Chapter 10 - Guyton Flashcards
Pathway of heartbeat.
S-A node–>internodal pathway–>A-V node–>delayed in A-V node and bundle–>left and right bundles of Purkinje fibers
Where is the SA node located?
in the superior posterolateral wall of the right
atrium immediately below and slightly lateral to the opening of the superior vena cava
Which channels cause the action potential at the SA node?
slow calcium channels (potassium channels return it to resting potential)
Internodal pathways
transmits cardiac impulse through the atria; anterior, middle, and posterior; anterior interatrial bands carries impulses to left atrium
What delays cardiac impulse?
A-V node
Is most delay in the A-V node or bundle?
node (0.09 sec vs bundle 0.04 sec)
Purkinje system
fast conduction, many gap junctions at intercalated discs, spreads potential to ventricles
Transmission time between A-V bundles and last of ventricular fibers is:
0.06 sec (QRS time)
Sinus node is pacemaker because?
faster discharge rate, approx. 70-80 bpm (A-V node 40-60 bpm, Purkinje 15-40 bpm)
What is the ectopic pacemaker?
portion of heart with more rapid discharge than the SA node, occurs when transmission between SA and AV is disrupted (A-V block)
Parasympathetic effects on heart rate.
parasympathetic (vagal) nerves that innervate SA and AV releases Ach which causes hyperpolarization by increased potassium permeability (slows or stops heart rate - ventricular escape)
Sympathetic effect on heart rate.
releases NE at nerve ending, increased sinus node discharge, increased rate of contraction and force of contraction in atria and ventricles
Main Arrival Times
SA node 0.0 sec; AV node 0.03 sec; AV bundle 0.12 sec; Ventricular septum 0.16 sec
What causes self-excitation of the sinus node?
the inherent leakiness of the sinus nodal fibers to sodium and calcium ions causes their self-excitation (high sodium ion concentration in the extracellular fluid outside the nodal fiber, as well as a moderate number of already open sodium channels)
