USI Summer Session Chapter 18 A&P - The Heart part 6

Regulation of Stroke Volume
degree of stretch of cardiac muscle cells before they contract (Frank-Starling law of the heart)

•Cardiac muscle exhibits a length-tension relationship
•At rest, cardiac muscle cells are shorter than optimal length
•Slow heartbeat and exercise increase venous return
•Increased venous return distends (stretches) the ventricles and increases contraction force
Contractile strength at a given muscle length, independent of muscle stretch and EDV
Increase Contractility
Increased Ca2+ influx due to sympathetic stimulation

Hormones (thyroxine, glucagon, and epinephrine)
Decrease contractility

Increased extracellular K+

Calcium channel blockers
Pressure that must be overcome for ventricles to eject blood
increases afterload

resulting in increased ESV and reduced SV
Sympathetic nervous system
activated by emotional or physical stressors

Norepinephrine causes the pacemaker to fire more rapidly (and at the same time increases contractility)
Parasympathetic nervous system
opposes sympathetic effects

Acetylcholine hyperpolarizes pacemaker cells by opening K+ channels

The heart at rest exhibits vagal tone (parasympathetic)
Atrial (Bainbridge) reflex
a sympathetic reflex initiated by increased venous return

Stretch of the atrial walls stimulates the SA node

Also stimulates atrial stretch receptors activating sympathetic reflexes