Cardiac Conduction System

Conduction System of the Heart

• Sinoatrial (SA) node (pacemaker) → internodal bundles and fibers → right atrium (RA)

• SA node → interatrial bundle and fibers → left atrium (LA)

• Internodal bundle/fibers → atrioventricular (AV) node

• AV node → atrioventricular (AV) bundle (bundle of His)

• AV bundle → right and left AV bundle branches

  • Right branch → subendocardial branches (Purkinje fibers) → lateral wall and septum of right ventricle (RV)

  • Left branch → subendocardial branches (Purkinje fibers) → septum and lateral wall of left ventricle (LV)

The Cardiac Cycle

• One heartbeat is produced

• One cycle of atrial and ventricular contraction followed by relaxation

  • Systole = myocardium contracting, "working"

  • Diastole = myocardium relaxing and repolarizing

• Each chamber goes through systole and diastole, but not at the same time

  • Example: When the atria are in systole, the ventricles will be in diastole

Normal Heart Sounds

• Produced by heart valves snapping shut

• One cardiac cycle produces two distinct heart sounds

  • "Lub" after atrial systole → tricuspid and mitral valves snap shut

  • "Dub" after ventricular systole → pulmonary and aortic valves snap shut

Abnormal Heart Sounds

• Extra sounds heard when the 2 AV valves or 2 semilunar valves are not closing simultaneously

• Valvular insufficiency: one or more valves don't close all the way → murmur

• Valvular stenosis (narrowing): one or more valves won't open all the way → murmur

Stroke Volume

• Volume of blood being ejected by the left ventricle (mL)

• Represents strength of the heartbeat

• Determined by 2 factors:

  • Preload: volume of blood received from atrium

  • Afterload: physical resistance by the aorta the ventricle is ejecting blood into

• Can also be affected by the length of cardiac muscle cells

Cardiac Output (CO)

• Volume of blood ejected out of the left ventricle over a unit of time, usually a minute

• Determined by:

  • Stroke volume (SV) = systolic discharge

  • Heart rate (HR)

• Calculation: CO = SV × HR

Heart Rate

• Normal rate for each species is set internally (rate of spontaneous SA node depolarization)

• Outside control comes through the autonomic nervous system

Arteries

• Carry blood away from the heart

  • Deoxygenated blood to lungs for oxygenation (only during pulmonary circulation)

  • Oxygenated blood throughout the body (systemic circulation)

• Usually come in pairs

• 2 types:

  • Elastic: aorta is the largest elastic artery

  • Muscular: arterioles are the smallest branches of the arterial tree

Capillaries

• Microscopic blood vessels from branching arterioles

• Occur in groups called capillary beds or networks

• Walls are one endothelial cell thick

  • Exchange of gases and nutrients occurs at this level

Veins

• Carry blood toward the heart

  • Oxygenated blood from the lungs (only during pulmonary circulation)

  • Deoxygenated blood and waste materials from throughout the body (systemic circulation)

• Capillaries join together to form venules

  • Venules join together to form veins

• One-way valves and muscular movements

• All systemic veins drain into the vena cava

Blood Vessels: A Summary

• General path: heart → arteries → arterioles → capillaries → venules → veins → heart

• Note: Veins = oxygenated blood TO heart / Artery = deoxygenated blood AWAY from heart (the only exception, referring to pulmonary vessels)