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General Schemaatic of HR Control
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Regulation of Cardiac Cycle
Autonomic nerve impulses alter the activities of the S-A and A-V nodes
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Overview of short-term control mechanisms
From: Introduction to Cardiovascular physiology. J.R. Levick. Arnold 4th edition (2003)
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Heart rateAutonomic regulation (medullary CV center): Receives input from higher brain centers and variety of sensory receptors
– Proprioceptors
– Chemoreceptors
– Baroreceptors
• Sympathetic output ↑HR and contractility
• Parasympathetic impulses
↓ HR– Little effect on
contractility
(does not innervate ventricular myocardium)
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Heart rate• Several factors contribute to
regulation of heart rate:– Chemical regulation
• Cardiac activity depressed by– Hypoxia
– Acidosis
– Alkalosis
• Hormones– Catecholamines and thyroid hormones
increase HR and contractility
• Cations– Alterations in balance of K+, Na+ and Ca2+
alter HR and contractility
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Heart rate• Several other factors contribute to
regulation of heart rate:• Age
• Gender– Female HR higher
• Physical fitness– Resting bradycardia
• Body temperature– Increase causes SA node to discharge
more rapidly
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PNS
• Vagus nerve (via ACh) ↓ HR by ↓ slow inflow of Na+ and Ca++ and by ↑ the subsequent outflow of potassium (K+).
• Acts at SA and AV nodes.
• May treat SNS-driven heart attack by gagging or massage of carotid arteries activate vagal reflexes PNS counteracts SNS.
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The Cardiovascular Stress Response
• Get the heart to beat faster: ↑ SNS tone, ↓ PNS tone
• Norepinephrine (NE) and epinephrine (Epi) ↑ slow inflow of Na+ and Ca++ increase rate of re-excitation in SA node.
• This ↑ Ca++ also increases contractility.• SNS terminals also excite AV node and
whole myocardium: enhances contractility everywhere.
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Summary of long term BP control
• Cardiac output and BP depend on renal control of extra-cellular fluid volume via:– Pressure natriuresis, (increased renal filtration)– Changes in:
• Vasopressin• Aldosterone• Atrial natiuretic peptide
All under the control of altered cardiovascular receptor signaling
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Vasopressin• Enhances water retention• Causes vasoconstriction• Secretion increased by aortic
baroreceptors and atrial sensors
http://www.cvphysiology.com/Blood%20Pressure/BP016.htm
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Baroreceptor reflexBlood pressure
falls
Aortic archAortic arch Carotid sinusCarotid sinus
Constriction of veins &
arterioles
Constriction of veins &
arterioles
Increased stroke volume
Increased stroke volume
Increased heartrate
Increased heartrate
VasoconstrictionVasoconstriction Cardiac stimulationCardiac stimulation Cardiac inhibitionCardiac inhibition
Nucleus tractus solitariusNucleus tractus solitarius
Increased peripheral resistance
Increased cardiac output
Increased blood pressure
Neural integrationNeural integration
SensorsSensors
EffectorsEffectors