Heart

Principles of Anatomy and Physiology

Introduction

  • Purpose of this Chapter:

    • Learn about the components of the cardiovascular system

    • Focus on the anatomy and physiology of the heart

    • Learn about the cardiac cycle

    • Discuss various factors that influence heart rate and force of contraction

Location of the Heart

General Location

  • The heart is located in the mediastinum of the thoracic cavity.

Pericardium

Structure of the Pericardium

  • The heart is enclosed and held in place by the pericardium.

  • Components:

    • Outer fibrous pericardium: Provides protection and anchors heart to surrounding structures.

    • Inner serous pericardium: Consists of two layers:

    • Visceral layer: Adheres to the heart.

    • Parietal layer: Lines the fibrous pericardium.

  • Serous cavity: A fluid-filled space between visceral and parietal layers that allows for smooth heart movement.

Cardiac Muscle Anatomy

Overview of Cardiac Muscle Structure

  • Anatomical Overview:

    • The cardiac muscle structure includes various bundles:

    • Superior vena cava

    • Aorta

    • Pulmonary trunk

    • Superficial and deep muscle bundles in the atria and ventricles.

Layers of the Heart Wall

  • The wall of the heart consists of three layers:

    • Epicardium: The outermost layer, also part of the pericardium.

    • Myocardium: The muscular middle layer responsible for heart contractions.

    • Endocardium: The inner layer lining the chambers and valves of the heart.

Clinical Connections

Inflammatory Conditions of the Heart

  • Pericarditis: Inflammation of the pericardium.

    • Acute pericarditis: Sudden onset, often linked to viral infection.

    • Chronic pericarditis: Gradual onset, associated with fluid build-up around the pericardium.

  • Myocarditis: Inflammation of the myocardium, typically a complication of viral infections.

  • Endocarditis: Inflammation of the endocardium, usually due to bacterial infection, often affecting heart valves.

Chambers of the Heart

Structure of Heart Chambers

  • The heart consists of four chambers:

    • Two upper atria: (Right and Left Atrium)

    • Two lower ventricles: (Right and Left Ventricle)

Function of Each Chamber

  • Right Atrium: Receives blood from the

    • Superior vena cava (SVC)

    • Inferior vena cava (IVC)

    • Coronary sinus.

  • Right Ventricle: Receives blood from the right atrium and sends it to the lungs for oxygenation.

  • Left Atrium: Receives blood from the pulmonary veins.

  • Left Ventricle: Receives blood from the left atrium and pumps oxygen-rich blood to the entire body. The left ventricle wall is thicker than that of the right due to increased pressure requirement for systemic circulation.

Fibrous Skeleton

Functions

  • The fibrous skeleton serves multiple functions:

    • Foundation for heart valve attachment.

    • Insertion point for cardiac muscle bundles.

    • Prevents overstretching of heart valves.

    • Acts as an electrical insulator between atria and ventricles.

Heart Valves and Blood Circulation

Function of Valves

  • Heart valves open and close in response to pressure changes during heart contractions:

    • Atrioventricular valves (Right and Left): Prevent backflow from the ventricles into the atria.

    • Semilunar valves (Right and Left): Prevent backflow from the arteries into the ventricles.

Blood Flow Through the Heart

  1. Blood enters the right atrium from the superior and inferior vena cavae.

  2. Blood flows from the right atrium to the right ventricle through the right atrioventricular valve.

  3. Right ventricle pumps blood to the lungs through the pulmonary valve.

  4. Oxygen-rich blood returns to the left atrium from the lungs through the pulmonary veins.

  5. Blood flows from the left atrium to the left ventricle through the left atrioventricular valve.

  6. The left ventricle pumps blood to the body through the aorta.

Coronary Circulation

Overview

  • The coronary circulation involves blood flow through coronary arteries and veins:

    • Coronary arteries: Deliver oxygenated blood and nutrients to the myocardium.

    • Arise from the ascending aorta.

    • Coronary veins: Remove carbon dioxide and waste products from the myocardium, converging at the coronary sinus.

Clinical Connections Regarding Myocardial Health

Conditions Affecting Myocardial Health

  • Myocardial Ischemia: Reduced blood flow causing

    • Hypoxia (reduced oxygen supply).

    • Angina pectoris (chest pain).

  • Myocardial Infarction: Heart attack due to tissue death from interrupted blood supply, with treatments including thrombolytic injection, coronary angioplasty, and coronary artery bypass grafting.

Cardiac Muscle Tissue

Structure of Cardiac Muscle Fibers

  • Key components include:

    • Intercalated discs: Connect cardiac muscle fibers for synchronized contractions.

    • Mitochondria: Provide energy through ATP production.

    • Arrangement of thin (actin) and thick (myosin) filaments for contraction.

The Conduction System

Functionality

  • Cardiac muscle cells are self-excitable and autorhythmic.

  • The conduction system facilitates spontaneous action potentials triggering heart contractions.

Pathway of Electrical Conduction

  1. Sinuatrial (SA) node: Acts as the primary pacemaker; generates action potentials.

  2. Atrioventricular (AV) node: Receives impulses from the SA node.

  3. Atrioventricular bundle (Bundle of His): Transmits impulses from the AV node through interventricular septum.

  4. Right and left bundle branches: Conduct impulses to ventricles.

  5. Subendocardial conducting network (Purkinje fibers): Distributes impulses throughout the ventricles.

Artificial Pacemakers

  • Devices that emit electrical currents to stimulate heart contractions; includes a battery and pulse generator connected via leads threaded into heart chambers.

Influences on the Conduction System

  • SA node autorhythmic fibers: Natural pacemaker, can be influenced by:

    • Nervous system signals.

    • Hormones (e.g., epinephrine) modify heart rate and contraction force but do not establish the basic rhythm.

Cardiac Physiology

Action Potential Characteristics

  • In a ventricular fiber, the action potential consists of:

    • Rapid depolarization

    • Plateau phase

    • Repolarization.

Electrocardiogram (EKG or ECG)

  • An EKG records the electrical changes accompanying each heartbeat, providing insights into heart rhythm and function.

Cardiac Cycle

Definition and Components

  • The cardiac cycle refers to the events of contraction (systole) and relaxation (diastole) of the heart chambers.

  • Components include:

    • Electrical events

    • Pressure changes

    • Heart sounds

    • Volume changes

    • Mechanical events.

Events of the Cardiac Cycle

  1. Atrial systole and ventricular diastole: Atria contract filling ventricles with blood.

  2. Ventricular systole: Ventricles contract, pumping blood out to arteries.

  3. Ventricular diastole: Ventricles relax and fill with blood from atria.

Cardiac Output (CO)

Definition and Calculation

  • Cardiac Output (CO): Volume of blood ejected from the ventricles into the aorta or pulmonary trunk per minute.

  • Calculation: CO ext{ (mL/min)} = SV ext{ (mL/beat)} imes HR ext{ (beats/min)}

    • Stroke Volume (SV): Volume of blood pumped out per beat.

Regulation of Cardiac Output

Stroke Volume Regulation

  • Three primary factors that regulate SV:

    • Preload: Degree of stretch of cardiac muscle before contraction.

    • Contractility: Forcefulness of contraction.

    • Afterload: The pressure against which the heart must work to eject blood.

Heart Rate Regulation

  • Various factors regulate heart rate, including:

    • Autonomic nervous system activity.

    • Hormonal influences.

    • Ion concentrations (e.g., Ca2+).

    • Age, gender, physical fitness, and temperature.

Exercise and Heart Health

  • Regular aerobic exercise benefits include:

    • Increased cardiac output

    • Increased HDL cholesterol

    • Decreased triglycerides

    • Improved lung function

    • Decreased blood pressure

    • Weight control.

Help for Failing Hearts

Devices and Procedures

Mechanical and Electrical Support Solutions

  1. Intra-aortic balloon pump (IABP): Improves coronary blood flow and reduces workload on heart.

  2. Ventricular Assist Devices (VADs): Mechanical pumps to support weak ventricles, potentially serving as bridges to transplantation.

    • Types:

      • LVAD: Supports left ventricle.

      • RVAD: Supports right ventricle.

      • BVAD: Supports both ventricles.

  3. Cardiomyoplasty: A piece of the patient's muscle is attached to the heart, contracting to assist with heart function.

Development of the Heart

Developmental Stages

  1. Begin as cardiogenic mesenchyme at 19-21 days in fetal development.

  2. Formation of heart primordial tubes leading to fusion into a heart tube.

  3. Regions of heart structure develop (atrial and ventricular formation, septum development).

Disorders: Homeostatic Imbalances

  • Common heart disorders include:

    • Coronary artery disease

    • Atherosclerotic plaques

    • Congenital heart defects

    • Arrhythmias

    • Congestive heart failure.