What are in biliary secretions?
Well, first there are bile salts.
There are cholesterol, lecithin,
bilirubin as well as ions.
The gallbladder’s primary function is it is
a storehouse of these biliary secretions,
but similar to detergent, they
concentrate this amount of constituents.
And it does by dehydrating what
is in the gallbladder itself.
So you can concentrate that
somewhere between 5 to 20%.
Okay, how do the biliary
secretions get to the gallbladder?
Well, they’re secreted by
hepatocytes, which are liver cells.
They are collected through
these small little canaliculi
into terminal bile ducts,
into larger bile ducts,
into even larger bile ducts, until finally
you get bile flow down to the gallbladder.
Now, we have these biliary
constituents in the gallbladder,
how do they get out into
the small intestine?
Well, first is done
through the cystic duct.
There are some biliary
secretions that are
released from the right
and left hepatic ducts.
These come down the
common hepatic duct
into the bile duct and finally
out of the sphincter of Oddi.
The sphincter of Oddi will be
one of primary regulatory spots
in which if you relax the sphincter
of Oddi, make it larger,
you’re able to release the
constituents from both
the gallbladder and
from the hepatocytes.
It’s released into the duodenum or
upper part of the small intestines.
Now, how do you concentrate these
substances within the gallbladder itself.
To do that, you’re going to have to
undergo large amount ion exchange.
So let’s start talking
through that process.
First, anytime you want to dehydrate
something, which means to remove water,
the first ion you need
to deal with is sodium.
So sodium is exchanged with hydrogen ions,
then it enters the cytosol
and is removed out of the basolateral
membrane by the sodium-potassium ATPase.
Now, potassium is then also
allowed to move out of the cell
to form a little of
a circular motion
to help facilitate the
Also, you’re going to have to
move chloride into the cytosol.
And that is done via the exchanging
of chloride for bicarbonate
that enters into cytosol.
And as it enters into the cytosol,
it’s allowed to leave or go through the
basolateral membrane via chloride channel.
Finally, with sodium and chloride on the
basolateral side of the hepatobiliary site,
then you can get water
movement to cross
because water is only to cross the cell
in response to an osmotic gradient.
And so you need to have had the sodium and
chloride to be able to be moved across
the cell first before the
water will follow it.
So bile is constantly being
produced by the liver
and is transported to the
gallbladder for storage,
concentrated there, and then finally
will be able to be released.
How it’s released is primarily
in the intestinal phase.
And cholecystokinin will
be the primary mediator
of the release of bile
from the gallbladder.
And how does it do that?
Well, it first has to overcome
any inhibitory effects
of somatostatin and
But it’s worked through
this particular process.
Where bile acids are released,
the sphincter of Oddi
will need to be opened
so that the bile acids will be
able to enter the small intestine.
Cholecystokinin does two things.
One is it helps to contract the gallbladder
and then relax the sphincter of Oddi.
So between those two processes, it
creates enough of a pressure gradient
that will have the bile
flow from the gallbladder
all the way out the sphincter of Oddi
into the small intestine or the duodenum.
The vagus or cranial nerve number
X also causes a weak stimulation
of contraction of the
However, it’s not in the same
level as the cholecystokinin is.