Did you know that your chances of developing a kidney stone
in your lifetime are 1 in 10? In 1995, more than 3 million
people in the United States had some type of kidney condition
such as an infection, kidney stones or cancer. More than
300,000 people suffer from renal failure each year and undergo
dialysis or await a kidney transplant.
But what do your kidneys do? Why are they so important?
Don't they just produce urine? In this edition of How Stuff
Works, we will take a close look at our kidneys and
find out exactly what they do.
A Look at Your Kidneys
kidneys are two bean-shaped organs, each about the size
of your fist. They are located in the middle of your back,
just below your rib cage, on either side of your spine. Your
kidneys weigh about 0.5 percent of your total body weight.
Although the kidneys are small organs by weight, they receive
a huge amount -- 20 percent -- of the blood pumped
by the heart. The
large blood supply to your kidneys enables them to do the
Regulate the composition of your blood
keep the concentrations of various ions and other
important substances constant
keep the volume of water in your body constant
remove wastes from your body (urea, ammonia, drugs,
keep the acid/base concentration of your blood
receive the blood from the renal artery, process it,
return the processed blood to the body through the renal
vein and remove the wastes and other unwanted substances
in the urine. Urine flows from the kidneys through the ureters
to the bladder. In the bladder, the urine is stored until it
is excreted from the body through the urethra.
To understand how the kidney does these impressive jobs, we
need to take a closer look inside the organ.
Inside the Kidney If you were to cut a
kidney in half, you would see the following parts:
renal capsule - a thin, outer membrane that helps
protect the kidney
cortex - a lightly colored outer region
medulla - a darker, reddish-brown, inner region
renal pelvis - a flat, funnel shaped cavity that
collects the urine into the ureters
Diagram showing the parts of the kidney and
If you look closely at the cortex and medulla, you can see
many tiny, tubular structures that stretch across both regions
perpendicular to the surface of the kidney. In each kidney,
there are one million of these structures, called nephrons.
The nephron is the basic unit of the kidney. It is a
long thin tube that is closed at one end, has two twisted
regions interspaced with a long hair-pin loop, ends in a long
straight portion and is surrounded by capillaries.
The parts of the nephron are as follows:
Bowman's capsule - closed end at the beginning of
the nephron. It is located in the cortex.
Proximal convoluted tubule or proximal
tubule - first twisted region after the Bowman's
capsule. It is also in the cortex.
Loop of Henle - long, hairpin loop after the
proximal tubule. It extends from the cortex down into the
medulla and back.
Distal convoluted tubule or distal tubule
- second twisted portion of the nephron after the loop of
Henle. It is also in the cortex.
Collecting duct - long straight portion after the
distal tubule that is the open end of the nephron. It
extends from the cortex down through the medulla.
Each part of the nephron has different types of cells with
different properties -- this is important in understanding how
the kidney regulates the composition of the blood.
The nephron has a unique blood supply compared to other
Afferent arteriole - connects the renal artery
with the glomerular capillaries.
Glomerular capillaries - coiled capillaries that
are inside the Bowman's capsule.
Efferent arteriole - connects the glomerular
capillaries with the peritubular capillaries.
Peritubular capillaries - located after the
glomerular capillaries and surrounding the proximal tubule,
loop of Henle, and distal tubule.
Interlobular veins - drain the peritubular
capillaries into the renal vein.
The kidney is the
only organ of the body in which two capillary beds, in series,
connect arteries with veins. This arrangement is important for
maintaining a constant blood flow through and around the
nephron despite fluctuations in systemic blood pressure.
Now that we know the anatomy of the kidney and the nephron,
let's look at how these parts allow the kidney to do its jobs.
Regulating Blood Composition Regulating the
composition of the blood involves the following:
Keeping the concentrations of various ions and other
important substances constant.
Keeping the volume of water in your body constant.
Removing wastes from your body.
Keeping the acid/base concentration of your blood
The kidney does this by a combination of
It filters 20 percent of the plasma and non-cell
elements from the blood into the inside of the nephron (the
It reabsorbs the components that the body needs
from the lumen back into the blood.
It secretes some unwanted components from the
blood into the lumen of the nephron.
(fluid, ions, small molecules) that has not been reabsorbed
from the lumen gets swept away to form the urine, which
ultimately leaves the body. Through these processes, the blood
is maintained with the proper composition, and excess or
unwanted substances are removed from the blood into the urine.
Kidney Processes The kidneys regulate blood
composition by three main processes:
Blood in Urine
filtrate only includes small molecules and water. No red blood
cells get filtered. Therefore, no blood appears in
the urine under normal conditions. If you find blood in
your urine, you should contact your physician as soon as
possible because it could be a sign of kidney problems.
In the nephron, approximately
20 percent of the blood gets filtered under pressure
through the walls of the glomerular capillaries and Bowman's
capsule. The filtrate is composed of water, ions (e.g. sodium,
potassium, chloride), glucose and small proteins (less than
30,000 daltons -- a dalton is a unit of molecular weight). The
rate of filtration is approximately 125 ml/min or 45 gallons
(180 liters) each day. Considering that you have 7 to 8 liters
of blood in your body, this means that your entire blood
volume gets filtered approximately 20 to 25 times each day!
Also, the amount of any substance that gets filtered is the
product of the concentration of that substance in the blood
and the rate of filtration. So the higher the concentration,
the greater the amount filtered or the greater the filtration
rate, the more substance gets filtered.
This filtration process is much like the making of espresso
or cappuccino. In a cappuccino machine, water is forced under
pressure through a fine sieve containing ground coffee; the
filtrate is the brewed coffee. The arrangement of the
glomerular capillaries in series with the
peritubular capillaries is important to maintain a
constant pressure in the glomerular capillaries, and thus a
constant rate of filtration, despite momentary fluctuations in
blood pressure. Once the filtrate has entered the Bowman's
capsule, it flows through the lumen of the nephron into the
Reabsorption Once inside
the lumen of the nephron, small molecules, such as ions,
glucose and amino acids,
get reabsorbed from the filtrate:
called transporters are located on the membranes
of the various cells of the nephron.
These transporters grab the small molecules from the
filtrate as it flows by them.
Each transporter grabs only one or two types of
molecules. For example, glucose is reabsorbed by a
transporter that also grabs sodium.
Transporters are concentrated in different parts of the
nephron. For example, most of the Na transporters are
located in the proximal tubule, while fewer ones are spread
out through other segments.
Some transporters require energy, usually in the form of
triphosphate (active transport), while others do
not (passive transport).
Water gets reabsorbed passively by osmosis
in response to the buildup of reabsorbed Na in spaces
between the cells that form the walls of the nephron.
Other molecules get reabsorbed passively when they are
caught up in the flow of water (solvent drag).
Reabsorption of most substances is related to the
reabsorption of Na, either directly, via sharing a
transporter, or indirectly via solvent drag, which is set up
by the reabsorption of Na.
Click on the menu items. The reabsorption process in various segments of the
The reabsorption process is similar to the "fish pond" game
that you see in some amusement parks or state fairs. In these
games, there is a "stream" that contains different colored
plastic fish with magnets. The little children playing the
game each have a fishing pole with an attached magnet to catch
the fish as they move by. Different colored fish have
different prize values associated with them, so some children
will be selective and try to grab the colored fish with the
highest prize value. Now suppose our nephron is the stream,
the filtered molecules are the various colored fish, and our
children are the transporters. Furthermore, each child is
fishing for a specific colored fish. Most children start at
the beginning of the stream and some spread out further
downstream. By the end of the stream, most of the fish have
been caught. This is what happens as the filtrate travels
through the nephron.
Two major factors affect the reabsorption process:
Concentration of small molecules in the filtrate - the
higher the concentration, the more molecules can be
reabsorbed. Like our children in the fish pond game, if you
increase the number of fish in the stream, the children will
have an easier time catching them.
In the kidney, this is true only to a certain extent
There is only a fixed number of transporters for a
given molecule present in the nephron.
There is a limit to how many molecules the
transporters can grab in a given period of time.
Rate of flow of the filtrate - flow rate affects the
time available for the transporters to reabsorb molecules.
As with our fish pond, if the stream moves by slowly, the
children will have more time to catch fish than if the
stream were moving faster.
To give you an idea of
the quantity of reabsorption across the nephron, let's look at
the sodium ion (Na) as an example:
Proximal tubule - reabsorbs 65 percent of filtered Na.
In addition, the proximal tubule passively reabsorbs about
2/3 of water and most other substances.
Loop of Henle - reabsorbs 25 percent of filtered Na.
Distal tubule - reabsorbs 8 percent of filtered Na.
Collecting duct - reabsorbs the remaining 2 percent only
if the hormone aldosterone
Secretion Some substances
are secreted from the plasma into the lumen by the cells of
the nephron. Examples of such substances are ammonia
(NH3). As in reabsorption,
there are transporters on the cells that can move these
specific substances into the lumen.
In the next section, we'll see what happens when you put
all of these processes together.
Balance Now let's put all of these processes
-- filtration, reabsorption and secretion -- together to
understand how the kidneys maintain a constant composition of
the blood. Let's say that you decide to eat several bags of
salty (NaCl) potato chips at one sitting. The Na will be
absorbed into your blood by your intestines, increasing the
concentration of Na in your blood. The increased Na in the
blood will be filtered into the nephron. While the Na
transporters will attempt to reabsorb all of the filtered Na,
it is likely that the amount will exceed their ability.
Therefore, excess Na will remain in the lumen; water will also
remain, due to osmosis. The excess Na will be excreted into
the urine and eliminated from the body. So whether a substance
remains in the blood depends on the amount filtered into the
nephron and the amount reabsorbed or secreted by various
Let's look at an another example: Why do you have to keep
taking repeated doses of any given medicine? Well, once you
take the medicine, it gets absorbed by the intestine into the
blood. The medicine in the blood acts on its target cell and
also gets filtered into the nephron. Most medicines do not
have transporters in the nephron to reabsorb them from the
filtrate. In fact, some transporters actively secrete
medicines into the nephron. Therefore, the medicine gets
eliminated in the urine and you must take another dosage
We have seen how the kidney can regulate ions and small
molecules and eliminate unwanted substances. In the next
section, we will see how the kidney maintains water balance.
Maintaining Water Volume Your kidneys have
the ability to conserve or waste water. For example, if you
drink a large glass of water, you will find that you will have
the urge to urinate within an hour or so. In contrast, if you
do not drink for a while, such as overnight, you will not
produce much urine and it will usually be very concentrated
(i.e. darker). How does your kidney know the difference? The
answer to this question involves two mechanisms:
The structure and transport properties of the loop of
Henle in the nephron.
The anti-diuretic hormone (ADH), also called
vasopressin, secreted by the pituitary gland.
Loop of Henle The
loop of Henle has a descending limb and an ascending limb. As
filtrate moves down the loop of Henle, water is reabsorbed,
but ions (Na,Cl) are not. The removal of water serves to
concentrate the Na and Cl in the lumen. Now, as the filtrate
moves up the other side (ascending limb), Na and Cl are
reabsorbed, but water is not. What these two transport
properties do is set up a concentration difference in NaCl
along the length of the loop, with the highest concentration
at the bottom and lowest concentration at the top. The loop of
Henle can then concentrate NaCl in the medulla. The longer the
loop, the bigger the concentration gradient. This also means
that the medulla tissue tends to be saltier than the cortex
Now, as the filtrate flows through the collecting ducts,
which go back down through the medulla, water can be
reabsorbed from the filtrate by osmosis. Water moves from an
area of low Na concentration (high water concentration) in the
collecting ducts to an area of high Na concentration (low
water concentration) in the medullary tissue. If you remove
water from the filtrate at this final stage, you can
concentrate the urine.
(ADH) ADH, which is secreted by the pituitary
gland, controls the ability of water to pass through the cells
in the walls of the collecting ducts. If no ADH is present,
then no water can pass through the walls of the ducts. The
more ADH present, the more water can pass through.
Specialized nerve cells, called osmoreceptors, in
the hypothalamus of the brain sense the Na
concentration of the blood. The nerve endings of these
osmoreceptors are located in the posterior pituitary gland and
secrete ADH. If the Na concentration of the blood is high, the
osmoreceptors secrete ADH. If the Na concentration of the
blood is low, they do not secrete ADH. In reality, there is
always some very low level of ADH secreted from the
Now let's look at how your kidneys maintain water balance.
Why You Urinate Soon After Drinking
a Large Glass of Water When you drink a large glass
of water, the water gets absorbed into the blood and the
The absorbed water increases the amount of water
filtered in the glomerulus.
The absorbed water in the blood reduces the Na
concentration a little.
The reduced Na concentration lowers the amount of Na
filtered in the glomerulus.
The nephron reabsorbs all of the reduced Na load and
some of the accompanying water, leaving excess water in the
The reduced Na concentration is sensed by the
The osmoreceptors do not secrete as much ADH.
Because the collecting ducts do not see as much ADH,
they do not allow much water to be reabsorbed in response to
the Na concentration gradient set up by the loop of Henle.
The excess water gets excreted in the urine.
When the excess water is excreted, the Na concentration
of the blood returns to normal.
Why You Have Concentrated Urine in
the Morning Typically, we do not drink water
overnight when we sleep. So, our intestines are not absorbing
Decreased water absorption by the intestine reduces the
amount of water in the blood.
Decreased water in the blood reduces the amount of water
filtered in the glomerulus.
Decreased water in the blood increases the Na
concentration in the blood.
Increased Na concentration in the blood increases the
amount of Na filtered in the glomerulus.
The nephron does not reabsorb all of the filtered Na,
and some water remains with it in the filtrate.
The increased Na concentration in the blood is sensed by
The osmoreceptors secrete ADH.
The collecting ducts see more ADH and allow water to be
reabsorbed in response to the Na concentration gradient set
up by the loop of Henle.
More water gets reabsorbed from the collecting ducts,
producing a concentrated urine. A little water is lost in
the urine because of the Na; we cannot excrete solid urine.
The removal of Na and increased reabsorption of water
help return the blood concentration of Na to normal.
So, the loop of Henle sets up the Na concentration
gradient across the medulla, allowing for water to be
reabsorbed from the collecting ducts, and ADH allows the water
to pass through those collecting ducts.
The only remaining aspect to the kidneys' regulation of
blood composition is how they maintain the acid/base balance
of the blood. We will look at this in the next section.
Altering Blood's Acid/Base Balance Your
blood maintains a constant concentration of hydrogen ion (pH)
by a chemical mixture of hydrogen ions and sodium
bicarbonate. The sodium bicarbonate is produced by the
carbon dioxide (CO 2)
formed in the cells as a byproduct of many chemical reactions.
The CO2 enters the blood in
the capillaries, where red blood
cells contain an enzyme called carbonic anhydrase
that helps combine CO 2 and
water (H 2O) to form
carbonic acid (H 2
CO3 ) quickly.
The carbonic acid formed then rapidly separates into
hydrogen ions (H+ ) and
bicarbonate ions (HCO3-). This
reaction can also proceed in the reverse direction, whereby
sodium bicarbonate plus hydrogen ion yields carbon dioxide and
Carbonic Anhydrase CO
2 + H 2 O
<---------> H 2
+ HCO 3
The correct pH is maintained by keeping the ratio of
hydrogen ion to bicarbonate in the blood constant. If you add
acid (hydrogen ion) to the blood, then you will reduce the
bicarbonate concentration and alter the pH of the blood.
Similarly, if you reduce the hydrogen ion by adding alkali,
you will increase the bicarbonate concentration and alter the
pH of the blood.
Now, the acid/base balance of our blood changes in response
to many things including:
- diets rich in meats provide acids to the bloods when
digested. In contrast, diets rich in fruits and vegetables
make our blood alkaline because they are rich in
- exercising muscles produce lactic acid that must be
eliminated from the body or metabolized.
- high altitude causes rapid breathing that makes our blood
alkaline. In contrast, certain lung
diseases that block the diffusion of oxygen can cause
the blood to be acidic.
The kidney can correct any
imbalances by removing excess acid (hydrogen ion) or bases
(bicarbonate) in the urine and restoring the
bicarbonate concentration in the blood to normal. The
kidney cells produce a constant amount of hydrogen ion and
bicarbonate because of their own cellular metabolism
(production of carbon dioxide). Through a carbonic anhydrase
reaction similar to the red blood cells, hydrogen ions get
produced and secreted into the lumen of the nephron. Also,
bicarbonate ions get produced and secreted into the blood. In
the lumen of the nephron, filtered bicarbonate combines with
secreted hydrogen ions to form carbon dioxide and water
(carbonic anhydrase is also present on the luminal surface of
the kidney cells). Whether the kidney removes hydrogen ions or
bicarbonate ions in the urine depends upon the amount of
bicarbonate filtered in the glomerulus from the blood relative
to the amount of hydrogen ions secreted by the kidney cells.
If the amount of filtered bicarbonate is greater than the
amount of secreted hydrogen ions, then bicarbonate will be
lost in the urine. Likewise, If the amount of secreted
hydrogen ion is greater than the amount of filtered
bicarbonate, then hydrogen ions will be lost in the urine
(i.e. acidic urine).
Let's consider a few examples:
Hydrogen ions added to the blood by breaking down a
meat-rich diet combine with bicarbonate in the blood and
form carbon dioxide and water.
This reaction reduces the bicarbonate concentration
and the pH in the blood.
The decreased bicarbonate concentration in the blood
reduces the amount of bicarbonate filtered in the
All of the filtered bicarbonate combines with the
hydrogen ion secreted by the kidney cells in the lumen to
form carbon dioxide and water.
Because the filtered load of bicarbonate was less than
the amount of hydrogen ion secreted by the kidney cells,
there is an excess of hydrogen ion in the urine.
The amount of bicarbonate secreted from the kidney
cells into the blood was equal to the hydrogen ion
secreted into the lumen and greater than the filtered load
of bicarbonate from the blood -- therefore, the blood has
a net gain of bicarbonate.
This process continues to lose hydrogen ions in the
urine and gain bicarbonate in the blood until the
concentrations of hydrogen (pH) and bicarbonate ions in
the blood are restored to normal.
Bicarbonate added to the blood from the fruit or
vegetable-rich diet combines with hydrogen ions to form
carbon dioxide and water.
This reaction reduces the hydrogen ion concentration
and increases the pH.
The increased bicarbonate concentration increases the
amount of bicarbonate filtered in the glomerulus.
The filtered bicarbonate exceeds the amount of
hydrogen ion secreted by the kidney cell, and excess
bicarbonate is lost in the urine.
The amount of bicarbonate secreted from the kidney
cells into the blood was equal to the hydrogen ions
secreted into the lumen and less than the filtered load of
bicarbonate from the blood -- therefore, the blood has a
net loss of bicarbonate.
This process continues to lose bicarbonate in the
urine and reduce the bicarbonate in the blood until the
concentrations of hydrogen (pH) and bicarbonate ions in
the blood are restored to normal.
we have seen how the kidneys regulate the composition of our
blood, let's look at how they help regulate our blood
Influencing Blood Pressure The blood
pressure in your body depends upon the following conditions:
The force of contraction of the heart - related to how
much the heart muscle gets stretched by the incoming blood.
The degree to which the arteries and arterioles
constrict - increases the resistance to blood flow, thus
requiring a higher blood pressure.
The circulating blood volume - the higher the
circulating blood volume, the more the heart muscle gets
stretched by the incoming blood.
influences blood pressure by:
Causing the arteries and veins to constrict.
Increasing the circulating blood volume.
How the Kidney Causes Blood Vessels
with chronic high blood pressure (hypertension)
often take a class of drugs called diuretics to
control their blood pressure. Diuretics reduce Na
reabsorption from the lumen of the nephron. Water
reabsorption is also reduced. Therefore, Na and water
are lost in the urine, which increases urine flow. The
decreased reabsorption of Na and water from the nephron
reduces blood volume, thereby reducing blood pressure.
Specialized cells are located
in a portion of the distal tubule located near and in the wall
of the afferent arteriole. The distal tubule cells (macula
densa) sense the Na in the filtrate, and the arterial
cells (juxtaglomerular cells) sense the blood pressure.
When the blood pressure drops, the amount of filtered Na also
drops. The juxtaglomerular cells sense the drop in blood
pressure and the decrease in Na is relayed to them by the
macula densa cells. The juxtaglomerular cells then release an
called renin. Renin converts another protein from the
blood called angiotensin I into angiotensin II.
Angiotensin II causes blood vessels to contract -- the
increased blood vessel constrictions elevate the blood
How the Kidney Increases the
Circulating Blood Volume Angiotensin II also
stimulates the adrenal gland to secrete a hormone called
aldosterone. Aldosterone stimulates more Na
reabsorption in the distal tubule, and water gets reabsorbed
along with the Na. The increased Na and water reabsorption
from the distal tubule reduces urine output and increases the
circulating blood volume. The increased blood volume helps
stretch the heart muscle and causes it to generate more
pressure with each beat, thereby increasing the blood
The actions taken by the kidney to regulate blood pressure
are especially important during traumatic injury, when they
are necessary to maintain blood pressure and conserve the loss
of fluids. Now that we have seen how the kidneys can affect
blood pressure, let's look at how the kidney plays an active
role in maintaining your body's calcium stores.
Kidneys and Calcium Your body stores calcium
in the bones, but also maintains a constant level of calcium
in the blood. If the blood calcium level falls, then the
parathyroid glands in your neck release a hormone called
parathyroid hormone. Parathyroid hormone increases
calcium reabsorption from the distal tubule of the nephron to
restore the blood calcium level. Parathyroid hormone also
stimulates calcium release from bone and calcium absorption
from the intestine.
In addition to parathyroid hormone, your body also requires
vitamin D to stimulate calcium absorption from the
kidney and intestine. Vitamin D is found in milk products. A
precursor to vitamin D (cholecalciferol) is made in the
skin and processed in the liver. However, the final step that
converts an inactive form of cholecalciferol into active
vitamin D occurs in the proximal tubule of the nephron. Once
activated, vitamin D stimulates calcium absorption from the
proximal tubule and from the intestine, thereby increasing
blood calcium levels.
Kidney stones are often caused by problems in the kidney's
ability to handle calcium. In addition, the kidney's role in
maintaining blood calcium is important in the bone disease
osteoporosis that afflicts many elderly people,
As you can see, the kidneys perform many functions that are
important to your body:
Controlling the composition of your blood and eliminate
wastes - filtration/reabsorption/secretion method.
Influencing blood pressure - renin secretion.
Helping to regulate your body's calcium - vitamin D
If the kidneys fail to function, then
renal dialysis methods (artificial filtration methods) can be
used to help you survive by cleansing the blood. This is
especially necessary when both kidneys fail. Although you have
two kidneys, it is possible to live with only one. One healthy
kidney can be donated and transplanted into a compatible
person with total kidney failure. Kidney transplants are a
common way to help those people survive and live a normal
For more information on the kidney, its functions and its
diseases, see the Links