The Urinary System
The kidneys, ureters, bladder and urethra, what a nephron actually does, how the kidneys control blood pressure and pH as well as fluid, and what changes in a patient on dialysis.
8 min read · Updated September 10, 2026
More Than Making Urine
The urinary system filters blood and produces urine, and describing it that way undersells it considerably. The kidneys are also the body's main regulator of fluid volume, electrolytes, blood pH and, over time, blood pressure. They produce a hormone that tells the marrow to make red blood cells, and they activate vitamin D.
A patient whose kidneys have stopped working has all of those problems at once, which is why renal failure touches nearly every system.
The Structures
Kidneys. Two, one on each side of the spine, at roughly the level of the lowest ribs. They sit retroperitoneal, meaning behind the lining of the abdominal cavity rather than inside it. The right kidney sits slightly lower than the left, because the liver takes the space above it.
Retroperitoneal position has a consequence. Bleeding from a kidney goes into that space rather than into the abdominal cavity, so the abdomen does not distend and the blood loss stays hidden. Blunt flank trauma with blood in the urine deserves attention for that reason.
Each kidney is protected behind by the lower ribs and by a surrounding layer of fat.
Ureters. Two narrow tubes carrying urine from each kidney down to the bladder, moving it along by peristalsis. A kidney stone lodged in a ureter causes some of the most severe pain in medicine, typically flank pain radiating toward the groin, coming in waves, with a patient who cannot find a comfortable position.
Bladder. A muscular sac in the pelvis storing urine. It holds several hundred milliliters comfortably. A full bladder rises above the pubic bone, where it becomes vulnerable to blunt trauma, and a full bladder in a pelvic fracture can rupture.
Urethra. Carries urine out. Short in females, considerably longer in males, where it passes through the prostate. That length difference is why urinary tract infections are far more common in females, and why urethral injury accompanies pelvic fracture more often in males.
Quick check
A patient with blunt flank trauma has blood in the urine, and the abdomen is not distended. Why can a bleeding kidney stay hidden?
Frequently asked questions
What do the kidneys actually do?
They filter waste out of the blood and make urine, and they also regulate fluid volume, electrolytes including potassium and sodium, blood pH, and blood pressure through the renin system. They release erythropoietin, which tells the marrow to make red cells, and they activate vitamin D.
Where are the kidneys located?
Retroperitoneal, meaning behind the abdominal lining rather than inside the abdominal cavity, one on each side of the spine at roughly the level of the lowest ribs. The right sits slightly lower than the left because the liver occupies the space above it.
Why is a missed dialysis session an emergency?
A patient whose kidneys have stopped working depends on dialysis to remove fluid and potassium. Missing a session leaves both in the body, so you get fluid overload with crackles and shortness of breath, and a rising potassium that can produce lethal cardiac rhythms.
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