The Importance of the Kidneys and Urinary Tract
Our bodies produce several kinds of wastes, including sweat,
carbon dioxide gas, feces (stool), and urine. These wastes exit the
body in different ways. Sweat is released through pores in the
skin. Water vapor and carbon dioxide are exhaled from the lungs.
And undigested food materials are formed into feces in the
intestines and excreted from the body as solid waste in bowel
movements.
Urine, which is produced by the kidneys, contains the byproducts
of metabolism - salts, toxins, and water - that end up in the
blood. The kidneys and urinary tract (which includes the kidneys,
ureters, bladder, and urethra) filter and eliminate these waste
substances from our blood. Without the kidneys, waste products and
toxins would soon build up in the blood to dangerous levels.
In addition to eliminating wastes, the kidneys and urinary tract
also regulate many important body functions. For example, the
kidneys monitor and maintain the body's balance of water,
ensuring that our tissues receive enough water to function properly
and be healthy.
When doctors take a
urine sample
, the results reveal how well the kidneys are working. For example,
blood, protein, or white blood cells in the urine may indicate
injury, inflammation, or infection of the kidneys, and glucose in
the urine may be an indication of
diabetes
.
What They Do
Although the two kidneys work together to perform many vital
functions, people can live a normal, healthy life with just one
kidney. In fact, some people are born with just one of these
bean-shaped organs. If one kidney is removed, the remaining one
will enlarge within a few months to take over the role of filtering
blood on its own.
Every minute, more than 1 quart (about 1 liter) of blood goes to
the kidneys. About one fifth of the blood pumped from the heart
goes to the kidneys at any one time.
In addition to filtering blood, producing urine, and ensuring
that body tissues receive enough water, the kidneys also regulate
blood pressure and the level of vital salts in the blood. By
regulating salt levels through production of an enzyme called renin
(as well as other substances), the kidneys ensure that blood
pressure is regulated.
The kidneys also secrete the hormone erythropoietin, which
stimulates and controls red blood cell production (red blood cells
carry oxygen throughout the body). In addition, the kidneys help
regulate the acid-base balance (or the pH) of the blood and body
fluids, which is necessary for the body to function normally.
How They Work
The kidneys are located just under the ribcage in the back, one
on each side. The right kidney is located below the liver, so
it's a little lower than the left one. Each adult kidney is
about the size of a fist. Each has an outer layer called the
cortex, which contains the filtering units.
The center part of the kidney, the medulla has 10 to 15
fan-shaped structures called pyramids. These drain urine into
cup-shaped tubes called calyxes. A layer of fat surrounds the
kidneys to cushion and help hold them in place.
Here's how the kidneys filter blood: Blood travels to each
kidney through the renal artery, which enters the kidney at the
hilus, the indentation in the kidney that gives it its bean shape.
As it enters the cortex, the artery branches to envelope the
nephrons - 1 million tiny filtering units in each kidney that
remove the harmful substances from the blood.
Each of the nephrons contain a filter called the glomerulus,
which contains a network of tiny blood vessels known as
capillaries. The fluid filtered from the blood by the glomerulus
then travels down a tiny tube-like structure called a tubule, which
adjusts the level of salts, water, and wastes that are excreted in
the urine.
Filtered blood leaves the kidney through the renal vein and
flows back to the heart.
The continuous blood supply entering and leaving the kidneys
gives the kidneys their dark red color. While the blood is in the
kidneys, water and some of the other blood components (such as
acids, glucose, and other nutrients) are reabsorbed back into the
bloodstream. Left behind is urine. Urine is a concentrated solution
of waste material containing water, urea, a waste product that
forms when proteins are broken down), salts, amino acids,
byproducts of bile from the liver, ammonia, and any substances that
cannot be reabsorbed into the blood. Urine also contains urochrome,
a pigmented blood product that gives urine its yellowish color.
The renal pelvis, located near the hilus, collects the urine
flowing from the calyxes. From the renal pelvis, urine is
transported out of the kidneys through the ureters, tubes that
carry the urine out of each kidney to be stored in the urinary
bladder - a muscular collection sac in the lower abdomen.
The bladder expands as it fills and can hold about 2 cups (half
a liter) of urine at any given time (an average adult produces
about 6 cups, or 11/2 liters, of urine per day). An adult needs to
produce and excrete at least one third of this amount in order to
adequately clear waste products from the body. Producing too much
or not enough urine may indicate illness.
When the bladder is full, nerve endings in its wall send
impulses to the brain. When a person is ready to urinate, the
bladder walls contract and the sphincter (a ring-like muscle that
guards the exit from the bladder to the urethra) relaxes. The urine
is ejected from the bladder and out of the body through the
urethra, another tube-like structure. The male urethra ends at the
tip of the penis; the female urethra ends just above the vaginal
opening.
Problems of the Kidneys and Urinary Tract
Like other systems in the body, the entire urinary tract is
subject to diseases and disorders.
In kids, the more common problems include:
Congenital problems of the urinary tract.
As a fetus develops in the womb, any part of the urinary tract can
grow to an abnormal size or in an abnormal shape or position. One
common congenital abnormality (an abnormality that exists at birth)
is duplication of the ureters, in which a kidney has two ureters
coming from it instead of one. This defect occurs in about 1 out of
every 125 births and can cause the kidney to develop problems with
repeated infections and scarring over time.
Another congenital problem is horseshoe kidney, where the two
kidneys are fused (connected) into one arched kidney that usually
functions normally, but is more prone to develop problems later in
life. This condition is found in 1 out of every 500 births.
Glomerulonephritis
is an inflammation of the glomeruli, the parts of the filtering
units (nephrons) of the kidney that contain a network of
capillaries (tiny blood vessels). The most common form is
post-streptococcal glomerulonephritis, which usually occurs in
young children following a case of
strep throat
. Most kids with this type of nephritis recover fully, but a few
can have permanent kidney damage that eventually requires dialysis
or a kidney transplant.
High blood pressure
(hypertension)
can result when the kidneys are impaired by disease. The kidneys
control blood pressure by regulating the amount of salt in the body
and by producing the enzyme renin that, along with other
substances, controls the constriction of muscle cells in the walls
of the blood vessels.
Kidney (renal) failure
can be acute (sudden) or chronic (occurring over time and usually
long lasting or permanent). In either form of kidney failure, the
kidneys slow down or stop filtering blood effectively, causing
waste products and toxic substances to build up in the blood.
Acute kidney failure may be due to many things, including a
bacterial infection, injury, shock, heart failure, poisoning, or
drug overdose. Treatment includes correcting the problem that led
to the failure and sometimes requires surgery or dialysis. Dialysis
involves using a machine or other artificial device to remove the
excess salts and water and other wastes from the body when the
kidneys are unable to perform this function.
Chronic kidney failure involves a deterioration of kidney
function over time. In children, it can result from acute kidney
failure that fails to improve, birth defects of the kidney,
chronic kidney diseases
, repeated kidney infections, or chronic severe high blood
pressure. If diagnosed early, chronic kidney failure in children
can be treated but usually not reversed. The child will usually
require a kidney transplant at some point in the future.
Kidney stones
(or nephrolithiasis) result from the buildup of crystallized salts
and minerals such as calcium in the urinary tract. Stones (also
called calculi) can also form after an infection. If kidney stones
are large enough to block the kidney or ureter, they can cause
severe abdominal pain. But the stones usually pass through the
urinary tract on their own. In some cases, they may need to be
removed surgically.
Nephritis
is any inflammation of the kidney. It can be caused by infection,
medications, an autoimmune disease (such as
lupus
), or it may be idiopathic (which means the exact cause may not be
known or understood). Nephritis is generally detected by protein
and blood in the urine.
Nephrotic syndrome
is a type of kidney disease that leads to loss of protein in the
urine and swelling of the face (often the eyes) or body (often
around the genitals). It is most common in children younger than 6
years old and is more prevalent in boys. Nephrotic syndrome is
often treated with steroids.
Urinary tract infections
(UTIs)
are usually caused by intestinal bacteria, such as
E. coli
, normally found in feces. These bacteria can cause infections
anywhere in the urinary tract, including the kidneys. Most UTIs
occur in the lower urinary tract, in the bladder and urethra. UTIs
occur in both boys and girls. However, uncircumcised males are
about 3 to 12 times more likely than circumcised males to
develop a UTI before age 1. Although uncircumcised males are about
3 to 12 times more likely than circumcised males to develop a UTI
before age 1, most experts don't believe that this is a strong
enough reason to recommend routine circumcision for boys. In
school-age children, girls are more likely to develop UTIs than
boys; this may be because girls have shorter urethras than
boys.
Vesicoureteral reflux (VUR)
is a condition in which urine abnormally flows backward (or
refluxes) from the bladder into the ureters. It may even reach the
kidneys, where infection and scarring can occur over time. VUR
occurs in 1% of children and tends to run in families. It's
often detected after a young child has a first urinary tract
infection. Most kids outgrow mild forms of VUR, but some can
develop permanent kidney damage and kidney failure later in
life.
Wilms' tumor
is the most common kidney cancer occurring in children. It is
diagnosed most commonly between 2 and 5 years of age and affects
males and females equally.
Reviewed by:
Laszlo Hopp, MD
Date reviewed: March 2009
Note: All information is for educational purposes only. For specific medical advice,
diagnoses, and treatment, consult your doctor.
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