The human kidney is a remarkable organ. While I couldn’t hope to cover all aspects of kidney function in a short column like this, I thought an overview might be of interest. Most humans have two kidneys, bean-shaped organs about the size of an adult fist, located in the mid back behind the stomach. Their basic function is to keep the composition of the blood balanced to maintain good health. They filter water and toxins (i.e. natural toxins like urea, some drugs and medications, and some poisons) and maintain proper acid/base balance. The kidneys filter about 120-150 quarts of water every 24 hours to make one to two quarts of urine daily. Each kidney is a system of millions of tiny filters called nephrons. The first part of a nephron is the glomerulus, which separates blood cells and large molecules from toxins and fluids. From there, the remaining fluid and toxins pass through the tubule. The tubules remove minerals (phosphorus, calcium, magnesium) and restore them to the bloodstream, plus they remove more toxins. The urine carries toxins out to the ureters, into the bladder, then out of the body via the urethra.
A routine blood test measures kidney function by measuring the levels of creatinine (derived from creatine, a muscle protein) and blood urea nitrogen (BUN) and calculates the glomerular filtration rate, or GFR. Most of us start with a GFR of 60-70, which declines naturally with age. Chronic kidney disease (CKD) may lead to kidney failure where the GFR falls below 15, at which point dialysis may be indicated. The indications for dialysis are not the absolute lab values, but rather the development of symptoms of kidney failure that cannot be corrected. These symptoms include hyperkalemia (high potassium), fluid overload, uremic encephalopathy (brain dysfunction caused by high BUN), uremic pericarditis (inflammation and thickening of the lining of the heart), and acidosis (the blood becomes too acidic). It is estimated that one in three Americans are at risk of developing CKD and failure. Risk factors include diabetes, hypertension, obstructions (i.e. stones), inflammatory diseases, and intravenous drug use. Kidney transplants are one of the most common surgeries in the U.S.
Besides filtering the blood, the kidneys also make certain hormones like erythropoietin and renin. Those with kidney failure become anemic because there is no erythropoietin to stimulate the bone marrow to make red blood cells. Renin, which in a series of steps is converted to angiotensin, is a vital part of maintaining proper blood pressure and is where ACE inhibitors (lisinopril) and ARB blockers (losartan) work to lower blood pressure (in response to high blood pressure, the kidneys produce more renin). In order to keep your kidneys healthy, it is important to keep yourself well hydrated. Also, as both vitamin C and calcium may cause kidney stones, if you take supplements that contain either or both of these molecules, be sure not to exceed the recommended daily dosage or RDA.
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