The Urinary System

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.

The Nephron

Each kidney contains roughly one million nephrons, and the nephron is where the actual work happens. Three steps.

Filtration. Blood enters a tuft of capillaries called the glomerulus, sitting inside a cup called Bowman's capsule. Pressure pushes water and small dissolved substances out of the blood into the capsule. Cells and large proteins are too big to pass, so they stay in the blood. Protein or blood appearing in urine therefore means the filter itself is damaged.

The kidneys receive roughly 20 to 25 percent of the body's cardiac output and filter about 180 liters of fluid every day.

Reabsorption. The filtrate flows through a long looping tubule surrounded by capillaries, and nearly everything useful is pulled back into the blood: water, glucose, sodium, and other electrolytes. Of that 180 liters, all but about 1 to 2 liters comes back.

Glucose is normally reabsorbed completely. In a patient whose blood glucose is very high the mechanism is overwhelmed, glucose spills into the urine, and it pulls water out with it. That is the excessive urination of untreated diabetes, and it is why those patients arrive dehydrated.

Secretion. The tubule actively adds additional waste, excess potassium and hydrogen ions from the blood into the filtrate. This is a major route for excreting drugs, and it is one of the kidney's main tools for controlling blood pH.

What remains flows into collecting ducts and out to the ureter as urine.

Fluid and Blood Pressure Control

The kidneys sense blood flow through themselves. Low flow means low blood pressure or low volume, and the kidney responds.

Cells in the kidney release renin, which starts a chain reaction producing angiotensin II. That substance is a powerful vasoconstrictor, raising blood pressure directly, and it also triggers release of aldosterone from the adrenal cortex.

Aldosterone tells the kidney to hold onto sodium, and water follows sodium, so volume is retained. Antidiuretic hormone from the pituitary works alongside it, telling the kidney to hold onto water directly.

The practical result: a patient who is losing volume produces less urine. Urine output is a real time measure of perfusion, which is why it is monitored so closely in the hospital.

That same system is also why chronic kidney disease so commonly comes with high blood pressure. A kidney that is not perfused well keeps signaling for more pressure whether the body needs it or not.

pH and Potassium

The kidneys and the lungs together control blood pH, which the body holds between roughly 7.35 and 7.45.

The lungs act in seconds to minutes by adjusting how much carbon dioxide is exhaled, since carbon dioxide forms acid in the blood. The kidneys act over hours to days by excreting hydrogen ions and retaining bicarbonate. The lungs are the fast correction and the kidneys are the durable one.

Potassium is the other kidney responsibility with immediate consequences. Almost all potassium in the body sits inside cells, and blood levels are held in a narrow range because cardiac muscle depends on that gradient to conduct properly. The kidney is the main route for getting rid of excess potassium.

Lose that route and potassium climbs. High blood potassium produces muscle weakness and characteristic changes on the cardiac monitor, and it can produce lethal rhythms. It is one of the reversible causes considered in cardiac arrest.

Renal Failure and Dialysis

Acute kidney injury develops over hours to days and is often reversible if the cause is corrected. Causes divide into three groups by where the problem sits: not enough blood reaching the kidney, as in shock or dehydration; damage to the kidney tissue itself, as from certain drugs or from myoglobin released by crushed muscle; or an obstruction downstream blocking outflow.

Chronic kidney disease develops over months to years, most often from long standing diabetes or high blood pressure, and it is not reversible. End stage disease requires dialysis or transplant.

Dialysis does mechanically what the kidney did biologically: removes waste, removes excess fluid, and removes potassium. Hemodialysis runs blood through a machine, usually three times a week, through a surgically created fistula or graft in the arm.

A dialysis fistula has some specific handling. It should have a palpable thrill and an audible bruit, which confirm it is flowing. Do not take a blood pressure on that arm and do not start a line in it.

A missed dialysis session is a real emergency, and it is one of the more common calls involving these patients. The fluid that should have come off stays on, producing shortness of breath and crackles as fluid backs into the lungs. The potassium that should have come off stays in, threatening the heart rhythm. Waste products accumulate and the patient becomes nauseated, weak and confused.

Patients can also become unwell during or just after dialysis from the opposite direction, having had too much fluid removed too quickly, which produces low blood pressure, cramping, weakness and dizziness.

What to Take Away

Kidneys filter, reabsorb and secrete, and they also control volume, electrolytes, pH and blood pressure. They sit retroperitoneal, so kidney bleeding is hidden bleeding. Filtration happens at the glomerulus, and protein or blood in the urine means the filter is damaged. Falling urine output is an early sign of falling perfusion. A patient who has missed dialysis carries both fluid overload and a rising potassium, and both are dangerous.