The Renin-Angiotensin-Aldosterone System (RAAS)
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The smaller system that controls blood pressure is the tubuloglomerular feedback system. |
Your body has a huge system involved in the sensation
and control of blood pressure, not only within the kidneys, but all over
the body - especially in times of great need. This is in contrast to a
smaller system called tubuloglomerular feedback, which you can think of
as the system that senses and controls blood pressure and glomerular
filtration rate within the kidneys on a moment-by-moment basis. When
called upon, this smaller system can also rev up the really big system
I'm about to get into. So, what is this really big system?
Could I
get a drumroll? The most important system involved in the regulation of
systemic blood pressure, renal blood flow and glomerular filtration
rate is called the
renin-angiotensin-aldosterone system, or (RAAS) for short.
The Release of Renin
When
systemic hypotension, or low blood pressure throughout the body,
occurs, receptors in your blood vessels called baroreceptors sense this
change. Cells of the kidney's juxtaglomerular apparatus get involved as
well. Detection by one or both of these mechanisms leads juxtaglomerular
cells in the kidneys to release an enzyme called
renin. Renin is
an enzyme released by the juxtaglomerular cells of the kidneys in
response to low blood pressure, causing the transformation of
angiotensinogen to angiotensin I.
Angiotensinogen & Angiotensin I
Angiotensinogen
is a precursor protein made in the liver for a hormone called
angiotensin I. Essentially, renin catalyzes a reaction that converts the
angiotensinogen protein into
angiotensin I, which is a precursor
hormone that is converted to an active hormone called angiotensin II by
an enzyme known as angiotensin-converting enzyme in the lungs. Wow,
that was a mouthful! Let's break this down.
Here's how to
remember what becomes what. Angiotensinogen's purpose is to serve as a
precursor to angiotensin I. Angiotensinogen is cleaved, or broken apart,
by renin. Since it's broken apart, it gets smaller and becomes shorter
in name as well. Therefore, it's now called plain old angiotensin I.
Angiotensin I decides to have a little kid and name it after itself.
Therefore, when angiotensin I is converted in the lungs by an enzyme
called ACE, it becomes angiotensin junior - or more technically,
angiotensin II.
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Angiotensin I is converted into angiotensin II in the lungs by the enzyme ACE.
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Angiotensin-Converting Enzyme (ACE) & Angiotensin II
It
bears repeating that the angiotensin-converting enzyme, or ACE for
short, is an enzyme located mainly in the lungs that converts
angiotensin I into angiotensin II. Once angiotensin II is made, it can
have a big effect on the body. Namely,
angiotensin II is a
vasoconstrictive hormone that increases systemic blood pressure, renal
perfusion pressure and the glomerular filtration rate.
Angiotensin
II not only constricts blood vessels all over the body in order to
increase systemic blood pressure, it also works in the kidneys in order
to maintain blood pressure in the glomerulus so that the glomerular
filtration rate stays normal even in the face of low blood pressure.
Control of the Glomerular Filtration Rate (GFR) by the RAAS
Let's
see how this works with a familiar example. If you connect a hose to a
faucet and turn the faucet on, a certain pressure will be exerted by the
water on the walls of the hose. Likewise, blood running through the
glomerulus (our hose) does the same thing. If the faucet is turned down a
bit due to hypotension, there is less water running through the hose
and therefore less pressure being exerted on the hose. If this were to
happen in our glomerulus due to hypotension, this would be very bad. We
need to maintain pressure in the glomerulus at a certain level if we
want to filter our blood enough to stay alive.
To maintain
pressure in the glomerulus and therefore keep the glomerular filtration
rate steady, angiotensin II constricts both the efferent and afferent
arteriole, but with a much greater effect on the efferent arteriole.
Remember, the
effect of angiotensin II is greater on the
efferent
arteriole. This means that the blood entering the glomerulus has a much
harder time leaving it because the exit is far smaller than the
entrance. This causes a backup of blood in the glomerulus, increases the
pressure within it and, therefore, keeps the GFR at an appropriate
rate.
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Angiotensin II constricts the afferent and efferent arterioles.
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In addition, angiotensin II increases the absorption
of sodium in the renal tubule. Since water follows sodium, it increases
the amount of fluid in the blood vessels, further causing an increase in
blood pressure in addition to the vasoconstriction that already
occurred.
Aldosterone
Angiotensin does some other important things that you must remember. It causes the release of a hormone called
aldosterone
from the adrenal glands. Aldosterone is a hormone that increases the
absorption of water from the distal convoluted tubule and collecting
duct of the kidney's nephrons.
Aldosterone has many other
functions, including the secretion of potassium into urine. However, for
this lesson, you should understand that aldosterone causes the
absorption of sodium out of the renal tubule's filtrate and into the
blood. Since water follows sodium, more water is reabsorbed back into
the blood in order to increase the blood pressure.
Anti-Diuretic Hormone (ADH)
As
if constricting blood vessels and releasing aldosterone to retain water
and sodium weren't enough, angiotensin II also causes the release of a
hormone called
anti-diuretic hormone, commonly called vasopressin, or ADH for short.
ADH
is a hormone released from the posterior pituitary gland that causes an
increase in blood pressure. ADH vasoconstricts our blood vessels, which
causes increased blood pressure. It also increases water absorption
from the distal tubule and collecting ducts. Now that you know what it
does, it's easy to remember this because 'anti' in 'anti-diuretic' means
'against,' and 'diuretic' means 'excess urine production' that occurs
thanks to water loss. Hence, ADH is against the loss of water in urine
from your body!
Vasopressin raises blood pressure by vasoconstricting blood vessels and increasing water absorption.
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Negative Feedback Mechanisms & Loops on the RAAS
Finally,
you must understand that if the RAAS system was to run wild without any
inhibitory control, it would actually kill you. This is why there are
several mechanisms in place that try to control the RAAS system so that
it doesn't go into overdrive. We'll go over some of the most important
aspects of this inhibitory feedback system.
First off, increasing
levels of angiotensin II are sensed by your body, and this by itself
suppresses renin release. This is called an inhibitory
feedback loop.
As angiotensin II increases, renin decreases, which means angiotensin
II also decreases since it depends on renin for its production.
Secondly,
as blood pressure is restored back to normal thanks to vasoconstriction
and water retention, baroreceptors in your body's blood vessels sense
the increased pressure and send signals to the juxtaglomerular cells to
stop secreting renin. This signaling eventually stops the entire RAAS
cascade. No renin means no angiotensin II and therefore no aldosterone
or ADH either.