Nick Travis, age 12, of White Plains, N.Y., for his question:
HOW DO KIDNEYS PRODUCE URINE?
Humans have two kidneys performing many vital functions, of which the most important is the production of urine. This fluid carries various waste materials out of the body.
Human kidneys are made up of three layers. These layers are, in order, the cortex on the outside of the organ, the medulla and the pelvis.
Blood flows into the medulla through the renal artery. In the medula and cortex, the renal artery branches into increasingly smaller arteries. Each of these arteries ends in a blood filtration unit called a nephron.
Two healthy kidneys contain a total of about 2 million nephrons, which filter about 50 gallons of blood each day.
A nephron is made up of a group of tiny blood vessels, the glomerulus, surrounded by Bowman's capsule, a two layer membrane that opens into a convoluted tubule. Pressure forces much of the blood plasma or fluid portion of the blood through the glomerulus and into Bowman's capsule.
The filtered plasma, which contains water and dissolved chemicals, then passes into the convoluted tubule. The portion of the blood that remains in the glomerulus flows into small vessels called capillaries, which surround the convoluted tubule.
As the filtered plasma flows through the tubule, certain substances needed by the body are absorbed by the cells of the tubule wall. These substances, which include amino acids, glucose and about 99 percent of the water, then rejoin the blood in the capillaries. The capillaries return the blood to the heart by way of the renal vein.
Substances not absorbed in the tubule are wastes that the body cannot use. These substances, which include urea, uric acid and excess water, make up urine.
Healthy kidneys produce from one to two quarts of urine daily.
The urine passes from the convoluted tubules into larger collecting tubules and then into the pelvis layer of the kidney. A tube called the ureter carries urine from each kidney into a storage area called the urinary bladder.
Urine collects in the bladder until it passes out of the body through another tube, the urethra.
The kidneys look like purplish brown kidney beans and are about the size of an adults fist. They lie below the middle of the back on each side of the spine.
The right kidney, located under the liver, is a little lower than the left one. Some poeple are born with only one kidney, but they still can lead normal lives.
In addition to producing urine, the kidneys secrete a hormone called erythropoietin, which controls the production of red blood cells. The kidneys also help maintain the blood pressure of the body.