Blood Pressure and Regulation. Neural.pd

Blood Pressure physiology

1. Cardiovascular physiology- Blood Pressure
Figure1 expansion and elastic recoil of arterial wall during systole and diastole
Lecture
Objectives : -Definition of arterial blood pressure and the meaning of diastolic and
systolic pressure -Methods of measurement of BP.

.
Arterial blood pressure : means the force exerted by the blood against any unit are
the vessel wall. Blood pressure almost alwaysis measured in millimeters of mercury (mm
Hg) because the mercury manometer has been used since antiquity as the standard reference
for measuring pressure .When one says that pressure in a vessel is 50mmHg this means that
the force exerted is sufficient to push a column of mercury up to the level 50mm high . The
pressure in the aorta and in the brachial and other large arteries in a young adult human rises
to a peak value (systolic pressure) of about 120 mm Hg during each heart cycle and falls to
a minimum (diastolic pressure) of about 70 mm Hg . Systolic pressure is produced by
ejection of blood into aorta during left ventricular systole while diastolic pressure is
produced as a result of the elastic recoil of the aorta during ventricular diastole, fig.1. The
combination of stiffness and flexibility of arterial wall enables arteries to perform one of
their major functions—acting as pressure reservoirs (storage sites for pressure) to ensure a
continual, smooth flow of blood through the vasculature even when the heart is not pumping
blood (diastole).The arterial pressure is conventionally written as systolic pressure over
diastolic pressure, for example, 120/70 mm Hg.
Mechanism of regulation of BP.
Venous pressure

2. arterial pressureisthe product ofthe cardiac output and the peripheral resistance, it is
affected by conditions that affect either or both of these factors.
BP=CO*PVR ( CO: cardiac output, PVR: peripheral vascular resistance)
The pulse pressure, the difference between the systolic and diastolic pressures, is normally
about 40 mm Hg.The mean blood pressure is the average pressure throughout the cardiac
cycle. Mean BP=diastolic BP+1/3pulse pressure.
Methods of Measuring Blood Pressure:
1-Invasive method: a cannula is inserted into an artery, the arterial pressure can be
measured directly . This technique involves direct measurement of arterial pressure by
inserting a cannula needle in a suitable artery. The cannula must be connected to a sterile,
fluid-filled system, which is connected to an electronic patient monitor. The advantage of
this system is that a patient’s blood pressure is constantly monitored beat-by-beat, and a
waveform can be displayed.
2-Non invasive methods: BP measured by sphygmomanometer. It includes :
I-palpatory method.
II-Auscultatory method
Regulation of arterial blood pressure arterial pressure is not regulatednot by a single
pressure controlling system but instead by several interrelated systems:
Reflex mechansims for controlling arterial blood pressure
(1) the baroreceptor feedback mechanism: Baroreceptors are stretch receptors that are
extremely abundant in two locations: the aortic arch, and the carotid sinuses of the carotid
arteries, which. Thus arterial baroreceptors are also called sinoaortic baroreceptors. These
baroreceptors are placed because pressure in the aorta affects blood flow to every organ in
the systemic circuit, and because pressure in the carotid arteries affects blood flow to the
brain . Signals from the “carotid baroreceptors” are transmitted through glossopharyngeal
nerves to the medulla. Signals from the“aortic baroreceptors” in the arch of the aorta are
transmitted through the vagus nerves also to the medulla. When arterial BP increases cause
stimulation of baroreceptors ,so signals transmitted to the medulla, secondary signals inhibit
the vasoconstrictor centerof the medulla and stimulate cardiac inhibitory center(CIC) and
vasodilator center(VDC). The net effects are (1) vasodilatationof the veins and arterioles .
(2) decreased heartrate and strength of heart contraction. Therefore, excitationof the

3. vasoconstriction
Increased BP
Vasomotor center
Increased HR and
contractility of heart
Fig-2 Baroreceptor system for controlling arterial pressure.
baroreceptors by highpressure in thearteries reflexly causes the arterial pressure to decrease
because of both a decrease in peripheral resistance and a decrease in cardiac output.
Conversely, low pressure has opposite effects by inhibition of baroreceptors causing the
pressure to rise back toward the normal.
(2)The chemoreceptor mechanism: The chemoreceptors are chemosensitive cells to oxygen
lack, carbon dioxide excess, and hydrogen ion excess. They are located in several small
chemoreceptor organs (two carotid bodies, one of which lies in the bifurcation of each
common carotid artery, and aortic bodies adjacent to the aorta). The chemoreceptors
excite nerve fibers that pass through Hering nerve and vagus nerve into vasomotor center.
Whenever the arterial pressure falls below a critical level, the chemoreceptors become
stimulated because diminished blood flow causes decreased oxygen as well as excess
buildup of carbon dioxide and hydrogen ions that are not removed by the slowly flowing
blood.The signals transmitted from the chemoreceptors excite the vasomotor center, and
this elevates the arterial pressure back toward normal. However, this chemoreceptor reflex
is not a powerful arterial pressure controller until the arterial pressure falls below 80 mm
Hg. (3) the central nervous system ischemic mechanism: when blood flow to the
vasomotor center in the lower brain stem becomes decreased severely enough to cause
cerebral
Decreased BP
baroreceptors

4. ischemia—the vasoconstrictor andcardioaccelerator neurons in the vasomotor center respond
to the ischemia and maintain cerebral blood flow.This response is due to failure of
slowly flowing blood to carry carbon dioxide away from vasomotor center. This excess
CO2 has potent effect on sympathetic nervous control areas in the brain΄s medulla. It
is a powerfull response .This CNS ischemic response occur when BP falls below
60mmHg. Therefore it is not one of the usual mechanisms for regulating normal
arterial pressure .Instead , it operates principally as an emergency arterial pressure
control system that acts rapidly and powerfully to prevent further decrease in arterial
pressure whenever blod flow to brain decreases dangerously .
4.Control of Blood Pressure by Low-Pressure Baroreceptors (Volume Receptors) In
addition to the arterial baroreceptors, which monitor systemic arterial pressure, other
baroreceptors monitor pressures elsewhere in the cardiovascular system, specifically on
the low-pressure side of the circulation. Particularly important are baroreceptors in the
walls of large systemic veins and in the walls of the right atrium. These receptors have
receptor endings that respond to stretch; because of their locations, they monitor pressure
on the venous side of the systemic circulation and act directly to detect changes in blood
volume. With stretching of these receptors the atrial reflex causes reflex dilatation of
afferent arterioles with resultant increase of filtration of fluid into the renal tubules.
Signals from atrial reflexes cause reduction in secretion of Antidiuretic hormone that
cause reduction in reabsorption from the renal tubules .The combination of these two
effects causes rapid loss of fluid into the urine . Atrial reflex also causes release of atrial
natriuretic peptide that acts on the kidneys and increase loss of fluid in urine . Atrial reflex
control of heart rate called (Bainbridge reflex) that occurs due to increase blood volume
and atrial stretching that cause increase in heart rate to prevent damming of blood in the
veins ,atria and pulmonary circulation.
-Renal role in long term regulation arterial blood pressure: involves
the renin-angiotensin system, rennin is synthesized and stored in the juxtaglomerular cells
of
the kidney, these cells located in the wall of the afferent arterioles proximal to the
glomeruli.

5. Renin(kidney)
Angiotensinogen Angiotensin I
ACE(lungA)CE(lung)
AngiotensinII
:decreased BP stimulate secretion of renin enzyme from kidney this enzyme act on
angiotensinogen (synthesized by the liver) forming angiotensinI which is converted to
angiotensinII by angiotensin converting enzyme which is occur in the lung .this
angiotensinII has potent vasoconstrictor effect.angiotensinII has two principal effects that
can elevate arterial pressure. The first of these, vasoconstriction in many areas of the
body, occurs rapidly. Vasoconstriction occurs intensely in the arterioles so increases the
total peripheral resistance, thereby raising the arterial BP. The second principal means by
which angiotensin increases the arterial pressure is to decrease excretionof both salt and
water by the kidneys. This slowly increases the extracellular fluid volume, which then
increases the arterial pressure during subsequent hours and days. AngiotensinII that is
formed from rennin-angiotensin system causes the kidney retain both salt and water in
two major ways: (fig-3) 1. AngiotensinII acts directly on the kidneys to cause salt and
water retention. 2. AngiotensinII stimulates the adrenal glands to secrete aldosterone, and
the aldosterone in turn increases salt and water reabsorption by the kidney tubules.
Decreased arterial pressure

U Secretionof
aldosterone
from adrenal
gland
Increased arterial blood
pressure
Vasoconstriction
Renalretention of
salt and water
Figure -3 Renin angiotensin system

6. Veins: A VolumeReservoir
Unlike arteries, which function as pressure reservoirs, veins function as volume reservoirs, a
property that is related to vessel compliance. Because veins are thin walled and easily
stretched, they have high compliance.veins can accommodate a large increase in blood
volume with little change in blood pressure, which makes them good at storing volume. Thus
veins can hold a larger volume of blood than arteries . In fact, the veins in the human body
contain a substantially greater volume of blood than do the arteries , even though the
pressure within veins is much lower than that within arteries.
Factors That Influence Venous Pressure and Venous Return
The driving force for venous return is the pressure gradient between the peripheral veins and
the right atrium. Increases in venous pressure enhance venous return,
four factors that affect venous pressure: the skeletal muscle pump, the respiratory pump,
blood volume, and venomotor tone
-Muscle pump: every time one moves the legs by contraction of the muscles which
compress the veins adjacent to them and this squeezes the blood out of the veins. the
valves in the veins are arranged so that the direction of venous blood flow can be only
toward the heart.
-Thoracic Pump: During inspiration, your diaphragm pulls downward and your rib cage
expands, which lowers pressure in the thoracic cavity and raises pressure in the
abdominal cavity. This action creates a pressure gradient that promotes the movement of
blood from abdominal veins to the central veins located in the thoracic cavity, thereby
increasing blood flow toward the heart. When you exhale, thoracic pressure rises and
abdominal pressure falls. This creates a pressure gradient that would tend to favor the
backward movement of blood from the central veins to the abdominal veins, but such
backward flow is prevented by the closure of valves in the abdominal veins.the rise in
thoracic pressure drives the forward movement of blood from the central veins to the
heart .

7. References : Ganong textbook of physiology , Guyton and Hall textbook of physiology.
th
-Blood Volume Therelationship between blood volume and venous pressure is a simple
one: An increase in blood volume produces an increase in venous pressure, and a decrease
in blood volume produces a decrease in venous pressure. -Venomotor Tone : The smooth
muscle in the walls of veins contracts or relaxes in response to input from the sympathetic
nervous system and certain chemical agents. In terms of neural control, venous smooth
muscle contains _ adrenergic receptors and activity of the sympathetic nervous system
triggers increased contractile activity, with a resulting rise in tension referred to as
venomotor tone. Central venous pressure: it is the pressure in the right atrium as Blood
from all the systemic veins flows into the right atrium of the heart; Right atrial pressure is
regulated by a balance between (1) the ability of the heart to pump blood outof the right
atrium and ventricle into the lungs . (2)the tendency for blood to flow from the peripheral
veins into the right atrium. If the right heart is pumping strongly, the right atrial pressure
decreases. Conversely, weakness of the heart elevates the right atrial pressure. Also, any
effect that causes rapid inflow of blood into the right atrium from the peripheral veins
elevates the right atrial pressure. Right atrial pressure can rise to abnormal levels as in
heart failure or massive transfusion of blood .