Kidneys – Anatomy, Nerve Supply, Function

The kidneys are bean-shaped organs, with medial concavity and lateral convexity, weighing anywhere from 150 to 200 g in males and about 120 to 135 g in females. The dimensions are usually a length of 10 to 12 cm, a width of 5 to 7 cm, and a thickness of 3 to 5 cm. Each kidney is about the size of a closed fist. They are located retroperitoneally on the posterior abdominal wall and are found between the transverse processes of T12 and L3. Both of the upper poles are usually oriented slightly medially and posteriorly relative to the lower poles. If the upper renal poles are oriented laterally, this could suggest a horseshoe kidney or a superior pole renal mass.  Further, the right kidney is usually slightly more inferior in position than the left kidney, likely because of the liver.[rx]

The following are the kidneys relative to surrounding organs

• Superiorly – on top of each kidney and separated by renal fascia, are the suprarenal glands (adrenal glands), the right pyramidal suprarenal gland oriented apically on the right kidney and the left crescentic suprarenal gland oriented more medially on the left kidney
• The right kidney is posterior to the ascending colon, the second part of the duodenum medially, and the liver, separated by the hepatorenal recess
• The left kidney is posterior to the descending colon, its renal hilum lateral to the tail of the pancreas, superomedial aspect adjacent to the greater curvature of the stomach, and left upper pole adjacent to the spleen and connected by splenorenal ligaments
• Posteriorly the diaphragm rests over the upper third of each kidney with the 12th rib passing posteriorly over the upper pole. The kidneys usually sit located over the medial aspect of the psoas muscle and the lateral aspect of the quadratus lumborum. The proximal ureters will typically pass over the psoas muscle on their way to the bony pelvis.[rx]
• At the medial margin of each kidney lies the renal hilum, where the renal artery enters, and the renal pelvis and vein leave the renal sinus. The renal vein is found anterior to the renal artery, which is anterior to the renal pelvis. The renal pelvis is the flattened, superior end of the ureter. It receives 2 or 3 major calyces, each of which receives 2 or 3 minor calyces. The minor calyces are indented by the renal papillae, which are the apices of the renal pyramids. A pyramid and its cortical tissue comprise a lobe.[rx]
• Each kidney is covered by a two-layered capsule and is surrounded by perinephric fat, Gerota’s fascia, Zuckerkandl fascia, and perinephric fat. The entire area immediately involving the kidneys is considered the retroperitoneum.[rx][rx]

Structure and Function of Kidneys

The kidney is composed of two regions: the cortex and medulla.

• The cortex is composed of renal corpuscles,
• convoluted tubules,
• straight tubules,
• collecting tubules,
• collecting ducts, and vasculature.
• Medullary rays, comprised of straight tubules and collecting ducts, extend into the cortex from the medulla.
• The medulla also contains the vasa recta, a network of capillaries integral to the countercurrent exchange system.
• Pyramids are conical structures formed by the collecting of tubules in the medulla, oriented with the base towards the cortex and apices towards the hilum.
• The papillae at the apices of the pyramids extend into minor calyces and drain via the collecting ducts at their tips, the area cribrosa. A collecting duct and the group of nephrons that it drains is referred to as a lobule.[rx]

Blood Supply of Kidneys

About 20% of the total cardiac output goes to the kidneys.

• These highly vascular organs are supplied via the renal arteries, which branch off of the aorta inferior to the superior mesenteric artery and enter the hilum of the kidney at L2.
• The longer right renal artery passes posterior to the inferior vena cava (IVC).
• Both of the renal arteries divide close to the renal hilum giving off five segmental arteries. The first branch is the posterior segmental artery, and it supplies the posterior segment of the kidney.

The remaining 4 main segmental arteries all arise from the anterior branch of the renal artery and are named according to the segment of the kidney they supply

• The superior segmental artery,
• The anterosuperior segmental artery,
• The anteroinferior segmental artery, and
• The inferior segmental artery.
• Accessory renal arteries, leftover embryologically in approximately 25% of people due to failure of vessel degeneration, may arise from the aorta or renal artery and usually enter the poles.[rx][rx][rx]

Nerves of Kidneys

Innervation to the kidneys and a portion of the proximal ureters and suprarenal glands,[rx][rx] is communicated via the

• Renal nerve plexus consisting of sympathetic and parasympathetic fibers.
• Splanchnic nerves.
• Sympathetic efferents nerve
• The sensory renal afferent nerves are situated primarily in the renal pelvis, following along the renal artery or proximal ureter to the pelvic wall, and serve an important role in sympathetic outflow and blood pressure regulation. Some travel along the ureter or pelvis in a parallel fashion, while others are oriented circumferentially. They innervate the pelvic wall, renal artery, and renal vein. No sensory fibers are found in the medulla, and few can be observed in the cortex. [rx][rx]

References