Cardiac Muscle and Heart Conduction System Overview

Autorhythmic fibers

One of the two types of cardiac muscle fibers.

Contractile fibers

Make up the majority of heart tissue.

Sinoatrial (SA) Node

The component of the heart's conduction system known as the pacemaker.

Components of the heart's conduction system

SA node → Internodal fibers → AV node → AV bundle (Bundle of His) → Bundle branches → Purkinje fibers.

P wave

Represents atrial depolarization on an ECG tracing.

Atrial repolarization

Cannot be seen on a standard ECG tracing because it is hidden within the QRS complex.

QRS complex

Represents ventricular depolarization and atrial repolarization on an ECG.

AV node

Slows the action potential to allow the atria to finish contracting.

Purkinje fibers

Wrap around the myocardium and allow unison contraction of the ventricles.

Phases of the cardiac cycle

1) Ventricular filling, 2) Atrial contraction, 3) Isovolumetric contraction, 4) Ventricular ejection, 5) Isovolumetric relaxation.

Isovolumetric contraction phase

AV and semilunar valves are closed. Pressure increases in the ventricles, despite the blood volume remaining unchanged.

Percentage of blood remaining in ventricles after contraction

Approximately 40% of blood remains in each ventricle after contraction.

Valves open during ventricular filling

The atrioventricular (AV) valves.

Trigger for opening of semilunar valves

When pressure in the ventricles exceeds the pressure in the pulmonary trunk and aorta.

Order of blood flow through blood vessels

Arteries → Arterioles → Capillaries → Venules → Veins.

Reabsorption

The movement of small substances from interstitial fluid back INTO capillaries at their distal ends.

Fenestration pores

Small holes or pores through the endothelium of capillaries.

Interstitial fluid

The fluid surrounding the cells of the body (also known as tissue fluid).

Formula for calculating cardiac output

CO = SV × HR (Stroke Volume × Heart Rate).

Effect of increasing blood volume on blood pressure

As blood volume increases, blood pressure increases.

Effect of vasodilation of arteries on blood pressure

Decreases resistance to blood flow, which decreases blood pressure.

Effect of increased parasympathetic impulses to the heart on blood pressure

Decreases blood pressure.

Effect of Aldosterone on blood pressure

Increases blood pressure by increasing blood volume through sodium and water retention.

Flowchart for blood pressure regulation

Renin → Angiotensin II → Aldosterone and ADH (Increases blood volume) & vasoconstriction.

Effect of Antidiuretic Hormone (ADH) on blood pressure

Increased ADH release increases blood pressure by increasing water reabsorption in the kidneys, which increases blood volume.

Effect of constriction of systemic arterioles on blood pressure

Significant constriction of systemic arterioles increases blood pressure by increasing resistance to blood flow.