from the T11/T12 vertebrae superiorly to the L2 vertebra inferiorly.
• The right kidney extends from approximately the T12/L1 vertebrae superiorly to the L3 vertebra inferiorly.

In adults, the kidneys are approximately 3.94 to 4.72 inches (10 - 12 cm) long, 1.97 to 2.76 inches (5 - 7 cm) wide, and 0.79 to 1.18 inches (2 - 3 cm) thick. Each kidney weighs around 3.88 to 7.05 ounces (110 - 200 grams). Please note that these are average parameters, and they differ depending on gender, age and demographic characteristics.

As mentioned previously, the kidneys produce urine, and their main functions include eliminating excess bodily fluids, salts and metabolic by-products. Therefore, the kidneys regulate blood osmolarity, acid-base (pH) balance, blood volume and pressure and filter the blood of foreign substances. Besides that, they also take part in vitamin D metabolism by participating in the production of calcitriol. Also, these organs release erythropoietin when necessary, which further stimulates red blood cell production. Overall, the kidneys are an essential organ regulating body homeostasis.

Note: In an hour, the kidneys filter around 1.98 - 2.11 gallons (7.5 - 8 liters) of blood. In 24 hours, they equal 47.55 - 50.72 gallons (180 - 192 liters). Overall, the kidneys filter blood up to 300 times per day, producing around 0.26 - 0.4 gallons (1 - 1.5 liters) of urine.

Renal coverings

The kidneys are enclosed by a fibrous capsule, which is surrounded by a unique arrangement of fascia and fat. From the innermost to the outermost, these coverings are arranged in the following order:

• Fibrous capsule - it is closely adherent to the surface of the kidney;
• Perinephric (perirenal) fat - covers the renal capsule and completely surrounds the kidney;
• Renal fascia - it is a condensation of the surrounding connective tissue that lies outside the perinephric fat; it encloses not only the kidney but also its adjacent adrenal (suprarenal) gland (both organs are found within different compartments as they are separated by a thin and weak septum); the renal fascia has two layers - anterior and posterior; the anterior layer is also called the anterior renal or Gerota's fascia, while the posterior layer is known as the posterior renal or Zuckerkandl fascia; both layers cover the kidney from the corresponding aspect and fuse laterally and superiorly;
• Paranephric (pararenal) fat - lies outside the renal fascia and is a part of retroperitoneal fat; this fat accumulates posteriorly and posterolaterally to each kidney and separates it from the abdominal wall muscles.

The perinephric fat, renal fascia and paranephric fat support the kidneys and hold them in their position against the posterior abdominal wall.