Blood Cleaning by the Kidneys
This page is about processes performed by the kidneys in order
to filter (clean) blood.
Filtration also called "Ultra-filtration",
Reabsorption also called "Selective Re-Absorption"
As these occur in the kidney
nephrons, it is best to read this page after viewing
the page about the
structure of a kidney nephron.
(1) Glomerular Filtration
Blood enters the kidney via the renal
many times (Renal Artery -> Segmental Arteries -> Interlobar
Arteries -> Arcuate Arteries -> Interlobular Arteries -> Afferent
Arterioles), eventually forming many afferent arterioles,
each of which delivers blood to an individual
The diameter of the afferent (incoming) arteriole is greater
than the diameter of the efferent arteriole (by which
blood leaves the glomerulus). The pressure of the
blood inside the glomerulus is increased due to the
difference in diameter of the incoming and out-going
This increased blood pressure helps
to force the following components of the blood out of the glomerular
- Most of the water;
- Most/all of the salts;
- Most/all of the glucose;
- Most/all of the urea.
The above are filtered in preference to other components
of blood based on particle size. (Water and
solutes of relative molecular mass less than 68,000 form
the filtrate.) Blood cells and plasma proteins are not filtered through
the glomerular capillaries because they are relatively
larger in physical size.
The water and salts that have been forced out of the glomerular
capillaries pass into the Bowman's Capsule and are called
the glomerular filtrate.
This glomerular filtrate is formed at a rate of above 125
minute in humans. This volume is approx.
20% of the plasma delivered during that time. (Again:
It contains all
present in the
blood except blood cells and most proteins - which are
too large to cross the basement membrane of the glomerulus.)
Click here to read more about the Glomerular
Filtration Rate (GFR) - link coming soon.
The glomerular filtrate passes from the renal
corpuscle to the renal
(2) Tubular Reabsorption
Only about 1% of the glomerular fitrate actually leaves the
body because the rest (the other 99%) is reabsorbed into the
blood while it passes through the renal tubules and ducts.
This is called tubular reabsorption and
occurs via three mechanisms. They are:
- Diffusion, and
- Active Transport.
Reabsorption varies according to the body's needs, enabling
the body to retain most of its nutrients.
The processes of tubular reabsorption occur in the following
order (see also the diagram
of a kidney nephron to trace the flow of fluid through
the 3 stages outlined below):
In the PCT
Most of the volume of the fitrate solution is reabsobed
in the proximal convoluted
tubule (PCT). This includes some water
and most/all of the glucose (except in the case
Most of the energy consumed by the kidneys is used in the
reabsorption of sodium ions (Na+), which
are solutes - that is, they are dissolved in the water
component of the fitrate solution.
As the concentration of Na+ in the filtrate
solution is high (about the same as the concentration
of Na+ in blood plasma), Na+
moves from the tubular fluid into the cells of the
PCT. In the cases of many Na+ ions this
occurs with the help of symporters.
Symporters simultaneously facilitate passage through
the PCT membrane of both Na+ and another
substances/solutes. Other such substances that are
reabsorbed with Na+ in this way include
glucose (an important type of sugar), amino
acids, lactic acid, and bicarbonate ions (HCO3-).
These then move on through cells via diffusion
and/or other transport processes.
A short way to summarize the above is to say that solutes
are selectively moved from the glomular filtrate to
by active transport.
(However, almost all glucose and amino acids, and
high but variable
of ions, are
reabsorbed again later - see the next section, below).
Following the movement of solutes (including Na+ ),
water is then also reabsorbed by osmosis.
About 80% of the filtrate volume is reabsorbed in this
As this part of the reabsorption process is not controlled
it is sometimes called obligatory
In the Loop of Henle
The remaining water (together with the dissolved salts
and urea) passes from the PCT into the descending limb
of Henle. It then passes along the Loop of Henle, and up
the ascending limb of Henle.
The different permeability properties of the two limbs
of the Loop of Henle, together with their counterflow arrangement,
allows a countercurrent multiplication to
generate a high solute concentration in the tissue fluid
of the medulla
(that is, outside of the tubules). The highest solute concentrations
are generated deep in the medulla. This is explained as
- Descending Limb of Loop of Henle
The epithelium lining of the descending limb of Henle is
relatively permeable to water - but much much less permeable
to the salts Na+ and Cl-, and to
urea. Therefore water gradually moves from the descending
limb and into the interstitium (surrounding the tubules)
as fluid flows through this part of the system of renal
- Thin Ascending Limb of Loop of Henle
The thin ascending limb of Henle differs from the descending
limb in that it is impermeable to water (so the water
that is inside the tubule at this stage generally remains
inside it), but is highly permeable to Na+ and
and somewhat permeable to urea. Therefore while
the tubular fluid flows back towards the renal cortex,
Cl- (which are more concentrated in the tubular
fluid than in the interstital fluid) diffuse from the
tubules into the interstitium. Some urea also enters
the tubules at this stage - but the loss of NaCl from
the tubular fluid greatly exceeds the gain in urea.
- Thick Ascending Limb of Loop of Henle
ascending limb of Henle (and its continuation into the
of the DCT), reabsorbs NaCl from the tubular fluid via
a different transport process from that of the thin ascending
limb of Henle.
The overall effect of the processes outlined above is
that the concentation of the fluid inside the renal tubules
that form the Loop of Henle is highest at the deepest part
of the renal medulla, and is less concentrated in the renal
cortex. This is what is meant by the "concentration gradient"
of the Loop of Henle. The term "counter-current" is also
used in descriptions of the Loop of Henle - and refers
to the tubular fluid flowing in opposite directions along
the descending and ascending limbs (as indicated by the
thin red arrows in the diagram above.
In the DCT
The water, urea, and salts contained within
the ascending limb of Henle eventually pass into the distal
convoluted tubule (DCT).
The DCT reacts to the amount of anti-diuretic
hormone (ADH) in the blood:
- The more ADH is present
in the blood, the more water is re-absorbed into it.
This happens because the presence of ADH in the blood
causes the cells in the last section
of the DCT (and associated tubules and collecting ducts)
more permeable to water, therefore they allow more
water to pass from the tubular fluid back into the blood.
This results in more concentrated
- The opposite is also true, i.e. if the level of ADH
in the blood is reduced then the cells in the latter
sections of the DCT (and associated tubules and collecting
ducts) becomes less permeable to water therefore less
water is able to pass from the tubular fluid back into
the blood - which results in less concentrated urine.
The amount of ADH in the blood may be affected by conditions such as diabetes insipidus, or by consumption of diuretics*
in the diet (*substances that occur in some foods and
(3) Tubular Secretion
The third process by which the kidneys clean blood (regulating
its composition and volume) is called tubular
secretion and involves
substances being added to the tubular fluid. This removes
excessive quantities of certain dissolved substances from the
body, and also maintains the blood at a normal healthy pH (which
is typically in
to pH 7.45).
The substances that are secreted into the tubular fluid (for
removal from the body) include:
- Potassium ions (K+),
- Hydrogen ions (H+),
- Ammonium ions (NH4+),
- some hormones, and
- some drugs (e.g. penicillin).
Tubular secretion occurs from the epithelial cells that line
the renal tubules and collecting ducts.
It is the tubular secretion of H+ and NH4+
from the blood into the tubular fluid (i.e. urine
- which is then excreted from the body via the ureter,
bladder, and urethra) that helps to keep blood pH
at its normal level. The movement of these ions also
helps to conserve sodium bicarbonate (NaHCO3).
The typical pH of urine is about 6.
... and finally
Urine formed via the three processes outlined above trickles
into the kidney pelvis. At this final stage it is only
approx. 1% of the originally filtered volume but includes
concentrations of ions.
The typical volume of urine produced by an average adult
is around 1.5 - 2.0 dm3 per
Quick Summary: The Processes of Blood Filtration by the Kidneys
- Glomerular Filtration
- Tubular Reabsorption
- Tubular Secretion