Heart-III

Anatomy and Physiology of the Cardiovascular System

Lecture Overview

Course: MCB 246 Anatomy and PhysiologyInstructor: Dr. Bharathi AravamudanTopic: Cardiovascular System (The Heart- III)Textbook Reference: Chapter 19

Learning Outcomes

By the end of this lecture, students should be able to:

  • Describe different segments of the electrocardiogram (ECG) and interpret their significance.

  • Recall events of the cardiac cycle and understand their physiological implications.

  • Define key terms:

    • Stroke Volume (SV): The volume of blood pumped from the ventricle with each heartbeat.

    • Cardiac Output (CO): The total volume of blood pumped by the heart per minute, essential for maintaining systemic circulation.

    • Venous Return: The volume of blood returning to the heart through the veins.

  • Discuss influences on heart rate, including neural, hormonal, and lifestyle factors.

  • Understand development of the heart, highlighting key changes from embryonic stages to adulthood.

  • Appreciate the clinical significance of the discussed concepts, particularly in diagnosing and treating cardiovascular diseases.

Lecture Material Sources

  • Required Reading: Chapter 19; Pages 759 - 760

  • In-class Lecture Slides posted on Canvas (36 content slides)

Electrocardiogram (ECG)

Definition and Purpose

Electrocardiogram (ECG/EKG): A diagnostic tool that employs skin electrodes to measure the electrical activity of the heart muscle cells. It provides vital information on heart rate, rhythm, and electrical conduction pathways, aiding in the diagnosis of various cardiac abnormalities.

Waves of ECG

  • P wave: Reflects atrial depolarization, initiated by the spontaneous discharge of the SA node, triggering atrial contraction.

  • QRS complex: Represents the rapid depolarization of the ventricles, occurring simultaneously with atrial repolarization, which is typically obscured on the ECG.

  • T wave: Associated with ventricular repolarization, marking the end of the contraction cycle for the ventricles.

ECG Segments

  • P-Q Segment: Corresponds to atrial contraction and indicates a plateau phase where no electrical change occurs. It plays a crucial role in understanding atrial mechanics.

  • S-T Segment: Reflects the plateau of ventricular contraction, essential for assessing myocardial oxygen demand.

Intervals in ECG

  • P-R interval: The duration from the beginning of the P wave to the start of the QRS complex; it reflects the time taken for the depolarization wave to travel from the atria to the ventricles, indicating the conduction time through the AV node.

  • Q-T interval: Measured from the beginning of the QRS complex to the end of the T wave; it shows the total duration of ventricular action potentials and is important in assessing the risk of ventricular arrhythmias.

Cardiac Cycle Overview

Definition

The cardiac cycle encompasses all heart events from the onset of one heartbeat to the start of the next, divided into two main phases:

  • Systole: The contraction phase where blood is propelled from the chambers of the heart.

  • Diastole: The relaxation phase where the heart chambers fill with blood, crucial for maintaining adequate cardiac function.

Phases of the Cardiac Cycle

  1. Atrial Relaxation and Ventricular Filling:

    • All chambers are relaxed, allowing blood to return to both atria from the systemic and pulmonary circulation.

    • Atrioventricular (AV) valves remain open, facilitating passive filling of the ventricles with blood.

  2. Atrial Contraction and Ventricular Filling:

    • Atria contract, actively pushing additional blood into the ventricles to complete ventricular filling.

  3. Isovolumic Contraction:

    • Ventricles begin to contract, leading to an increase in pressure, causing the AV valves to close but no blood is ejected yet since the semilunar valves remain closed.

  4. Ventricular Ejection:

    • Ventricles contract fully, ejecting blood into the aorta and pulmonary artery; the Stroke Volume (SV) is quantitatively defined as the volume of blood ejected per heartbeat, typically around 70 mL in a healthy adult.

  5. Isovolumic Relaxation:

    • Ventricles relax after ejection; all valves are closed until ventricular pressure decreases significantly, preparing for the next filling phase.

Heart Rate and Cardiac Output

Cardiac Output (CO)

Definition: The amount of blood ejected by a single ventricle in one minute, a critical measure of heart function.Formula: CO = Heart Rate (HR) x Stroke Volume (SV)Example: If the heart rate is 75 beats/min and stroke volume is 70 mL/beat, then CO = 75 x 70 = 5.25 L/min, indicating the volume of blood pumped into systemic circulation by the heart.

Heart Rate Regulation

Influenced by:

  • Chronotropic Agents: Substances or mechanisms that affect the activity of SA (sinoatrial) and AV (atrioventricular) nodes.

    • Positive Agents: Sympathetic stimulation and hormones like epinephrine increase heart rate.

    • Negative Agents: Parasympathetic activity mediated by the vagus nerve decreases heart rate.

Stroke Volume and Influences

Factors Affecting Stroke Volume:

  1. Preload: The volume of blood returning to the heart; it directly influences end-diastolic volume (EDV), which affects the force of contraction.

  2. Contractility: The intrinsic strength of heart muscle contraction influenced by inotropic agents such as calcium ion availability; greater contractility results in higher SV.

  3. Afterload: The pressure against which the ventricles must work to eject blood; it is influenced by systemic vascular resistance and arterial pressure.

Frank-Starling Law

States that an increase in venous return leads to an increase in the stretch of ventricles, causing stronger contractions due to optimal overlap of actin and myosin filaments, enhancing stroke volume.

Development of the Heart

Key Stages

  • Weeks 3-4: The formation of two heart tubes from lateral mesoderm; fusion creates a primitive heart tube that is capable of rhythmic contractions.

  • Weeks 5-8: Development of septation, resulting in four distinct chambers of the heart; formation of septum primum and septum secundum within the interatrial septum, creating left and right atria.

  • Foramen Ovale: A vital shunt allowing blood to bypass pulmonary circulation in fetal development; this structure closes after birth, becoming fossa ovalis, critical in childhood physiology.

Common Congenital Disorders

  • Atrial Septal Defect: Characterized by an abnormal opening between the atria, leading to altered blood flow dynamics, which can cause right atrial and ventricular enlargement.

  • Ventricular Septal Defect: An incomplete formation of the septum between ventricles, potentially leading to volume overload of the pulmonary circulation.

  • Tetralogy of Fallot: A complex condition involving four heart defects that lead to reduced blood flow to the lungs and oxygen-poor blood sent to systemic circulation.