Cardiovascular System Study Guide

NURS 231: Cardiovascular System Part 1

Learning Objectives

Chapter 18 Overview

1. Cardiac Muscle and Intrinsic Conduction System

   • Describe the structure and function of cardiac muscle and understand its intrinsic conduction system (Pages 684-688).

2. Modifying Heart Rhythm

   • Understand how to modify the basic rhythm of the heart (Pages 688-689).

3. Action Potentials in Heart Muscle

   • Compare and contrast action potentials in different regions of the heart: Autorhythmic cells (Page 687) and contractile muscle cells (Pages 689-690).

4. ECG Interpretation

   • Describe a typical electrocardiogram (ECG) (Pages 690-692).

5. Electrical and Mechanical Events

   • Relate the electrical and mechanical events of the heart with blood flow (Pages 694-695).

6. Heart Sounds

   • What are heart sounds? (Pages 693, 696).

7. Cardiac Output

   • Define cardiac output and how to regulate heart rate and stroke volume (Pages 696-699).

Heart Anatomy Review

Chambers of the Heart

• Receiving chambers

  • Right Atrium: Receives blood returning from the systemic circuit.

  • Left Atrium: Receives blood returning from the pulmonary circuit.

• Pumping chambers

  • Right Ventricle: Pumps blood through the pulmonary circuit.

  • Left Ventricle: Pumps blood through the systemic circuit.

Interventricular Septum

• The interventricular septum separates the right ventricle and left ventricle.

Blood Flow Through the Heart

• Oxygen-poor blood returns via:

  • Superior vena cava (SVC)

  • Inferior vena cava (IVC)

  • Coronary sinuses

• Blood Pathway:

  1. Right Atrium → Tricuspid Valve → Right Ventricle → Pulmonary Semilunar Valve → Pulmonary Trunk → Pulmonary arteries to the lungs.

  2. Left Atrium → Mitral Valve → Left Ventricle → Aortic Semilunar Valve → Aorta → Oxygen-rich blood delivered to body tissues.

  3. Oxygen-rich blood returns to the heart via four pulmonary veins.

Types of Muscle Tissue: Cardiac Muscle

• Characteristics of Cardiac Muscle:

  • Involuntary: Not under conscious control.

  • Striated: Has striations similar to skeletal muscle.

  • Uninuclear: Typically has one nucleus per cell but can have two.

  • Short, branched, interconnected: Structure that facilitates communication and functioning as a unit.

Intercalated Discs in Cardiac Muscle

• Function: Intercalated discs link adjacent cardiac cells.

  • Desmosomes: Prevent cells from separating during contraction.

  • Gap Junctions: Allow ions to pass between cells, promoting synchronized contraction and coordination of the myocardium.

Differences Between Skeletal and Cardiac Muscle

Electrical Events of the Heart

• The heart can depolarize and contract without neural input.

• Rhythm modification can be influenced by the autonomic nervous system (ANS).

• The coordinated heartbeat relies on:

  1. Gap Junctions: Electrically link heart cells.

  2. Intrinsic Cardiac Conduction System: A network of noncontractile (autorhythmic) cells that initiates and distributes impulses for coordinated depolarization and contraction.

The Intrinsic Conduction System: Setting the Basic Rhythm

1. Pacemaker Cells: Initiate the action potentials.

   • Cardiac pacemaker cells feature unstable resting membrane potentials, known as pacemaker potentials or prepotentials.

2. Three Phases of Action Potential:

   1. Pacemaker Potential: The interior of the cell becomes more positive as Na⁺ channels open, leading to depolarization.

   2. Depolarization: Ca²⁺ channels open at a threshold around -40 mV, causing a significant influx of Ca²⁺.

   3. Repolarization: K⁺ channels open, allowing efflux of K⁺, resulting in the cell becoming more negative.

3. Sequence of Excitation: Impulses pass across the heart in approximately 0.22 seconds:

   • Sinoatrial (SA) node → Atrioventricular (AV) node → AV Bundle → Right and Left Bundle Branches → Purkinje Fibers.

SA Node:

• The pacemaker of the heart located in the right atrial wall; depolarizes faster than the rest of the myocardium, spreading impulses across the atria to the AV node.

AV Node:

• Delays impulse by approximately 0.1 seconds to allow for atrial contraction before ventricular contraction; this is due to smaller diameter fibers with fewer gap junctions.

AV Bundle (Bundle of His):

• The only electrical connection between atria and ventricles.

Right and Left Bundle Branches:

• Pathways in the interventricular septum that carry impulses towards the heart's apex.

Purkinje Fibers:

• Cause contraction of the ventricles; the system is more elaborate on the left side of the heart.

Modifying Basic Rhythm: Extrinsic Innervation of the Heart

• Heartbeat modified by ANS:

  • Cardioacceleratory Center: Sympathetic trunk increases rate and force, stimulating SA and AV nodes and coronary arteries.

  • Cardioinhibitory Center: Parasympathetic signals via vagus nerve decrease the heart rate by inhibiting SA and AV nodes.

Action Potentials of Contractile Cardiac Muscle Cells

1. Depolarization Phase: Fast voltage-gated Na⁺ channels open, leading to rapid influx of Na⁺ (from -90 mV to +30 mV).

2. Plateau Phase: Slow Ca²⁺ channels remain open following Na⁺ channels, prolonging depolarization.

3. Repolarization Phase: After approximately 200 ms, Ca²⁺ channels close and K⁺ channels open, resulting in rapid efflux of K⁺, returning to resting membrane potential; Ca²⁺ is pumped back into SR and out of the cell into the extracellular space.

Electrocardiography (ECG)

• Definition: Electrocardiograph detects electrical currents generated by the heart, producing a graphic recording called an electrocardiogram (ECG or EKG).

• Components of ECG:

  • P Wave: Depolarization of the SA node and atria, with atrial contraction commencing after the P wave begins.

  • QRS Complex: Represents ventricular depolarization preceding ventricular contraction; meanwhile, atrial repolarization occurs.

  • T Wave: Indicates ventricular repolarization, with completion of depolarization seen after the T wave.

  • P-R Interval: Time from the start of atrial excitation to the beginning of ventricular excitation.

  • S-T Segment: Entire ventricular myocardium depolarized.

  • Q-T Interval: Time from beginning of ventricular depolarization through ventricular repolarization.

Diagnostic Uses of ECG

• ECG can diagnose various disorders:

  • Prolonged Q-T intervals indicate repolarization abnormalities, increasing the risk of ventricular arrhythmias.

  • Enlarged R waves may suggest enlarged ventricles.

  • Junctional blocks, flutters, and fibrillations can also be detected.

Mechanical Events of the Heart

• Systole: Period of heart contraction.

• Diastole: Period of heart relaxation.

• Cardiac Cycle: Represents blood flow through the heart during one complete heartbeat, involving atrial and ventricular systole and diastole. The cycle includes pressure and volume changes in the heart, with mechanical events following electrical activities seen on the ECG.

Phases of the Cardiac Cycle

1. Ventricular Filling (Mid-to-Late Diastole): Pressure is low, blood flows into atria passively, followed by atrial contraction leading to blood flow into the ventricles. This phase ends with end-diastolic volume (EDV).

2. Ventricular Systole: Atria relax while ventricles contract; AV valves are closed during this isovolumetric contraction phase. The ejection phase occurs as pressure forces semilunar valves open.

3. Isovolumetric Relaxation (Early Diastole): End-systolic volume (ESV) refers to the blood volume remaining in the ventricles after systole. As blood backflows into the aorta and pulmonary trunk, SL valves close, and AV valves open, beginning the cycle anew.

Understanding the Relationship of Pressure and Electrical Events

• The P wave (atrial depolarization) precedes atrial contraction, the QRS complex (ventricular depolarization) precedes ventricular contraction, and the T wave (ventricular repolarization) precedes ventricular relaxation. Pressure changes drive volume changes, directly influencing blood flow through the heart phases.

Summary of Heart Sounds

• Heart sounds are associated with the closure of heart valves:

  • The first heart sound (lub) is linked to the closing of AV valves at the beginning of ventricular systole.

  • The second sound (dub) correlates to the closure of SL valves at the beginning of ventricular diastole, with a brief pause indicating heart relaxation.

Cardiac Output (CO)

• Definition: Cardiac Output is the amount of blood pumped by each ventricle per minute.

• Stroke Volume (SV): The volume of blood ejected from one ventricle per beat.

• Relationship:

  • ext{Cardiac Output} (CO) = ext{Heart Rate} (HR) imes ext{Stroke Volume} (SV)

  • Average CO at rest is approximately 5.25 L/min, calculated as:

  • ext{CO} = ext{HR} imes ext{SV} = 75 ext{ beats/min} imes 70 ext{ ml/beat}

Factors Affecting Stroke Volume

• Preload: Degree to which cardiac muscle cells are stretched before contraction; slow heart rates and exercise enhance venous return, increasing contraction force and stretching the ventricles.

• Contractility: Refers to the strength of contraction at a specific muscle length; it can be positively influenced by sympathetic release of epinephrine, which heightens Ca²⁺ influx and increases cross-bridge formations, or negatively affected by acidosis and calcium channel blockers.

• Afterload: This is the back pressure exerted by arterial blood that structures must overcome to eject blood. Conditions like hypertension increase afterload and lead to higher ESV and reduced SV.

Regulation Summary for Stroke Volume

• Exercise increases venous return due to enhanced sympathetic activity and skeletal muscle/respiratory pumps, leading to an increase in preload and stroke volume.

Heart Rate Regulation via the Autonomic Nervous System (ANS)

• Sympathetic Nervous System: Activated through emotional or physical stressors leads to the release of norepinephrine, which binds to beta-1 adrenergic receptors on the heart, resulting in:

  • Increased heart rate due to rapid pacemaker firing.

  • Decreased end-diastolic volume (fill time).

  • Increased contractility resulting in reduced end-systolic volume.

• Parasympathetic Nervous System: Counteracts sympathetic effects by releasing acetylcholine, hyperpolarizing pacemaker cells by opening K⁺ channels. The vagus nerve's parasympathetic tone predominates at rest with little to no effect on contractility.

Effects of Other Factors on Heart Rate

• Age: Heart rate is fastest in fetuses, decreasing with age.

• Gender: Females generally have a faster heart rate than males.

• Exercise: Increases heart rate; trained athletes may exhibit a slower resting heart rate.

• Body Temperature: Increased body temperature correlates to increased heart rates.

Conclusion

In summary, understanding the cardiovascular system necessitates knowledge of the heart's anatomy, muscle types, electrical and mechanical processes, regulation, and output — all of which are interconnected and vital for maintaining effective blood flow and systemic homeostasis. This foundational knowledge assists in recognizing the physiological implications of various heart conditions and the body's responses during physical activity or stress.