Blood Cleaning by the Kidneys - Part (2)
(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
- Thick Ascending Limb of Loop of Henle
ascending limb of Henle (and its continuation into
the first part
of the DCT), reabsorbs NaCl from the tubular fluid
via a different transport process from that of the
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
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
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
Click here to view the next page: Blood Cleaning
by the Kidneys - Part
(3): Tubular Secretion.