Section 2.3

The heart has two populations of cardiac myocytes (heart cells):

Nonpacemaker cells and Pacemaker cells

Pacemaker cells

(about 1 percent) – Specialized cells with the property of autorhythmicity.

Nonpacemaker cells

(about 99 percent) – Regular contractile myocytes that contract when stimulated by electrical impulses.

Autorhythmicity

Autorhythmicity means that pacemaker cells can generate their own electrical impulses without needing signals from the nervous system.

Sinoatrial (SA) Node

• Location: Upper right atrium near the superior vena cava.

• Function: Main pacemaker of the heart; initiates each heartbeat and sets the heart rate.

• Rhythm type: Normal heartbeat generated by the SA node is called sinus rhythm.

Atrioventricular (AV) Node

• Location: Lower right atrium near the tricuspid valve.

• Function: Backup pacemaker; generates about 40 beats per minute.

• Electrical role: Acts as the gateway for impulses to enter the ventricles.

Purkinje System

• Function: Transmits impulses from the AV node to the ventricles.

  • Atrioventricular (AV) Bundle or Bundle of His – Fibers in the lower interatrial septum and upper interventricular septum that carry impulses from the AV node toward the ventricles.

  • Right and Left Bundle Branches – Carry impulses down each side of the interventricular septum toward the apex.

  • Purkinje Fibers – Located just beneath the endocardium (subendocardial location) before penetrating into the ventricular walls; distribute excitation to ventricular muscle cells.

  • The Purkinje fibers can act as a last-resort pacemaker, producing idioventricular rhythm

Depolarization

triggers contraction (systole)

Repolarization

triggers relaxation (diastole)

P wave

Produced when a signal from the SA node spreads through the atria, causing atrial depolarization. At the end of the P wave, both atria have depolarized and contract (atrial systole).

T wave

Represents ventricular repolarization. At the end of the T wave, both ventricles have repolarized and relax. Ventricles take longer to repolarize than depolarize, so the T wave is smaller, wider, and more rounded than the QRS complex.

Cardiac Output (CO)

Amount of blood each ventricle pumps in one minute

Peripheral Resistance (PR)

Impedance to blood flow in blood vessels

Blood Volume (BV)

Total blood in the vascular system at any given time

Systolic Pressure

Peak arterial pressure during ventricular contraction (systole)
Normal: 100 to 120 mmHg

Diastolic Pressure

Lowest arterial pressure during ventricular relaxation (diastole)
Normal: 60 to 80 mmHg