Short and Long term regulation of blood pressure

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Short and Long term regulation of blood pressure
Applied Sciences Lecture Course

• Mahesh Nirmalan MD FRCA PhD
• Consultant, Critical Care Medicine
• Manchester Royal Infirmary

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A late response to blood loss is..

1. Activation to chemo-receptors
2. Release of Aldosterone from the adrenal gland
3. Activation of baro-receptors
4. Sympathetic activation
5. Initiation of flight fright response

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Vasomotor centre is located in the

1. Spinal cord
2. Pre-frontal cortex
3. Frontal cortex
4. Cerebellum
5. Brain stem

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Background Tissue blood flow

• Local metabolic factors
• Calibre of the vessels
• Driving pressure the pressure gradient
• Importance of mean arterial pressure
• Maintaining arterial pressure is a physiological
  imperative
• BP is maintained sometimes at the expense of
  regional blood flow

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Tissue blood flow is frequently independent of
blood pressure Auto-regulation
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Blood pressure is a poor surrogate for tissue
blood flow
Serosal blood flow sigmoid colon pre aortic
cross clamp MAP 68
Serosal blood flow sigmoid colon post aortic
cross clamp MAP 90
MAP is a poor surrogate of tissue blood flow
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Objectives

• Physiology of arterial pressure
• Short term regulation of blood pressure
• Intermediate term
• Long term regulation of blood pressure
• Clinical examples
• Vascular remodelling

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Blood Pressure Cardiac Output x Total
Peripheral Resistance V
I x
R
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Typical resistance arteriole
post-ganglionic sympathetic neuron
internal elastic lamina
VSMC
10 µm
endothelium
From Professor Arthur Weston, UoM
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Aortic pressure
Aortic flow
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Arterial pressure is a product of a dynamic
interplay between the force generated by the
ventricles and the peripheral resistance vessels
Ventricular vascular coupling
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Rise of the blood pressure curve dependent on the
characteristic Compliance of the Aorta
Heart in systolic Phase
Compliance
Aortic valve open
SV
P(t)
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Aortic compliance ? Diastole
Heart in diastolic Phase
Fall of the blood pressure curve dependent on the
characteristic Compliance of the Aorta
Aortic valve closed
P(t)
Rev Steven Hales 1733 Otto Frank 1899
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Arteriosclerosis increases the systolic pressure
and lowers the diastolic pressure
Nirmalan Pinsky Year Book of ICEM 2010
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Vasoconstriction?systolic diastolic pressure
Baseline
6 mg Ephedrine
Post Ephedrine
Nirmalan Pinsky Year Book of ICEM 2010
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Arterial pressure is a product of a dynamic
interplay between the force generated by the
ventricles and the peripheral resistance vessels
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Regulation of blood pressure?
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Overview of systems
Autonomic nervous system Fluid shifts Renal and
endocrine responses
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Short term regulation of BP

• Neurally mediated
• Baro receptors, chemo receptors
• Both arms of the ANS sympathetic and
  parasympathetic
• Ischaemic CNS response

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Case study

• 20 year old medical student on the ward
  attempting venepuncture
• Suddenly looked pale, clammy and vacant
• Then vomited and collapsed with loss of
  consciousness
• Noted to have very slow, thready pulse
• Rapidly recovered

Diagnosis Vasovagal syncope
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Gravity mediated increase in venous
return Baro-receptor mediated vasoconstriction Res
toration of SV and BP Return of consciousness
Stress relaxation of vessels
Immediate and within seconds Involved in
maintaining BP during ordinary physiological
perturbations Powerful Hypovolaemic Shock Never
complete Not sustainable
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Receptors
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• Baro-receptors
• Nerve endings in all large thoracic and neck
  arteries
• 2 major populations Carotid sinus and Arch of
  the aorta
• Activation on stretch
• Carotid Herings nerve to Glossopharyngeal nerve
  to tractus solitarius in brainstem
• Aortic Vagus nerve to tractus solitarius
• If baro-receptors sense increased BP? Secondary
  signals from tractus solitarius ?Inhibition of
  vasoconstrictor centre and excitation of vagal
  parasympathetic centre

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• Baro-receptors
• Important in maintaining postural blood
  pressure
• (When standing from lying down ? strong
  sympathetic discharge)
• Long term changes in blood pressure result
  in resetting of baroreflexes (i.e. not
  influential)

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Case report

• 72 year old lady
• Collapsing in street, often when just about to
  cross road
• History of ischaemic heart disease and asthma
• Clinical examination unremarkable
• Postural blood pressures normal
• Referred for tilt table testing and carotid sinus
  massage

Cardio-inhibitory Carotid Sinus Hypersensitivity
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Chemo-receptors

• Sensitive to low O2, high CO2 and acidosis
• Chemoreceptor organs
• 2 carotid bodies (one each bifurcation)
• 1-3 aortic bodies (adjacent to aorta)
• Separate blood supply
• Reduction in blood flow (reduction in pressure
  lt80mmHg) causes metabolic stimulation
• Excitatory effect on vasomotor centre

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• Vasomotor centre
• Vasoconstrictor area
• origin of excitatory pre-ganglionic
  vasoconstrictor neurones
• Vasodilator area
• internal inhibition of vasoconstrictor area
• Sensory area
• input from vagus and glossopharyngeal nerves
  modulate vasoconstrictor/dilator area activity

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• Anatomy of the autonomic nervous control of blood
  pressure
• Sympathetic vasomotor nerve fibres leave cord
  through (TL) spinal nerves
• Sympathetic chain ?
• 1)Sympathetic nerves (viscera)
• 2)Spinal nerves (vasculature)
• Vagus nerve (PNS)

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• To raise the arterial pressure
• SNS release NA from nerve terminals
• NA acts on the a adrenergic receptors of the VSMC
• All arterioles constricted
• Veins strongly constricted
• Heart directly stimulated

Not innervated capillaries, precapillary
sphincters and metarterioles
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.Its all about intracellular calcium
Cardiovascular Physiology Concepts, Richard E
Klabunde. www.cvphysiology.com
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Beta-adrenoceptors
Heart Increased contractility
Blood vessel reduced contractility
Cardiovascular Physiology Concepts, Richard E
Klabunde. www.cvphysiology.com
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Drug Receptor Clinical use
Adrenaline ß1 ß2 gt a1 a2 Anaphylactic shock, Cardiogenic shock, Cardiac arrest
Noradrenaline ß1 a1 gtß2 a2 Septic shock
Dopamine ß1 ß2 gt a1 Cardiogenic shock
Dobutamine ß1 gt ß2 gt a1 Cardiogenic shock
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Intermediate responses to changes in blood
pressure

• Within minutes
• Starling forces at the level of the capillary bed
• Redistribution of fluid within the
  vascular-interstitial compartments

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Atrial volume reflex
Reduced secretion of anti-diuretic hormone from
hypothalamus
Atrial stretch due to pressure
Reflex dilation of renal afferent arteriole
Reduction in water resorbtion from renal tubule
Fluid loss by kidneys
Increased glomerular capillary pressure
Increased filtration of fluid into renal tubule
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Long term control The kidneys

• Pressure diuresis and pressure natriuresis
• Renin Angiotensin- Aldosterone system
• Almost unlimited capacity
• Not a neural reflex
• Restores BP to precise limits

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Pressure diuresis or pressure natriuresis
A primitive adaptive system
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http//en.wikibooks.org/wiki/Human_Physiology/The_
Urinary_System
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Vascular Remodelling in common diseases
Schofield, Circulation 20021063037-43
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Peri-vascular adipose tissue (PVAT)

• Role in vascular tone?

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Summary

• Maintaining BP within a tight limit is a
  physiological imperative.
• Immediate, Intermediate and delayed systems
• Neural reflexes, fluid shifts and renal
  mechanisms
• Age related changes to the vessel tone causes a
  gradual increase in BP as we age
• Vascular remodelling Hypertension, diabetes

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Acknowledgements

• Dr Adam Greenstein
• Dr Reza Aghamohammadzadeh

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??Supporting lecture on HypertensionProf A M
Heagerty (Professor of Medicine, UoM) Available
on-line