CRAMMER
Talk - 13
REGULATION OF BLOOD
PRESSURE
A normal BP is necessary for maintaining
a normal blood flow.
This is necessary for normal perfusion of
tissues.
But due to various reasons the BP
changes from moment to moment even in
normal individuals.
However, the BP is brought back to the
desired level.
The process by which BP is maintained at
a normal desired level is called regulation
of BP.
BP is regulated by manipulating the
factors that determine BP.
Role of Medullary centers
These centers, situated in the medulla
oblongata, are most important in
regulation of CVS functions.
These are involved in moment-to-moment
regulation of CVS to achieve homeostasis.
There are different areas in this group as
follows:
Vasomotor center (VMC)
Cardio-inhibitory center (CIC)
Nucleus of tractus solitarius (NTS)
Vasomotor center (VMC)
These are situated in the medulla at the
level of the lower end of the floor of the
fourth ventricle on either side of the
midline. It has two parts ―
the lateral ‘pressor area’ and
the medial ‘depressor area’.
Pressor area
Also called C1 or vasoconstrictor area
The neurons in this area send fibers to all
levels of spinal cord, where they excite
vasoconstrictor neurons
These neurons show inherent tonic activity,
i.e., they discharge rhythmically in a tonic
fashion and maintain the normal HR and BP
Stimulation of pressor area leads to ↑ed
discharge of the whole sympathetic
outflow → vasoconstriction and rise of
arterial BP, ↑ed HR and ↑ed CO
Depressor area
also called vasodilator area
neurons here send fibers to pressor area
and inhibit vasoconstrictor activity of pressor
area.
Stimulation of this area causes vasodilatation
and es HR → es BP.
Thus, the VMC can produce either:
(i) ↑ in sympathetic activity due to ↑ in
activity of pressor area or
(ii) in sympathetic activity due to ↑ in
activity of depressor area which in turn
inhibit the pressor area.
Cardio -inhibitory center (CIC)
It is also situated in the floor of the fourth
ventricle and is actually composed of the
Nucleus ambiguus and
Dorsal nucleus of vagus
From here, efferent parasympathetic fibers
(cardio-inhibitory) of vagus originate
Its stimulation is similar to vagal
stimulation and leads to ed HR
If the CIC is inhibited or destroyed, the HR
rises immediately..
Nucleus of tractus solitarius
(NTS)
It receives the afferents coming from
different parts of the body and projects to
the VMC, CIC, and respiratory center and
thus plays a very important role in the
regulation of CVS.
Various mechanisms exist within the body to
regulate BP. These different mechanisms are ―
Short-term regulation ― Rapidly acting BP
regulatory ― Nervous regulatory mechanisms.
Intermediate acting arterial BP regulatory
mechanisms or intrinsic physical regulatory
mechanisms.
Long-term acting regulatory mechanisms or
autoregulation of BP by kidneys.
Miscellaneous mechanisms.
SHORT-TERM REGULATION
Rapidly acting BP Regulatory
Mechanisms
Nervous Regulatory Mechanisms ―
They are responsible for minute-to-minute
regulation of BP.
They become active within a minute or so
of change in BP.
They act rapidly and powerfully to correct
BP.
They are the first line of defense against
abnormal changes in BP
BAROCEPTOR MECHANISM
BARORECEPTOR REFLEX
SINOAORTIC REFLEX
The most important and best known reflex
regulating BP.
Also known as pressure buffer
mechanism as this reflex buffers or
prevents changes of BP.
Baroreceptors
Baroreceptors or Pressoreceptors are spray type
nerve endings.
They are located in the wall of almost every
major artery of the thoracic and neck regions,
They are extremely abundant in the carotid
sinus and arch of aorta
Baroreceptors
These are stretch receptors.
They are stimulated when they are
stretched.
They get stimulated when vessel wall is
stretched due to rise in BP, hence called
baroreceptors
They get stimulated when vessel wall is
stretched due to rise in BP, hence called
baroreceptors
They respond to changes in MAP and PP.
INNERVATION
Carotid sinus – sinus nerve, branch of
Glossopharyngeal nerve
Aortic arch – Vagus nerve
These nerves are also called as buffer nerves
because they are involved in buffering
changes in BP
MECHANISM OF ACTION
Thebaroreceptors are sensitive to stretch
and their discharge is caused by a rise of
pressure expanding the arterial wall
When baroreceptors are stimulated by rise in
MAP and PP, they send impulses to the NTS.
From here secondary signals are transmitted
that
Inhibit the VMC and
Stimulate the CIC
The overall effect is removal of vasomotor tone
leading to vasodilatation, in HR & ed force of
contraction.
This results in PR & CO and thus a fall in
BP.
Reverse changes are seen when MAP
falls ― BP → no stimulation of
baroreceptors → impulses to NTS →
impulses to VMC & CIC → ↑
vasoconstriction &↑HR → ↑PR & ↑CO →
↑BP.
Sectioning of buffer nerves ↑es BP to
300/200 mm Hg.
Bilateral lesion of NTS causes severe
hypertension that can be fatal.
These forms of hypertension are called
‘Neurogenic hypertension’.
The baroreceptors are said to be tonically
active.
↑ in pressure results in ↑ baroreceptor
discharge.
in pressure results in baroreceptor
discharge
When BP es baroreceptor discharge es
The baroreceptor signals enter the NTS of
medulla.
Secondary signals from NTS inhibit the VMC
and stimulate the CIC.
The net effect is removal of vasomotor tone and
vasodilatation through out and
in HR and strength of contraction →
• in BP.
When mean BP es, baroreceptor
discharge es, causing
less inhibition of VMC and
less stimulation of CIC
Thisresults in reflex ↑ in sympathetic
activity and reflex in vagal activity →
prevents any fall in BP.
RESETTING OF BARORECEPTORS
The baroreceptor mechanism is unimportant in
the long-term regulation of BP.
This is because the baroreceptors are fast
adapting receptors.
They get adapted in one or two days to whatever
BP they are exposed to.
That is, the baroreceptors are reset to the new
pressure and operate at this level.
The resetting of baroreceptors occurs for both
high BP and low BP.
