P: Cardiovascular System 6

Cardiovascular System

Overview

The cardiovascular system is responsible for the transport of blood, nutrients, gases, and waste products throughout the body. Its function is tightly regulated by electrical and mechanical activities that ensure effective circulation.

Electrical Activity of the Heart

Pacemaker Cells and Heart Rhythmicity

The heart's rhythmic contractions are coordinated by specialized pacemaker cells that generate and conduct electrical impulses. This process ensures the heart beats in a synchronized manner to pump blood efficiently.

• Sinoatrial (SA) Node: Known as the natural pacemaker of the heart, the SA node initiates spontaneous electrical impulses that trigger atrial contraction.

• Atrioventricular (AV) Node: Receives the impulse from the SA node and delays transmission, allowing the atria to finish contracting before the ventricles begin.

• Bundle of His: Transmits impulses from the AV node through the interventricular septum.

• Right and Left Bundle Branches: Conduct impulses to the right and left sides of the heart.

• Purkinje Fibers: Distribute the impulse rapidly throughout the ventricles, initiating ventricular systole from the apex upward, ejecting blood into the arteries.

Additional info: The coordinated sequence of electrical activation ensures efficient blood flow and prevents arrhythmias.

Generation of Electrical Impulses in the SA Node

Pacemaker cells in the SA node generate action potentials that set the heart rate. This process involves specific ion channels and changes in membrane potential.

• HCN Channels: Hyperpolarization-activated cyclic nucleotide-gated channels open during hyperpolarization, allowing Na+ ions to leak in slowly.

• Pacemaker Potential: The slow influx of Na+ causes a gentle upward drift in voltage, known as the pacemaker potential.

• Once the membrane potential reaches about -40 mV, voltage-gated Ca2+ channels open, causing a rapid influx of Ca2+ and generating an action potential (depolarization).

• Subsequently, K+ channels open, allowing K+ to leave the cell, repolarizing the membrane and making it negative again.

Equation:

$\text{Cardiac Output (CO)} = \text{Stroke Volume (SV)} \times \text{Heart Rate (HR)}$

Additional info: The pacemaker potential is crucial for the automaticity of the heart, allowing it to beat independently of nervous input.

Electrocardiogram (ECG or EKG)

Principles and Interpretation

An electrocardiogram (ECG or EKG) is a clinical tool used to record the heart's electrical activity. It provides valuable information about the timing and sequence of cardiac events.

• P wave: Represents atrial depolarization leading to atrial contraction.

• QRS complex: Represents ventricular depolarization leading to ventricular contraction.

• T wave: Represents ventricular repolarization leading to relaxation.

Additional info: The ECG is essential for diagnosing arrhythmias, conduction abnormalities, and myocardial infarction.

Clinical Applications of ECG

ECG is used to detect various cardiac conditions and monitor heart health.

• Arrhythmias: Irregular heart rhythms such as bradycardia (slow heart rate), tachycardia (fast heart rate), and fibrillation.

• Conduction Problems: Includes AV node blocks and bundle branch blocks.

• Heart Attack Detection: Changes in ECG patterns can indicate myocardial infarction.

Example: Atrial fibrillation is characterized by irregular, rapid electrical activity in the atria, visible as erratic baseline and absence of distinct P waves on ECG.

Summary Table: Cardiac Conduction System

Component	Location	Function
Sinoatrial (SA) Node	Right atrium	Initiates heartbeat, sets heart rate
Atrioventricular (AV) Node	Interatrial septum	Delays impulse, allows atrial contraction
Bundle of His	Interventricular septum	Transmits impulse to ventricles
Bundle Branches	Right and left sides of septum	Conduct impulse to Purkinje fibers
Purkinje Fibers	Ventricular walls	Distribute impulse, trigger ventricular contraction