Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.
The kidneys are a pair of bean-shaped organs that lie on either side of the spine in the lower middle of the back. Each kidney weighs about ¼ pound and contains approximately one million filtering units called nephrons. Each nephron is made of a glomerulus and a tubule. The glomerulus is a miniature filtering or sieving device while the tubule is a tiny tube like structure attached to the glomerulus.
The kidneys are connected to the urinary bladder by tubes
called ureters. Urine is stored in the urinary bladder until the bladder is emptied by urinating. The bladder is connected to the outside of the body by another tube like structure called the urethra.
Illustration of the kidneys, urinary tract, and bladder.
The main function of the kidneys is to remove waste products and excess water
from the blood. The kidneys process about 200 liters of blood every day and
produce about 2 liters of urine. The waste products are generated from normal metabolic processes including the breakdown of active tissues, ingested foods, and other substances. The kidneys allow consumption of a variety of foods, drugs, vitamins and supplements, additives, and excess fluids without worry that toxic by-products will build up to harmful levels. The kidney also plays a major role in regulating levels of various minerals such as calcium, sodium, and potassium in the blood.
As the first step in filtration, blood is delivered into the glomeruli by microscopic leaky blood vessels called capillaries. Here, blood is filtered of waste products and fluid while red blood cells, proteins, and large
molecules are retained in the capillaries. In addition to wastes, some useful
substances are also filtered out. The filtrate collects in a sac called
The tubules are the next step in the filtration process. The tubules are lined with highly functional cells which process the filtrate, reabsorbing water and chemicals useful to the body while secreting some additional waste products into the tubule.
The kidneys also produce certain hormones that have important functions in the body, including the following:
Active form of
vitamin D (calcitriol or 1,25
dihydroxy-vitamin D), which regulates absorption of calcium and phosphorus from foods, promoting formation of strong bone.
Erythropoietin (EPO), which stimulates the bone marrow to produce red blood cells.
Renin, which regulates blood volume and blood pressure.
Screening (looking) for early kidney disease in people who are not already known to have it. Kidney disease is common and is commonly insidious in onset. The burden of kidney disease in its earlier stages lies not only in the risk of progression but in the complications of decreased kidney function and the risk of heart disease.
In 2002 the National Kidney Foundation (NKF) set forth guidelines for kidney disease screening. It recommended that all individuals at increased risk for chronic kidney disease have their blood pressure measured and their blood and urine tested for signs of impaired kidney function. Those at increased risk for chronic kidney disease were defined as people with: