In article <19970402164900.LAA15548 at ladder01.news.aol.com> VIRdwm
wrote:
>I was wondering if anyone could tell me how the brain regulates
blood
>pressure? When you stand up how does the brain keep all of the
blood from
>pooling in the legs. (does it send a signal via long nerves to
blood
>vessels in the lower part of the body)
>
When you stand up, the blood pressure in the neck momentarily
drops. This drop is detected by the baroreceptors of the carotid
sinus. These are stretch receptors at a certain spot in the wall
of the carotid artery; they monitor the degree to which the artery
is distended by the blood inside.
The baroreceptors pass the message through the vagus nerve to the
lower brainstem, which responds by stimulating the sympathetic
nervous system. The result is threefold:
1. Sympathetic nerves from the stellate ganglion stimulate the
heart.
2. Sympathetic nerves all over the body (not just the lower body)
cause blood vessels to constrict.
3. The adrenal medulla (a special sympathetic ganglion on top of
the kidney) releases adrenaline into the blood, reinforcing both of
the previous effects above. In some species the adrenal medulla
release noradrenaline instead of, or in addition to, adrenaline.
The effects of the sympathetic nervous system are fairly diffuse.
There is also a parasympathetic nervous system whose effects are
targeted more specifically. Outgoing parasympathetic fibers in the
vagus nerve have the effect of reducing heart rate, so when you
stand up, these fibers are inhibited.
When you lie down again, all the same things happen in reverse.
The sympathetic nervous system quiets down, and the parasympathetic
fibers in the vagus nerve slow the heart.
Independent of the baroreceptors, your emotions can have a powerful
effect on blood pressure, acting through the same final pathways.
These are the main neural factors controlling blood pressure on a
moment-to-moment basis. In the longer term you also need to
control the total volume of blood.
There are stretch receptors in the walls of the atria of the heart.
These serve as monitors of blood volume. There are also stretch
receptors in the lower part of the carotid arteries, below the
carotid sinus. Their role is not well understood, at least not by
me. They pass their information through the vagus nerve to the
lower brainstem.
The brain controls blood volume partly through hormones (in
particular through the release of vasopressin from the posterior
pituitary) and partly by controlling drinking behavior.
There are also baroreceptors in the kidney, which as I recall
control the release of renin into the blood stream. However, you
asked about the brain.
CAVEAT: It's been almost 15 years since I worked with any of this
stuff. I may have garbled a few details or omitted more recent
findings, but in broad strokes I think the above is valid.
Scott McKellar jm407a at multi.sbc.com