What the Kidneys Actually Do, and Why Urine Is the Report Card

Most people know the kidneys make urine. Fewer know that urine is a byproduct of a much larger job. Understanding that job is the key to understanding every kidney test on this site, from a dipstick protein pad to an eGFR on a blood panel.

The filtration

Each kidney holds about a million filtering units called nephrons. Blood enters each one under pressure, and a fine sieve, the glomerulus, lets water and small molecules through while holding back cells and large proteins. Roughly 180 liters of fluid pass through those sieves every day. The body only has about five liters of blood, so the entire volume is filtered dozens of times.

Nearly all of that filtrate is reabsorbed. Water, glucose, salts, amino acids, and bicarbonate are pulled back into the blood along the length of the nephron. What is left, about one to two liters a day, is urine.

That two step design, filter everything and take back what you need, is why urine is such a sensitive readout. Anything that should have been held back at the sieve, like albumin, and anything that should have been reabsorbed, like glucose, shows up in urine before it shows up anywhere else.

The regulation

Filtration is the visible job. Regulation is the real one.

Water balance. The kidneys decide how concentrated urine will be, from very dilute after a liter of water to very concentrated after a night without any. Specific gravity on a urine strip is a direct measure of this decision. See specific gravity and hydration.

Salt and potassium. Blood sodium and potassium are held in narrow ranges by adjusting how much is reabsorbed. Most blood pressure medications work by changing these settings.

Acid balance. The body produces acid constantly from metabolism and diet. The kidneys excrete it and regenerate bicarbonate to neutralize the rest. Urine pH is the exhaust. See urine pH test strips.

Blood pressure. The kidneys release renin, which starts a hormone cascade that raises pressure, and they control blood volume through salt and water handling. High blood pressure damages kidneys, and damaged kidneys raise blood pressure. That loop is why the two conditions travel together.

Red blood cells. The kidneys make erythropoietin, the hormone that tells bone marrow to make red cells. Anemia is a common feature of advanced kidney disease.

Bone. The kidneys activate vitamin D and excrete phosphate. Bone disease follows when they cannot.

Drug clearance. Most medications, or their breakdown products, leave through the kidneys. Doses are adjusted to kidney function for that reason.

How they fail

Kidneys fail quietly because they have reserve. Half the nephrons can be lost before blood tests move. The two common causes in the US are diabetes and high blood pressure, which damage the sieves slowly over years. Less common causes include inflammatory diseases of the glomerulus, inherited conditions, obstruction, and drug toxicity.

The first sign, in most cases, is albumin leaking through damaged sieves. It appears in urine years before filtration rate falls. That is the reason urine albumin testing, not blood testing, is the front line of kidney screening, and the reason this site returns to it so often. See why kidney disease is found late.

Reading the report card

Each urine strip pad maps to one of the jobs above.

• Protein or albumin: the sieve. Leakage means damage. See protein in urine.
• Glucose: reabsorption. Sugar in urine means blood sugar overwhelmed it, or rarely that the reabsorption system itself failed. See glucose in urine.
• Specific gravity: water balance.
• pH: acid handling.
• Blood: the sieve or the plumbing downstream.
• Nitrite and leukocytes: infection in the plumbing.

And the blood test, eGFR, measures the total filtration rate, the sum of all the sieves still working. See eGFR explained.

What keeps them working

Controlled blood pressure. Controlled blood sugar. Adequate but not excessive water. Caution with anti inflammatory painkillers, which reduce kidney blood flow and are a common cause of acute injury. No smoking. A yearly albumin test if you have diabetes or hypertension.

None of that is exotic. It is the same list that protects the heart and the brain, which is not a coincidence. The kidneys are the organ that shows the damage first.