Unformatted text preview: 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology Boundless Anatomy and Physiology
Cardiovascular System: Blood Vessels Control of Blood Pressure 1/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology Role of the Cardiovascular Center
The cardiovascular system plays a role in body maintenance by
transporting hormones and nutrients and removing waste products. LEARNING OBJECTIVES Explain the role of the cardiovascular center in controlling
blood pressure KEY TAKEAWAYS Key Points The cardiovascular center is a part of the human brain
found in the medulla oblongata, responsible for
regulation of cardiac output.
Numerous receptors in the circulatory system can
detect changes in pH or stretch and signal these
changes to the cardiovascular center.
The cardiovascular center can alter heart rate and
stroke volume to increase blood pressure and ow.
Key Terms cardiovascular centre: A region of the brain responsible for nervous control of cardiac output. The cardiovascular center forms part of the autonomic nervous system
and is responsible for regulation of cardiac output. Located in the
medulla oblongata, the cardiovascular center contains three distinct 2/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology components: the cardioaccelerator center, the cardioinhibitor center, and
the vasomotor center.
The cardioaccelerator center stimulates cardiac function by regulating
heart rate and stroke volume via sympathetic stimulation from the
cardiac accelerator nerve. The cardioinhibitor center slows cardiac
function by decreasing heart rate and stroke volume via
parasympathetic stimulation from the vagus nerve. The vasomotor
center controls vessel tone or contraction of the smooth muscle in the
tunica media. Changes in diameter a ect peripheral resistance,
pressure, and ow, which in turn a ect cardiac output. The majority of
these neurons act via the release of the neurotransmitter norepinephrine
from sympathetic neurons. Although each center functions
independently, they are not anatomically distinct.
The cardiovascular center can respond to numerous stimuli. Hormones
such as epinephrine and norepinephrine or changes in pH such as
acidi cation due to carbon dioxide accumulation in a tissue during
exercise are detected by chemoreceptors. Baroreceptors that detect
stretch can also signal to the cardiovascular center to alter heart rate. 3/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology Human circulatory system: The cardiovascular system is composed largely
of the circulatory system, or the system of blood vessels that distributes
oxygen from the lungs throughout the body. Short-Term Neural Control
Neural regulation of blood pressure is achieved through the role of
cardiovascular centers and baroreceptor stimulation. 4/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology LEARNING OBJECTIVES Describe the role of baroreceptors and cardiovascular centers
(ANS) in blood pressure control KEY TAKEAWAYS Key Points The cardioaccelerator center, the cardioinhibitor center,
and the vasomotor center form the cardiovascular
center, a cluster of neurons that function independently
to regulate blood pressure and ow.
The release of the neurotransmitter norepinephrine
from sympathetic neurons directs the majority of
neurons associated with the cardiovascular center.
Baroreceptors respond to the degree of stretch caused
by the presence of blood; this stimulates impulses to be
sent to the cardiovascular center to regulate blood
pressure to achieve homeostasis when needed.
Key Terms autonomic nervous system: The part of the nervous system that regulates the involuntary activity of the
heart, intestines, and glands. These activities include
digestion, respiration, perspiration, metabolism, and
blood pressure modulation.
norepinephrine: A catecholamine with multiple roles including as a hormone and neurotransmitter. Areas of
the body that produce or are a ected by this substance
are described as noradrenergic.
sympathetic: Of or related to the part of the autonomic nervous system that under stress raises blood pressure 5/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology and heart rate, constricts blood vessels, and dilates the
baroreceptor: A nerve ending that is sensitive to changes in blood pressure.
parasympathetic: Of or relating to the part of the autonomic nervous system that inhibits or opposes the
e ects of the sympathetic nervous system. The autonomic nervous system plays a critical role in the regulation of
vascular homeostasis. The primary regulatory sites include the
cardiovascular centers in the brain that control both cardiac and vascular
Neurological regulation of blood pressure and ow depends on the
cardiovascular centers located in the medulla oblongata. This cluster of
neurons responds to changes in blood pressure as well as blood
concentrations of oxygen, carbon dioxide, and other factors such as pH. Baroreceptor Function
Baroreceptors are specialized stretch receptors located within thin areas
of blood vessels and heart chambers that respond to the degree of
stretch caused by the presence of blood. They send impulses to the
cardiovascular center to regulate blood pressure. Vascular
baroreceptors are found primarily in sinuses (small cavities) within the
aorta and carotid arteries. The aortic sinuses are found in the walls of the
ascending aorta just superior to the aortic valve, whereas the carotid
sinuses are located in the base of the internal carotid arteries. There are
also low-pressure baroreceptors located in the walls of the venae cavae
and right atrium.
When blood pressure increases, the baroreceptors are stretched more
tightly and initiate action potentials at a higher rate. At lower blood
pressures, the degree of stretch is lower and the rate of ring is slower.
When the cardiovascular center in the medulla oblongata receives this
input, it triggers a re ex that maintains homeostasis. 6/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology Baroreceptor Re exes: Increased blood pressure results in increased rates
of baroreceptor ring, whereas decreased blood pressure results in slower
rates of re, both initiating the homeostatic mechanism to restore blood
pressure. Baroreceptor Re exes
When blood pressure rises too high, baroreceptors re at a higher rate
and trigger parasympathetic stimulation of the heart. As a result, cardiac
output falls. Sympathetic stimulation of the peripheral arterioles will also
decrease, resulting in vasodilation. Combined, these activities cause
blood pressure to fall.
When blood pressure drops too low, the rate of baroreceptor ring
decreases. This triggers an increase in sympathetic stimulation of the
heart, causing cardiac output to increase. It also triggers sympathetic
stimulation of the peripheral vessels, resulting in vasoconstriction.
Combined, these activities cause blood pressure to rise.
The baroreceptors in the venae cavae and right atrium monitor blood
pressure as the blood returns to the heart from the systemic circulation.
Normally, blood ow into the aorta is the same as blood ow back into
the right atrium. If blood is returning to the right atrium more rapidly than 7/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology it is being ejected from the left ventricle, the atrial receptors will
stimulate the cardiovascular centers to increase sympathetic ring and
cardiac output until homeostasis is achieved. The opposite is also true.
This mechanism is referred to as the atrial re ex.
Other neural mechanisms can also have a signi cant impact on
cardiovascular function. These include the limbic system, which links
physiological responses to psychological stimuli, chemoreceptor
re exes, generalized sympathetic stimulation, and parasympathetic
stimulation. Short-Term Chemical Control
Blood pressure is controlled chemically through dilation or constriction
of the blood vessels by vasodilators and vasocontrictors. LEARNING OBJECTIVES Discuss chemical substances to control blood pressure KEY TAKEAWAYS Key Points Constriction or dilation of blood vessels alters
resistance, increasing or decreasing blood pressure
Generalized vasoconstriction usually results in an
increase in systemic blood pressure, but it may also
occur in speci c tissues, causing a localized reduction
in blood ow.
Vasoconstriction results from increased concentration
of calcium (Ca2+) ions within vascular smooth muscle. 8/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology When blood vessels dilate, the ow of blood is
increased due to a decrease in vascular resistance.
Therefore, dilation of arterial blood vessels (mainly the
arterioles ) causes a decrease in blood pressure.
Localized tissues increase blood ow in multiple ways,
including releasing vasodilators, primarily adenosine,
into the local intersitial uid, which di uses to capillary
beds provoking local vasodilation.
Key Terms vasodilation: The dilation (widening) of a blood vessel.
vasoconstriction: The constriction (narrowing) of a blood vessel. Many physical factors that in uence arterial pressure. Each may in turn
be in uenced by physiological factors such as diet, exercise, disease,
drugs or alcohol, stress, and obesity. In practice, each individual’s
autonomic nervous system responds to and regulates all of these
interacting factors so that the actual arterial pressure response varies
widely because of both split-second and slow-moving responses of the
nervous system and end organs. These responses are very e ective in
changing the variables and resulting blood pressure from moment to
moment. Chemical Vasoconstriction
Vasoconstriction is the narrowing of blood vessels resulting from
contraction of the muscular wall of the vessels, particularly the large
arteries and small arterioles. Generalized vasoconstriction usually results
in an increase in systemic blood pressure, but may also occur in speci c
tissues, causing a localized reduction in blood ow.
The mechanism that leads to vasoconstriction results from the increased
concentration of calcium (Ca2+ ions) and phosphorylated myosin within
vascular smooth muscle cells. When stimulated, a signal transduction
cascade leads to increased intracellular calcium from the sarcoplasmic 9/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology reticulum through IP3 mediated calcium release, as well as enhanced
calcium entry across the sarcolemma through calcium channels.
The rise in intracellular calcium interacts with calmodulin, which in turn
activates myosin light chain kinase. This enzyme is responsible for
phosphorylating the light chain of myosin to stimulate cross-bridge
cycling. Once elevated, the intracellular calcium concentration is
returned to its basal level through a variety of protein pumps and
calcium exchanges located on the plasma membrane and sarcoplasmic
reticulum. This reduction in calcium removes the stimulus necessary for
contraction allowing for a return to baseline.
Endogenous vasoconstrictors include ATP, epinephrine, and angiotensin
II. Vasoconstriction: Vasoconstriction of a microvessel by pericytes and
endothelial cells that encircle an erythrocyte (E). 10/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology Chemical Vasodilation
Vasodilation is the widening of blood vessels resulting from relaxation of
smooth muscle cells within the vessel walls, particularly in the large
veins, large arteries, and smaller arterioles. Generalized vasodilation
usually results in a decrease in systemic blood pressure, but may also
occur in speci c tissues causing a localized increase in blood ow.
The primary function of vasodilation is to increase blood ow in the body
to tissues that need it most. This is often in response to a localized need
for oxygen, but can occur when the tissue in question is not receiving
enough glucose, lipids, or other nutrients. Localized tissues increase
blood ow by several methods, including the release of vasodilators,
primarily adenosine, into the local interstitial uid, which di uses to
capillary beds provoking local vasodilation. Some physiologists have
suggested the lack of oxygen itself causes capillary beds to vasodilate
by the smooth muscle hypoxia of the vessels in the region.
As with vasoconstriction, vasodilation is modulated by calcium ion
concentration and myosin phosphorylation within vascular smooth
muscle cells. Dephosphorylation by myosin light-chain phosphatase and
induction of calcium symporters and antiporters that pump calcium
ions out of the intracellular compartment both contribute to smooth
muscle cell relaxation and therefore vasodilation. This is accomplished
through reuptake of ions into the sarcoplasmic reticulum via exchangers
and expulsion across the plasma membrane.
Endogenous vasodilators include arginine and lactic acid. Long-Term Renal Regulation
Consistent and long-term control of blood pressure is determined by the
renin-angiotensin system. LEARNING OBJECTIVES 11/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology Describe the long-term renal regulation of blood pressure KEY TAKEAWAYS Key Points When blood volume is low, renin, excreted by the
kidneys, stimulates production of angiotensin I, which is
converted into angiotensin II. This substance has many
e ects, including increase in blood pressure due to its
The cells that excrete renin are called juxtaglomerular
cells. When blood volume is low, juxtaglomerular cells in
the kidneys secrete renin directly into circulation.
Plasma renin then carries out the conversion of
angiotensinogen released by the liver to angiotensin I.
Aldosterone secretion from the adrenal cortex is
induced by angiotensin II and causes the tubules of the
kidneys to increase the reabsorption of sodium and
water into the blood, thereby increasing blood volume
and blood pressure.
Key Terms juxtaglomerular cells: The juxtaglomerular cells (JG cells, or granular cells) are cells in the kidney that
synthesize, store, and secrete the enzyme renin.
aldosterone: A mineralocorticoid hormone secreted by the adrenal cortex that regulates the balance of sodium
and potassium in the body.
adrenal cortex: The outer portion of the adrenal glands that produces hormones essential to homeostasis. Along with vessel morphology, blood viscosity is one of the key factors 12/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology in uencing resistance and hence blood pressure. A key modulator of
blood viscosity is the renin-angiotensin system (RAS) or the reninangiotensin-aldosterone system (RAAS), a hormone system that
regulates blood pressure and water balance.
When blood volume is low, juxtaglomerular cells in the kidneys secrete
renin directly into circulation. Plasma renin then carries out the
conversion of angiotensinogen released by the liver to angiotensin I.
Angiotensin I is subsequently converted to angiotensin II by the enzyme
angiotensin converting enzyme found in the lungs. Angiotensin II is a
potent vasoactive peptide that causes blood vessels to constrict,
resulting in increased blood pressure. Angiotensin II also stimulates the
secretion of the hormone aldosterone from the adrenal cortex.
Aldosterone causes the tubules of the kidneys to increase the
reabsorption of sodium and water into the blood. This increases the
volume of uid in the body, which also increases blood pressure. If the
renin-angiotensin-aldosterone system is too active, blood pressure will
be too high. Many drugs interrupt di erent steps in this system to lower
blood pressure. These drugs are one of the main ways to control high
blood pressure (hypertension), heart failure, kidney failure, and harmful
e ects of diabetes.
It is believed that angiotensin I may have some minor activity, but
angiotensin II is the major bioactive product. Angiotensin II has a variety
of e ects on the body: throughout the body, it is a potent vasoconstrictor
of arterioles. 13/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology The renin-angiotensin pathway: The gures outlines the origination of the
renin-angiotensin pathway molecules, as well as e ects on target organs
and systems. Checking Circulation
Checking circulation involves measurement of blood pressure and pulse
through a variety of invasive and noninvasive methods. LEARNING OBJECTIVES Choose the appropriate method for checking circulation KEY TAKEAWAYS Key Points Pulse rate is most commonly measured manually at the
wrist by a trained medical professional.
Arterial catheters and pulse oximetry allow for more
accurate and long-term measurement of pulse rate. 14/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology Heart rate can be measured directly by listening to the
heart through the chest.
Electrocardiography, which detects the electrical
pattern of the heart muscle through the skin, can be
used for more accurate or long-term measurements.
Arterial pressure is most commonly measured via a
Blood pressure values are generally reported in
millimeters of mercury (mmHg), though aneroid and
electronic devices do not use mercury.
The auscultatory method for determining blood
pressure uses a stethoscope and a
Key Terms electrocardiography: A measure of the electrical output of the heart detected through the skin.
sphygmomanometer: A device used to measure blood pressure. Circulatory health can be measured in a variety of ways as follows. Pulse
While a simple pulse rate measurement can be achieved by anyone,
trained medical sta are capable of much more accurate measurements.
Radial pulse is commonly measured using three ngers: the nger
closest to the heart is used to occlude the pulse pressure, the middle
nger is used to get a crude estimate of blood pressure, and the nger
most distal to the heart is used to nullify the e ect of the ulnar pulse as
the two arteries are connected via the palmar arches.
Where more accurate or long-term measurements are required, pulse
rate, pulse de cits, and much more physiologic data are readily
visualized by the use of one or more arterial catheters connected to a 15/26 2/19/2019 Control of Blood Pressure | Boundless Anatomy and Physiology transducer and oscilloscope. This invasive technique has been
commonly used in intensive care since the 1970’s. The rate of the pulse
is observed and measured by tactile or visual means on the outside of
an artery and recorded as beats per minute (BPM). The pulse may be
further indirectly observed under light absorbencies of varying
wavelengths with assigned and inexpensively-reproduced mathematical
ratios. Applied capture of variances of light signal from the blood
component hemoglobin under oxygenated vs. deoxygenated conditions
allows the technology of pulse oximetry. Heart Rate
Heart rate can be measured by
listening to the heart directly
though the chest, traditionally
using a stethoscope. For more
accurate or long-term
electrocardiography may be
During each heartbeat, a healthy
heart has an orderly progression
of depolarization that starts with
pacemaker cells in the sinoatrial ECG Graph: ECG graph of a normal
heartbeat. (SA) node, spreads out through
the atrium, passes through the
atrioventricular node down into the bundle of His and into the Purkinje
bers, and down and to the left throughout the ventricles. This organized
pattern of depolarization can be detected through electrodes placed on
the skin and recorded as the commonly seen ECG tracing. ECG provides
a very accurate means to measure heart rate, rhythm, and other factors
such as chamber sizing, as well as identifying possible regions of
damage. Blood Pressure
Arterial pressure is most commonly measured via a sphygmomanometer,
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