The two kidneys lie against the back wall of the abdomen, behind the peritoneum, at about the level of the lowest ribs. The right kidney sits slightly lower than the left because the liver is above it.
Outside layers
Each kidney is wrapped in a tough capsule and cushioned by fat. The renal artery, renal vein and ureter enter and leave at the hilum, the notch on the inner side.
Cortex and medulla
The outer cortex holds the filters. The inner medulla is made of cone-shaped pyramids full of tubules that concentrate urine.
The nephron
Each kidney has about one million nephrons. A nephron is a filter (the glomerulus inside Bowman's capsule) followed by a long tubule: proximal tubule, loop of Henle, distal tubule and collecting duct.
Drainage
Collecting ducts empty into the calyces, which join to form the renal pelvis. The renal pelvis narrows into the ureter, which carries urine to the bladder.
At the bedside
Kidney pain is felt in the flank and at the costovertebral angle. Tenderness when you tap that angle supports pyelonephritis or a kidney stone.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).