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 bloodstream. 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 tomaintain Homeostasis!!!

The one million nephrons in each human kidney are amazingly efficient atselectively removing wastes from the blood while at the same timeconserving water, salt ions, glucose and other needed materials. Thenephrons accomplish this task in 3 main steps; these 3 steps are also calledthe 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 membraneof 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 ureamolecules are all small enough to go through the membrane pores intoBowman's capsule. Blood cells (rbc, wbc and platelets) and proteins on theother 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 isidentical to blood plasma minus the blood cells and proteins. Filtrate is saidto be isotonic to blood plasma with respect to its concentration of water, saltions, glucose, amino acids and urea. The filtrate will then proceed fromBowman'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 theblood stream. That is, materials that could still be used by the body are sent

back to the blood. Reabsorption happens as filtrate passes sequentiallythrough the nephron. Materials re-entering the blood stream do so throughthe capillary network surrounding the nephrons. In short, "good" stuff is sentfrom the nephron back into the blood.

A. Proximal Convoluted Tubule (PCn:

As the filtrate enters the PCT approximately 80% of the salt ions (sodiumand potassium), glucose and amino acids are ACTIVELY

TRANSPORTED out of the PCT and back into the blood stream byspecial "pumping" cells located in the walls of the PCT. The process ofactive transport requires energy. Energy in the form of ATP is suppliedby 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 positivelycharged sodium and potassium ions (Na+, K+). As the concentration ofthe 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 ofHenle, 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. Collecting 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 is 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

water out of the collecting duct before it leaves

the kidney, It does so by creating a salty

environment in the medulla area of the

kidney.

The ascending loop of Henle actively

transports chlorine ions out of the filtrate with

carrier proteins. Chlorine builds up in the

fluids of the medulla by active transport.

Because it is a negative ion, it creates a cause

for the sodium ion, which is positive, to rush out

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

running out of salt. There is no need to worry.

The salt trick can continue because the

descending loop in its wisdom is stealing back

the salt that the ascending loop is so generously

releasing. This helps to keep a constant flow of

salt inside the loop for the ascending lope to

pump out . Because of the generosity gfthe

ascending loop and the stinginess of the

descending loop a salt trade or salt current is

established as the salt moves ouc of the

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.

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