D) Kidney


The kidneys are paired organs responsible for filtration of the blood, removal of waste products, fluid homeostasis and the production of several hormones. Each kidney is about 11 - 12 cm in length, and several cm thick. The kidney is covered by a thin capsule, which continues as the renal pelvis and also coats the ureters. The uncut surface is reddish-brown.

If bisected, the kidney has an outer cortical layer and a central medulla. The medulla is arranged as several pyramids; the cortex passes between the pyramids as renal columns. The pyramids of the medulla project into the calyces of the renal pelvis as papillae. The calyces coalesce to form the renal pelvis, which is then continuous inferiorly with the ureter.

Microscopic Structure

The middle part of the cortex of the kidney contains the glomeruli, small and delicate filtration units where fluid from the blood is siphoned off for processing in the renal tubules. The tubules descend, at first tortuously (in the cortex) and then straighten in the medulla. Each tubule then reascends (the 'loop of Henle') before combining with other tubules to form collecting ducts. These collecting ducts pass back to the medulla, where they empty into the renal calyces at the papilla of each pyramid.
The renal pelvis and calyces are lined by urothelium (transitional epithelium), with a surround smooth muscle layer and outer fibrous coat.


Each kidney has slightly different relations.
In relation to the right kidney:

  • The adrenal gland is superior, and the liver lies anterior and superior, separated by the peritoneum.
  • The descending duodenum is medial
  • The hepatic flexure is anterior to the inferior part of the gland

In relation to the left kidney:

  • The left adrenal is supero-medial
  • From lateral to medial, the spleen, pancreas and stomach are in relation to the anterior surface. The splenic flexure also passes near here.

For both kidneys, the psoas major muscle is medial and inferior.


The glomeruli and tubules of the kidney are complex structures involved in the filtration of plasma from the bloodstream, removal of waste products and resorption of required electrolytes. After birth it is impossible to create new glomeruli, perhaps explaining the loss of radiation tolerance seen following radiation.