Download CHAP 21b - Dr. Gerry Cronin

Autoregulation
• Two of the most important control points are
the pressure receptors (called baroreceptors)
located in the arch of the aorta and the carotid
sinus.
• There are also baroreceptors in the kidney and
the walls of the heart.
Autoregulation
• Stimulation of the baroreceptors in the carotid
sinus is called the carotid sinus reflex , and it
helps normalize blood pressure in the brain.
Stimulation of the aortic baroreceptors helps
normalize the systemic BP.
• When the blood pressure falls, baroreceptors are
stretched less, and the input is sensed in the
cardiovascular centers of the brain which respond
with decreased parasympathetic and increased
sympathetic stimulation. Blood pressure increases
do the opposite.
Autoregulation
• Another type of sensory receptor important to
the process of autoregulation of BP are the
chemoreceptors.
Autoregulation
• Chemoreceptors are found in the carotid bodies
(located close to baroreceptors of carotid sinus)
and aortic bodies (located in the aortic arch).
• When they detect hypoxia (low O2), hypercapnia
(high CO2), or acidosis (high H+), they signal the
cardiovascular centers.
• They increase sympathetic stimulation increasing
heart rate and respiratory rate, and vasoconstricting
the vessels (arterioles and veins) to increase BP.
Autoregulation
• The Renin-angiotensin-aldosterone (RAA)
system is an important endocrine component of
autoregulation.
• Renin is released by kidneys when blood
volume falls or blood flow decreases.
• It is subsequently converted into the active
hormone angiotensin II which raises BP
by vasoconstriction and by stimulating
secretion of aldosterone from the
adrenal glands.
Autoregulation
• Epinephrine and norepinephrine are also
released from the adrenal medulla as an
endocrine autoregulatory response to
sympathetic stimulation.
• They increase cardiac output by increasing rate and
force of heart contractions.
• Antidiuretic hormone (ADH) is released from the
posterior pituitary gland in response to
dehydration or decreased blood volume.
Autoregulation
• Atrial Naturetic Peptide (ANP) is a natural
diuretic polypeptide hormone released by cells
of the cardiac atria.
• ANP participates in autoregulation by:
• Lowering blood pressure (it causes a direct vasodilation)
• Reducing blood volume (by promoting loss of salt and
water as urine)
Circulation
• In an autoregulatory response, important
differences exist between the pulmonary and
systemic circulations:
• Systemic blood vessel walls dilate in response to
hypoxia (low O2) or acidosis to increase blood flow.
• The walls of the pulmonary blood vessels constrict
to a hypoxic or acidosis stimulus to ensure that
most blood flow is diverted to better ventilated
areas of the lung.
Circulation
• A measure of peripheral circulation can be done
by checking the pulse. The pulse is a result of the
alternate expansion and recoil of elastic arteries
after each systole.
– It is strongest in arteries closest to the heart and
becomes weaker further out.
– Normally the pulse
is the same as
the heart rate.
Circulation
• Blood pressure is the pressure in arteries generated by
the left ventricle during systole and the pressure
remaining in the arteries when the ventricle is in diastole.
Alterations Of Blood Pressure
• About 50 million Americans have hypertension
(HTN).
• It is the most common disorder
affecting the CV system
and is a major cause of
atherosclerotic vascular
disease (ASVD), heart
failure, kidney disease
and stroke.
Alterations Of Blood Pressure
• Hypertension is defined as an elevated systolic
blood
pressure (SBP), an elevated diastolic blood
pressure (DBP), or both. Depending on severity,
it is classified as pre-hypertension, Stage 1 HTN,
or stage 2 HTN.
Alterations Of Blood Pressure
• Hypotension is defined as any blood pressure
too low to allow sufficient blood flow (hypoperfusion) to meet the body's metabolic
demands (to maintain homeostasis).
• Many persons, especially some thin, young
women, have very low BP, yet experience no
dizziness, fatigue, or other symptoms – they are
not hypotensive, and in fact are probably very
healthy (cardiovascular wise).
• Hypotension leading to hypo-perfusion
(pressure and flow are related) of critical organs
results in shock
Shock And Homeostasis
• The 4 basic types of shock are:
•
•
•
•
Hypovolemic shock, due to decreased blood volume
Cardiogenic shock, due to poor heart function
Obstructive shock, due to obstruction of blood flow
Vascular shock, due to excess vasodilation - as seen
in cases of a massive allergy (anaphylaxis) or sepsis.
In the U.S., septic shock causes >100,000 deaths/yr.
and is the most common cause of death in hospital
critical care units.
Shock And Homeostasis
• The same negative feedback mechanism
discussed in autoregulation of blood
pressure/flow is activated to restore blood and
nutrient flow in cases of shock.
• Heart will respond with  rate and force of
contraction.
• Selective tissue beds will vasoconstrict to shunt
blood flow to those tissue most necessary to life
(brain).
• The other neural, hormonal, and chemical pathways
will be recruited to restore balance.
Shock and Homeostasis
• Heart rate & force increase
• Vasoconstriction or vasodilation
depending on type of shock
• ADH released  conserve water
• Renin released  Angiotensin II
• Aldosterone released  conserve Na+
• ANP inhibited
The body responds via negative feedback to restore homeostasis
Shock And Homeostasis
• Most cases of shock call for the administration of
extra fluids and emergency medications like
epinephrine to help restore perfusion to the tissues.
• If the body is not able to do this
quickly, with or without
outside help, organs will fail
(kidney failure, liver failure,
coma) and damage may
become permanent.
Circulatory Routes
• Blood vessels are organized into circulatory
routes that carry blood to specific parts of the
body.
• The pulmonary circulation leaves the right heart to
allow blood to be re-oxygenated and to off-load CO2.
• The systemic circulation leaves the left side of the
heart to supply the coronary, cerebral, renal,
digestive and hepatic circulations (among others).
The bronchial circulation provides oxygenated blood
to the lungs, not the pulmonary circulation, which
oxygenates blood!
Systemic Circulation - Arteries
• Aorta (one)
• Radial
• Brachiocephalic (one)
• Ulnar
• Common Carotid
• Bronchial (usually 3)
• External Carotid
• Renal
• Internal Carotid
• Iliac (common, internal,
• Subclavian
external)
• Axillary
• Femoral
• Brachial
• Popliteal
Systemic
Circulation
- Arteries
Systemic
Circulation
- Arteries
Systemic Circulation - Arteries
Systemic Circulation - Arteries
Systemic Circulation - Veins
• Vena Cava
• Median Cubital
• Brachiocephalic (two)
• Iliac (common,
• External Jugular
internal, external)
• Internal Jugular
• Femoral
• Subclavian
• Popliteal
• Axillary
• Saphenous
• Brachial
• Hepatic portal
Systemic
Circulation
- Veins
Systemic Circulation - Veins
Systemic Circulation - Veins
Systemic Circulation - Veins
Portal Circulation
• The hepatic portal system is designed to take
nutrient- rich venous blood from the digestive tract
capillaries, and transport it to the sinusoidal
capillaries of the liver.
• As it percolates through the liver sinusoids, the
hepatocytes of the liver, acting as the chemical factories
of the body, extract and add what they wish to maintain
homeostasis (extracting sugars, fats, proteins when
appropriate and then dumping them back into the
circulation when necessary).
Portal Circulation
Fetal Circulation
• The fetus has special circulatory requirements
because their lungs, kidneys and GI tract are
non-functional.
• The fetus derives its oxygen and
nutrients and eliminates wastes
through the maternal blood supply
by way of the placenta. Normally,
there is no maternal/fetal mixing;
the fetus is totally dependant on
capillary exchange.
Fetal Circulation
• Oxygenated blood leaves the placenta through
the umbilical vein. It then bypasses the liver via
the ductus venosus and dumps into the inferior
vena cava en route to the right heart.
• This oxygen-rich blood then
bypasses the lungs by
traveling to the left heart
through the foramen ovale.
Fetal Circulation
• Blood remaining in the right heart that manages
to flow through the right ventricle meets with
very high resistance from the closed and soggy
lungs.
• This blood is diverted into the
left-sided circulation by passing
through the ductus
arteriosus before returning
to the placenta via the
umbilical arteries.
Fetal circulation (before birth)
Neonatal Circulation After Birth
• At birth, the neonate’s lungs open and in just a
few seconds, there is a massive drop in
pulmonary vascular resistance.
• Blood now entering the right heart now sees lower
pressure looking into the lungs and has no
“incentive” to flow through the foremen ovale or the
ductus arteriosus.
• Another change also occurs very rapidly - the
umbilical cord is severed.
• And so begins the adult pattern of blood flow.
Neonatal Circulation After Birth
• Within hours, days, or weeks after birth, the
umbilical vein atrophies to become the
ligamentum teres.
• The ductus venosus atrophies to become the
ligamentum venosum.
• The foramen ovale becomes the closed fossa ovale.
• The ductus arteriosus atrophies to become the
ligamentum arteriosum.
• Umbilical arteries atrophy to become the medial
umbilical ligaments.
Neonatal Circulation After Birth
End of Chapter 21
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