Bill Moore, age 11, of Kingston, Ontario, Canada, for his question:
WHAT DO THE KIDNEYS DO?
Humans and all other vertebrates have two kidneys. The organ's !2~ most important function is the production of urine, the fluid which carries various waste materials out of the body.
.Your kidneys are located in the middle of your back on each side of the spine.
Human kidneys are made up of three layers: the cortex on the outside of the organ, the medulla and the pelvis. Blood flows into the medulla through the renal artery. In the cortex and medulla, the renal artery branches into increasingly smaller arteries.
Each of the tiny arteries end 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 every day.
A nephron is made up of tiny blood vessels called the glomerulus, which surround a two layer membrane called Bowman's capsule. The membrane opens into a convoluted tubule. Pressure pushes the fluid portion of the blood through the glomerulus and into Bowman's capsule.
As the fluid portion of the blood flows through the tubule, certain substances needed by the body are absorbed by the cells of the tubule wall. Included are amino acids, glucose and about 99 percent of the water. 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.
The urine passes from the convoluted tubules into larger collecting tubules and then into the pelvis layer of each 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. A healthy kidney will produce from one to two quarts of urine each day.
In addition to producing urine, the kidneys also secrete a hormone which controls red blood cell production.
Kidneys look like purplish brown kidney beans and are about the size of an adult's fist.
In addition to the production of urine and the secretion of the hormone which controls the production of red blood cells, the kidneys also help maintain the blood pressure of the body.
Some people are born with only one kidney, but they lead a normal life. Also, if one kidney is lost in an accident or because of disease, the remaining organ may enlarge and do the work of both. But a person couldn't live if both kidneys were lost, since wastes would accumulate in the body.
Many people who have lost their kidneys or who have suffered great kidney damage are kept alive by a dialysis machine. A tube connects this machine to an artery in the patient's arm. Blood then flows into the machine, which removes wastes. The machine is used several times each week.
Doctors have also replaced diseased kidneys with healthy ones in kidney transplants.