Renal regulation of pH: Acid-base balance
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Renal regulation of pH: Acid-base balance

October 6, 2019


Hello everyone, welcome back today we are
going to learn about the renal mechanism of acid-base balance that is renal
regulation of pH. We know there are three defense mechanism in acid-base balance
or regulation of pH. The first-line defense mechanism is the buffer
mechanism and the second line defense mechanism is respiratory regulation
so this renal regulation of pH is third line
defense mechanism or tertiary defense mechanism or regulatory mechanism of
acid-base balance. So compared to buffer mechanism and respiratory mechanism this
renal mechanism is very slow will take a long duration. So in the kidney the
functional unit of kidney is called nephron So imagine this is a nephron
there are arteries, anastomosis of arteries and efferent arteriole and afferent
arteriole, whatever blood present in the arteries filtered to this tubular lumen through Bowman’s capsule. So now we know that the pH of blood is around 7.35 to
7.45 or in an average it is 7.4 and the tubular fluid the beginning when it is
filtered from the plasma through the Bowman’s capsule to the proximal
convoluted tubules so the pH of this ultra filtrate is
approximately 7.4 but when it goes down and down through distal convoluted tubule
loops of Henle collecting and ultimately converted to urine then
the urine pH will become approximately 6. Generally, they say the pH of urine is
acidic. So it will range from around 4-8 The average is 6. How does this happen? This happens mainly by conserving the bases, conservation of base mainly in the
form of bicarbonate and secretion or excretion of acids. So this is the whole
process taking place in our kidney that means urine will be acidified, there
are many mechanism going to happen throughout this process. This acidification
or the regulation of pH by the kidney happens with 4 mechanisms. The first
mechanism is excretion or secretion of hydrogen ion, so this is mainly by
proximal convoluted tubule and secretion of hydrogen ions to the tubular lumen
by the tubular epithelial cell and the second mechanism of renal regulation of
pH is reabsorption of bicarbonate reabsorption of bicarbonate or recovery
or reclamation of bicarbonate. The third mechanism in renal regulation of pH is
excretion of titratable acids is mainly in the form of
phosphoric acid and the last mechanism in renal regulation of pH is excretion
of ammonium so these are the four main mechanism by which kidney regulates the
normal pH between 7.35 to 7.45. So we are going to study this mechanism one by one.
So now imagine this is a renal tubular cell
and this is the capillaries around the renal tubular cells or blood and this is
the actually tubular lumen. Now we are going to study the first mechanism that
is a secretion of hydrogen ion. The tubular fluid is nothing but ultrafiltrate of plasma usually contains sodium chloride , bicarbonate, phosphate,
glucose, amino acids through this process of formation of urine many compounds
which are required by the body are reabsorbed.
So whatever not required it will be secreted to the tubular lumen. Now in
the tubular epithelial cells or renal tubular cells as a result of tissue
metabolism, so there is a generation of carbon dioxide and water. So we know that
after glycolysis and TCA cycle there is a generation of carbon dioxide and water
this carbon dioxide and water with the help of carbonic anhydrase enzyme it
will be converted to carbonic acid. so the enzyme here is carbonic anhydrase so
this carbonic acid it is unstable is immediately converted to hydrogen ion
and bicarbonate this hydrogen ion will be secreted to the tubular fluid in
exchange with sodium, so the hydrogen ion will be secreted into the tubular fluid
at the same time the sodium will be going into the tubular epithelial cells
from the tubular fluid. So this is called antiport. So sodium is coming into the
tubular cell, whereas hydrogen will be secreted to tubular fluid. This is called antiport.
sodium hydrogen exchange here. So this is the actually the first mechanism where
hydrogen ion will be secreted into the tubular fluid, so you know that initial
ultrafiltrate the pH is almost same that of plasma but now it will be an
acidified so this bicarbonate and sodium together they will be diffused into
blood. So this is the first mechanism. The first mechanism is excretion of this hydrogen ion or excretion or secretion of hydrogen ion to the tubular
fluid by tubular cells. Now let’s see the second mechanism reabsorption of
bicarbonate. So we know that in the tubular lumen we have bicarbonate here
so which is actually filtered from the plasma and in the proximal convoluted
tubule we already have hydrogen ion, so in the first mechanism we
already learned there is a secretion of hydrogen ion by the tubular cell to the
tubular fluid or tubular lumen that takes place in proximal convoluted
tubule. In this ultrafiltrate, we know that there apart from sodium, there is also bicarbonate
in that tubular fluid. So now in order to acidify urine
our kidney tries to recover this bicarbonate
this is called conservation of base. So how this happens this bicarbonate and
this hydrogen ion they combine and they will form what is called carbonic acid
so this carbonic acid with the help of carbonic anhydrase enzyme, it will be
converted to carbon dioxide water, now this carbon dioxide will
diffuse into renal tubular cell so inside the renal tubular cell with the
help of water again it will convert it back to carbonic acid again this
carbonic acid is which is unstable it will be dissociated to give hydrogen ion
and bicarbonate now this bicarbonate along with sodium will be absorbed or
reabsorbed into the blood. This is actually reabsorption of bicarbonate, so remember
here this recovered or reabsorption of this bicarbonate into the blood is
the same bicarbonate which is filtered in the nephron or in the Bowman’s
capsule into the tubular lumen in the form of tubular fluid.
So this bicarbonate along with hydrogen ion they will form carbonic acid and
they will be converted back to carbon dioxide and water with the help of the
enzyme carbonic anhydrase and this carbon dioxide diffuses into the tubular
epithelial cell, in the tubular epithelial cell it combines with water to form
carbonic acid and this carbonic acid again converted to bicarbonate and
hydrogen ion so this bicarbonate which is actually formed from this bicarbonate
which is filtered to the tubular fluid and it will be recovered to the blood so
our kidney tries to recover this bicarbonate this is called conservation
of the base into the blood, this is reabsorption of bicarbonate or
reclamation of bicarbonate in order to acidify the urine. So this is second
the mechanism, it is called reabsorption of bicarbonate. We will see the third
mechanism that is excretion of acids, especially phosphoric acid we call titrable acids. So in tubular fluid we know that from the plasma phosphate also
filtered to the tubular fluid from the blood. So now this phosphate combines
with hydrogen ion. so already there is hydrogen ion secreted by the tubular
cell to the tubular fluid now this phosphate which is filtered combines
with this hydrogen ion to form phosphoric acid, so this phosphate is
actually a base, which is filtered from the plasma, now this will become an
acid, so already there is acidification or tubular fluid by secretion of
hydrogen ion by the tubular epithelial cell as we studied in the first
mechanism, here further the base phosphate will be converted to acid
phosphoric acid and it will be excreted in the urine. It can be excreted in the
urine as sodium dihydrogen phosphate, so this sodium is already filtered, so this
is the excretion of titratable acids. This is further acidification of urine.
The last mechanism is excretion of ammonium, here from the blood, we have glutamine it will
undergo deamination to give glutamate will you alpha-ketoglutarate and it will
release ammonia and this enters citric acid cycle and it will be converted to
carbon dioxide and water again this same process now this as a result of
deamination glutamine and glutamic acid the ammonia which is released, it will
diffuse into the tubule of fluid. So this ammonia will be released to tubular
fluid, now this ammonia combines with this hydrogen ion and to
give ammonium. So this ammonia is actually base whereas
this ammonium will be acidic or acid. So in the tubular fluid, we have chloride so
this chloride can combine with this ammonium and excreted in the urine as
ammonium chloride. So this is the last mechanism excretion of ammonium the end
result conservation of bicarbonate and secretion of hydrogen ion, so whatever
the pH the initial pH is around 7.4 it will be converted to their 6 so
generally, the pH of urine is acidic because there is acidification of
urine with all these four steps that is excretion or secretion of hydrogen ion,
reabsorption of bicarbonate or reclamation of bicarbonate, excretion of titratable
acids and excretion of ammonium. This is actually the renal regulation of pH. So
even though it is a slow process it will take a long duration but it is a
very very important in order to compensate acid-base order. Renal regulation of pH. Thanks for watching

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