1. General Schemaatic of HR Control

2. Regulation of Cardiac Cycle
Autonomic nerve impulses alter the activities of the S-A and A-V nodes

3. Overview of short-term control mechanisms
From: Introduction to Cardiovascular physiology. J.R. Levick. Arnold 4th edition (2003)

4. 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)

5. 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

6. 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

7. 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.

8. 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.

9. 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

10. Vasopressin Enhances water retention Causes vasoconstriction
Secretion increased by aortic baroreceptors and atrial sensors

11. 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