The workhorses of the urinary system are the kidneys which are the twin, bean-shaped organs in your body that clear harmful substances by filtering your blood. They’re like a water purification plant that helps clean the drinking water for a city. They also regulate blood pH, volume, pressure, osmolality as well as produce hormones.
The kidneys are located between the T12 and L3 vertebrae, and they’re partially protected by ribs 11 and 12--which are the floating ribs. The kidneys are roughly the size of a fist and are retroperitoneal, meaning they sit behind the peritoneal membrane alongside the vertebral column. The right kidney is pushed down by the liver so it sits slightly lower than the left kidney.
In the middle of each kidney there is an indentation that forms the renal hilum. This is the entry and exit point for the ureter, renal artery and renal vein, lymphatics, and nerves going into and coming out of the kidney.
The kidney is surrounded by three layers of tissue. On the outside is the renal fascia which is a thin layer of dense connective tissue that anchors the kidney to its surroundings. The middle layer, or the adipose capsule, is a fatty layer that protects the kidney from trauma. And the deepest layer, called the renal capsule, is a smooth, transparent sheet of dense connective tissue that gives the kidney its distinctive shape.
If you take a cross-section of the kidney, there are two main parts. The inner portion is the renal medulla and the outside rim is the renal cortex. The medulla is made up of 10 to 18 renal pyramids with the base of the pyramids facing the renal cortex and the tips of the pyramids, called renal papilla—or nipples, pointing towards the center of the kidney. The renal papilla project into minor calyces which join together to form major calyces which funnel into the renal pelvis. Urine collects in the renal pelvis and then heads out of the kidney through the ureter.