Download Control of heart rate

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