Intrinsic Conduction System

The heart comprises four chambers and various gross structures that facilitate blood flow.
A key boundary is the coronary sulcus, which serves as the physical separation between the atria and the ventricles.
The heart operates using an intrinsic conduction system composed of specialized structures including the sinoatrial node and the atrioventricular node.

The sinoatrial node, or SA node, functions as the heart's natural pacemaker.
Etymology of Sinus: The term "sinus" can refer to a large opening.
Anatomical Location: The SA node is located at the base of the superior vena cava within the atrium, situated at its largest opening.
Composition: It consists of specialized pacemaker cells that establish the basic rhythmic rate of the heart.
Cellular Connectivity:
- The SA node is connected to every muscle fiber in both the right and left atria.
- Communication occurs through cardiac muscle fibers linked via gap junctions and specific proteins known as connexon proteins.
Electrical Initiation:
- Upon excitation, the SA node initiates a wave of depolarization that travels across the atria.
- This electrical wave triggers the atria to contract, effectively squeezing blood into the ventricles.

Anatomical Location: The atrioventricular node, or AV node, is positioned at the junction between the atria and the ventricles, near the right AV valve (also known as the tricuspid valve).
Redundant Pacemaker Capability: If the SA node fails to fire, the AV node can take over as the pacemaker by auto-depolarizing.
The Electrical Pause:
- The signal originating from the SA node is funneled through the AV node, where it is "compressed" or paused.
- Duration of Delay: The signal is delayed for approximately $100\,ms$ (or $0.1\,s$ ).
- Biological Purpose: This pause provides sufficient time for the atria to finish contracting and squeezing blood into the expanding ventricles. It allow the atria to begin relaxing before the ventricles initiate their own contraction.
- Avoiding Inefficiency: Without this strategic delay, the ventricles would contract concurrently with or immediately after the atria, causing a "collision of blood" that would compromise the heart's efficiency as a pump.

Post-pause, the electrical signal moves into the AV bundle, commonly referred to as the bundle of His.
The signal then diverges into the right and left bundle branches.
Cellular Specialization of Bundle Branches:
- These branches are made of modified cardiac muscle cells.
- These cells contain very few contractile elements.
- They are characterized by a high volume of cytosol and a high concentration of ions.
- These attributes make them excellent conductors of electrical current or propagating potentials.
Signal Acceleration: The unique structure of these cells accelerates the electrical signal as it travels toward the apex of the heart.

Subendocardial Network: At the apex of the heart, the signal turns upward through the subendocardial network.
Purkinje Fibers: These specialized fibers transmit the signal throughout the walls of both the right and left ventricles.
Mechanical Direction of Contraction:
- By accelerating the signal to the apex first, the heart is able to contract from the apex upward.
- This creates a "wringing" motion that forces blood out of the ventricles efficiently after the atria have already begun to relax.
- This upward direction is crucial for moving blood from the bottom of the chambers toward the exit valves.

The cardiac skeleton is a critical but often overlooked component of heart anatomy made of connective tissue.
Structural Integration: It runs through the myocardium and thickens strategically at various points.
Dual Functionality:
- 1. Structural Anchor: It acts as the "tendon of the heart." Just as skeletal muscles use tendons to anchor to bone, cardiac muscle uses the cardiac skeleton to anchor back onto itself.
- 2. Electrical Insulation: It provides partitions that electrically isolate the atria and their signals from the ventricles and the signals propagating back up them.
Resulting Rhythm: The combination of the intrinsic conduction system and the insulation provided by the cardiac skeleton allows the heart to contract rhythmically, producing the distinct "lub-dup" sound.