Cardiovascular Physiology: The Heart as a Pump and the Cardiac Cycle

Cardiovascular Physiology

Introduction

This section covers the essential mechanisms by which the heart functions as a pump, the electrical and mechanical events of the cardiac cycle, and the interpretation of electrocardiograms (ECGs). Understanding these processes is fundamental to grasping how blood is circulated throughout the body and how cardiac health is assessed.

The Heart as a Pump

Electrical Signals and Conduction System

The heart's ability to pump blood relies on coordinated electrical signals that initiate muscle contraction. The conduction system ensures that these signals are distributed efficiently.

• Sinoatrial (SA) node: The primary pacemaker of the heart, located in the right atrium, sets the pace at approximately 70 beats per minute (bpm).

• Atrioventricular (AV) node: Receives signals from the SA node via the internodal pathway; can act as a secondary pacemaker at 50 bpm.

• Purkinje fibers: Specialized conducting fibers that transmit electrical signals rapidly through the ventricles; can act as pacemakers at 25–40 bpm under certain conditions.

• Pacemakers: Cells that set the heart rate by generating spontaneous action potentials.

Pathway of Electrical Conduction

• Electrical signals originate in the SA node and travel to the AV node.

• Signals are routed through the AV bundle (bundle of His) and then to the left and right bundle branches.

• Purkinje fibers distribute the signal to ventricular muscle cells, ensuring coordinated contraction from apex to base.

Figure 14.14: Electrical Conduction in Myocardial Cells

• Autorhythmic cells: Generate spontaneous action potentials.

• Action potentials spread to adjacent contractile cells via gap junctions in intercalated disks.

Figure 14.15: The Conducting System of the Heart

• Shows the anatomical locations of the SA node, AV node, bundle branches, and Purkinje fibers.

• Illustrates the sequence of electrical activation in the heart.

The Electrocardiogram (ECG)

Principles of ECG

An electrocardiogram (ECG) records the summed electrical activity generated by all the cells of the heart. It is not the same as a single action potential but reflects the overall electrical events during the cardiac cycle.

• Leads: Electrodes placed on the body to detect electrical signals; standard limb leads form an equilateral triangle around the heart.

• ECG waves:

  • P wave: Atrial depolarization

  • QRS complex: Ventricular depolarization (includes atrial repolarization)

  • T wave: Ventricular repolarization

• ECG segments:

  • P-R segment: AV nodal delay

  • T-P segment: Ventricular and atrial relaxation

Figure 14.16: The Electrocardiogram

• Shows the placement of leads and the resulting ECG trace.

• Demonstrates how the direction of deflection relates to the direction of electrical activity in the heart.

Interpretation of ECGs

• Heart rate: Determined by the time between two P waves or two Q waves.

  • Tachycardia: Faster than normal heart rate

  • Bradycardia: Slower than normal heart rate

• Rhythm: Regularity of the heartbeats

• Waveform: Presence and shape of normal waves

• QRS complex: Should be present for each P wave; P-R segment should be constant in length

• Pathologies: Cardiac arrhythmias can be detected by abnormal ECG patterns

Electrical and Mechanical Events of the Cardiac Cycle

Electrical Events

• Mechanical events (contraction and relaxation) lag behind electrical events (depolarization and repolarization).

• ECG begins with atrial depolarization (P wave), followed by atrial contraction.

• P-R segment: Signal passes through AV node and bundle.

• Q wave end: Ventricular contraction begins and continues through T wave.

Mechanical Events: The Cardiac Cycle

The cardiac cycle consists of alternating periods of contraction (systole) and relaxation (diastole) in the heart chambers.

• Diastole: Cardiac muscle relaxes, allowing chambers to fill with blood.

• Systole: Cardiac muscle contracts, ejecting blood from the chambers.

Five Phases of the Cardiac Cycle

1. Atrial and ventricular diastole: Both chambers are relaxed; AV valves open, ventricles fill with blood from veins.

2. Completion of ventricular filling (atrial systole): Atria contract, pushing the last 20% of blood into ventricles. End-diastolic volume (EDV): Volume in ventricle at the end of relaxation.

3. Early ventricular contraction and first heart sound: AV valves close, producing the "lub" sound; no blood enters or exits (isovolumic contraction); pressure increases.

4. Ventricular ejection: Semilunar valves open, blood is ejected into arteries. End-systolic volume (ESV): Volume in ventricle at the end of contraction.

5. Ventricular relaxation and second heart sound: Semilunar valves close, producing the "dup" sound; pressure drops, AV valves open when ventricular pressure falls below atrial pressure.

Figure 14.18: Mechanical Events of the Cardiac Cycle

• Illustrates the pressure-volume loop (PV loop) for the left ventricle, showing changes in pressure and volume during the cardiac cycle.

Figure 14.19: The Wiggers Diagram

• Integrates electrical, mechanical, and pressure events in the cardiac cycle.

• Shows the relationship between ECG waves, heart sounds, pressure changes, and volume changes.

Summary Table: Key Events of the Cardiac Cycle

Phase	Event	Valve Status	Volume Change
Atrial & Ventricular Diastole	Chambers relax, fill with blood	AV open, semilunar closed	Ventricular volume increases
Atrial Systole	Atria contract, complete filling	AV open, semilunar closed	EDV reached
Isovolumic Ventricular Contraction	Ventricles contract, first heart sound	AV closed, semilunar closed	No change
Ventricular Ejection	Blood ejected into arteries	AV closed, semilunar open	Volume decreases to ESV
Isovolumic Ventricular Relaxation	Ventricles relax, second heart sound	AV closed, semilunar closed	No change

Key Formulas

• Stroke Volume (SV): Amount of blood ejected per beat $SV = EDV - ESV$

• Cardiac Output (CO): Total blood pumped per minute $CO = SV \times HR$

Additional info:

• ECG analysis is crucial for diagnosing arrhythmias and other cardiac pathologies.

• The Wiggers diagram is a comprehensive tool for understanding the timing and relationships of cardiac events.