General Schemaatic of HR Control

Regulation of Cardiac Cycle

Autonomic nerve impulses alter the activities of the S-A and A-V nodes

Overview of short-term control mechanisms

From: Introduction to Cardiovascular physiology. J.R. Levick. Arnold 4th edition (2003)

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)

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

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

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.

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.

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

Vasopressin• Enhances water retention• Causes vasoconstriction• Secretion increased by aortic

baroreceptors and atrial sensors

http://www.cvphysiology.com/Blood%20Pressure/BP016.htm

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