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The Roles of the Nephron
of the 120 ml of blood that is filtered by the kidneys each minute, only I ml
(that's less than I%) turns into urine that will eventually leave the body
(after approximately 300 - 400 mis of it accumulates to fill the bladder!)
That leaves 119 ml of fluid called filtrate to be returned back to the blood
stream. Good thing, otherwise you would have to micturate (pee, urinate)
once every 3 minutes and drink 1 L of fluid every 10 minutes in order to
maintain Homeostasis!!!
The one million nephrons in each human kidney are amazingly efficient at
selectively removing wastes from the blood while at the same time
conserving water, salt ions, glucose and other needed materials. The
nephrons accomplish this task in 3 main steps; these 3 steps are also called
the 3 main roles of the nephron: Filtration, Reabsorption and Secretion.
Fiftratffln
Filtration is
aecomplished by the
movement of fluids from
the blood into the
Bowman's capsule.
Beabsorpttort
Reatuorptias wolves the
selective transfer of
essential solutes and
water back into the blood.
Secretion
Secretion Involves the
movement of wastes from
the blood into the
mphron.
1. Filtration
The renal artery carries blood into the kidney (approximately 600 mis of
blood enters a kidney each minute). The renal artery then branches into
arterioles which then branch intoaspecialized capillaries called the
glomerulus. Because of the great difference in diameter between the renal
artery and the glomerul us, blood entering the glomerulus is under very high
pressure. This pressure forces about 20% of the blood plasma (about 120m1
of the 600 ml) out of the glomerulus and across the membrane of Bowman's
capsule. Bowman's capsule acts to "filter" or separate some of the
substances that are located in blood plasma from others. This is because
some substances are small enough to fit through the pores of the membrane
of Bowman's capsule and some are too large and thus do not enter
Bowman's capsule with the rest of the blood plasma. Water, salt ions
(sodium, potassium and choride), glucose molecules, amino acids and urea
molecules are all small enough to go through the membrane pores into
Bowman's capsule. Blood cells (rbc, wbc and platelets) and proteins on the
other hand are too large to leave the capillaries or enter Bowman's capsule.
The fluid inside of Bowman's capsule gets a name change; it is now called
"filtrate" because it is blood plasma that has been filtered, This filtrate is
identical to blood plasma minus the blood cells and proteins. Filtrate is said
to be isotonic to blood plasma with respect to its concentration of water, salt
ions, glucose, amino acids and urea. The filtrate will then proceed from
Bowman's capsule through the rest of the nephron in the following order:
proximal convoluted tubule, loop of henle, distal convoluted tubule and
finally the collecting tubule. From the collecting tubule, the filtrate will enter
the pelvis of the kidney and be called urine.
2. Reabsorption
Useful materials such as sugars and salt ions are reabsorbed back into the
blood stream. That is, materials that could still be used by the body are sent
back to the blood. Reabsorption happens as filtrate passes sequentially
through the nephron. Materials re-entering the blood stream do so through
the capillary network surrounding the nephrons. In short, "good" stuff is sent
from the nephron back into the blood.
A. Proximal Convoluted Tubule (PCn:
As the filtrate enters the PCT approximately 80% of the salt ions (sodium
and potassium), glucose and amino acids are ACTIVELY
TRANSPORTED out of the PCT and back into the blood stream by
special "pumping" cells located in the walls of the PCT. The process of
active transport requires energy. Energy in the form of ATP is supplied
by the numerous mitochondria that are embedded in the walls of the
PCT. Because of ionic attraction, negatively charged chloride ions (CI-)
will flow passively out of the PCT as they are attracted by the positively
charged sodium and potassium ions (Na+, K+). As the concentration of
the above mentioned solute molecules drops inside of the PCT, water
then diffuses out of the PCT and into the capillary network passively by
the process of OSMOSIS. The lining of the PCT contains microvilli to
increase the surface area over which this reabsorption can occur.
B. Descending Loop of Henle:
As the filtrate travels into the descending Loop of Henle, both sodium
and potassium ions passively diffuse from the salty tissues of the
surrounding medulla BACK INTO the Loop of Henle. (Although this is
reabsorption of materials, the materials are not going back into the blood
stream at this point). At the same time, water continues to move out of
the Loop of Henle and into the capillary network by osmosis. The filtrate
at this point is more concentrated (hypertonic) with respect to salt ions
than it was in the PCT, both because water has been removed from it, and
because salt ions have been again added to it.
C. AscendineLoop of Henle
As the filtrate proceeds up into the ascending Loop of Henle, the choride
ions are actively pumped back out of the nephron. Because of ionic
attraction, sodium ions then passively follow the chloride ions out of the
tubule and into the tissues of the medulla. These ions only move into the
medulla and not back into the blood stream. Since the ascending Loop of
Henle is impermeable to water, water cannot leave this part of the
nephron. Because of this, the filtrate gets more dilute again.
Because the opposite happens in the ascending and descending Loops of
Henle, the process is called the COUNTER CURRENT
MECHANISM. The process is also known as the CHLORIDE SfUFT.
D. Distal Convoluted Tubule (DCT)
As the filtrate passes through this part of the nephron, water continues to
passively diffuse out of the nephron and back into the blood. Water
continues to diffuse out of the DCT because the surrounding tissues of
the medulla are now very salty due to so much sodium and chloride ions
accumulating there. The salty tissues attract the water out of the DCT
because the medulla is hypertonic with respect to salt concentration when
compared with the salt concentration of the filtrate (now hypotonic).
Water that enters the medulla will then diffuse back into the blood
stream. Because of the continual re-absorption of water, the filtrate
becomes more and more concentrated with wastes, mainly urea. The
amount of water that diffuses can be regulated by a hormone called
ADH. The amount of water that diffuses from the DCT back into the
blood depends on the needs of the body; if the body is dehydrated, more
water will go back into the blood, and less will be left in the nephron to
make less urine. The opposite occurs if the body is over hydrated.
E. Collectin g Tubule C
The same thing that occurs in the DCT also occurs in the CT
3. Secretion
Occurring at the same time as reabsorption is a process called secretion.
Secretion is when a cell releases a substance to its outside... in this case,
non-useful and toxic substances are ACTIVELY TRANSPORTED from the
blood into the nephron - usually in the regions of the distal and proximal
convoluted tubules. Substances which are secreted include excess acid (H+)
or base (OH-) ions, excess glucose (high glucose levels are found in diabetic
urine or urine of someone who has recently consumed a large amount of
sugar-this is the kidney's way of helping to ensure that the blood sugar
level doesn't get too high), ammonia, and drugs (this is why urine is used
from many drug tests - the breakdown of many drugs including marijuana,
cocaine, heroin, sleeping pills, codeine and many other medications can be
detected even in minute amounts in the urine). The process of secretion
ensures that materials that are potentially harmful to the body are quickly
disposed of by being "dumped" into the fluid that is about to become urine.
Secretion happens mainly in the regions of the DCT and CT but some also
occurs in the PCT. In short, secretion involves "bad" stuff being removed
from the blood being sent to the urine.
The Counter Current Mechanism of the
Nephron
The loop of Henle works toward the
goal of water conservation. Animals that live
in a terrestrial environment need to be careful
not to waste water. It is clearly a waste,if water
is in short supply, to release too much water
with the urine. As a result there needs to be a
mechanism to encourage water out of the urine
and back into the blood. The loop of Henle
creates that
mechanism in terrestrial
animals.
There i s no way of actively capturing
water in the urine that is passing through the
collecting ducts. It would almost seem too late
to capture the water that is already on its way
out of the body. However, the nifty nephron
creates a trick with its loop of lienle to get the
of the loop after chlorine. The sodium rushes
out by diffusion because of its 'fatal attraction'
to chlorine. The chlorine and the sodium ions
collect and dominate the fluids outside the loop
of I-fenle creating a salty environment. This
salty environment catches the attention of the
water that is passing through the nearby
collecting duct.
The collecting duct is
permeable to water but not permeable to the
salt.
Water can. not resist moving into the
salty medulla.
The salt creates an osmotic
pressure that pulls the water out of the collecting
duct by osmosis. (Water has a 'fatal attraction'
to salty solutions.) Once the water is out of the
duct it is no longer destined for elimination but
can' now be picked up by the nearby. ,Mood
capillaries and returned to be used by;.body
systems.
Meanwhile, back at the loop of`-•Henle,
trouble is starting. The ascending loop is
water out of the collecting duct before it leaves
running out of salt. There is no need to worry.
It does so by creating a salty
The salt trick can continue because the
environment in the medulla area of the
descending loop in its wisdom is stealing back
kidney.
the salt that the ascending loop is so generously
the kidney,
The ascending loop of Henle actively
releasing. This helps to keep a constant flow of
transports chlorine ions out of the filtrate with
salt inside the loop for the ascending lope to
Chlorine builds up in the
pump out . Because of the generosity gfthe
fluids of the medulla by active transport.
ascending loop and the stinginess of the
Because it is a negative ion, it creates a cause
descending loop a salt trade or salt current is
for the sodium ion, which is positive, to rush out
established as the salt moves ouc of the
carrier proteins.
ascen ding loop and into the descending loop,
This salt current established by the loop of
lienle maintains an environment that attracts
water out of the ducts containing urine and
back into the blood. This process is called the
counter current mechanism.
(ER 25
Nctwp
ruti"n
-i^ Urea _,
EXCRETION AND WE BALANCING OF WATER AN
i
Tubular
secretion
How Materials Move Into and Out of the Nephron As Urine Forms. As a
Al.
nephron extends through the kidney's cortex and medulla and dumps urine into the collecting duct,
various substances enter and leave the filtrate. Broken lines represent segments of the nephron wall
that are permeable to water, while solid lines represent wall segments impermeable to water. Narrow
arrows represent passive diffusion of materials into or out of the nephron tubule, while wide arrows
represent active transport against concentration gradients. Filtration activities are shown in blue,
tubu.l 'eabsorption activities in green, and tubular secretion in yellow. U ne is shown as yellow.
traces nephron function and material movements step by step.
The t
Cl
I
FI2u
Cw^^1ed
^- t
om)
wastes and needed
substances filtered
into capsule
`
blood with high
proportion of
waste products
•
0
•
•
•
.•
••
•
:`•a •
•
0
reabsorption of needed products
FU",t.Ic r1<3.N4 Cliff ysr aL ^G'`^
blood with lower
proportion of wastes
and reabsorbed
nutrients and ions
^"-
flow of flit:ate o: Jr -)e
^"" c CtIVe ::
r r, of Na4'
Pas ve Ci Ii nsW r of Na+
pd.tijl'Je L:t.`usif. it rd
H,C)
!ggmre5 indwam sodium concentration in hundred
i r iiinormality