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 rate Autonomic 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 reflex Sensors Neural integration Effectors Blood pressure falls Aortic arch Carotid sinus Sensors Vasoconstriction Cardiac stimulation Cardiac inhibition Nucleus tractus solitarius Neural integration Constriction of veins & arterioles Increased stroke volume Increased heart rate Effectors Increased peripheral resistance Increased cardiac output Increased blood pressure