Ch18_Heart_new2_slides

Chapter 18 - The Heart

Introduction to the Heart

• The heart is a four-chambered pump.

• Functions:

  • Moves blood around the body.

  • Right side: Receives deoxygenated blood from tissues and pumps it to the lungs.

  • Left side: Receives oxygenated blood from the lungs and pumps it to tissues.

Location of the Heart

Anatomical Position

• Enclosed within the mediastinum between the lungs.

• Apical orientation towards the left and inferiorly.

• Base where great vessels attach to the top.

Pericardium

• Pericardium: Double-layered sac enclosing the heart.

  • Fibrous Pericardium: External layer attached to the diaphragm.

  • Serous Pericardium:

    • Parietal layer: Lines fibrous pericardium.

    • Visceral layer (Epicardium): Covers the heart's surface.

• Pericardial Cavity: Contains pericardial fluid for lubrication.

Heart Wall Layers

• Myocardium is the thick, muscular middle layer of the heart wall. It is composed primarily of cardiac muscle tissue, which is unique for its ability to contract involuntarily and rhythmically.

• Endocardium is the inner lining of the heart chambers and valves, providing a smooth surface for blood flow and helping to prevent blood clots.

• Epicardium, also known as the visceral layer of the serous pericardium, is the outer layer of the heart wall and closely adheres to the myocardium.

Four Chambers

• Atria (A):

  • Right atrium (RA): Receives deoxygenated blood.

  • Left atrium (LA): Receives oxygenated blood.

• Ventricles (V):

  • Right ventricle (RV): Pumps deoxygenated blood to lungs.

  • Left ventricle (LV): Pumps oxygenated blood to the body.

Blood Flow Pathway

• Deoxygenated blood travels via the Superior/Inferior vena cava into the RA, then RV, to the pulmonary trunk (lungs).

• Oxygenated blood returns via pulmonary veins to the LA, then LV, to the aorta (body).

Coronary Circulation

• Coronary Circulation refers to the flow of blood to and from the tissues of the heart.

• The coronary arteries branch off from the aorta and supply oxygen-rich blood to the myocardium (heart muscle).

• Major coronary arteries include the left and right coronary arteries, which further divide into branches to supply different areas of the heart.

• Coronary Veins collect deoxygenated blood from the myocardium and drain it into the right atrium through the coronary sinus.

Systemic Circulation

• Systemic Circulation is the pathway through which oxygenated blood is distributed from the left side of the heart to the rest of the body.

• Blood travels from the left ventricle into the aorta, then branches out through arteries into systemic capillaries.

• Systemic Capillaries: Here, oxygen is delivered to tissues, and carbon dioxide is collected, leading to deoxygenation of blood.

• Systemic Veins: Deoxygenated blood returns to the heart via veins, ultimately entering the right atrium.

Pulmonary Circulation

• Pulmonary Circulation involves the process of moving deoxygenated blood from the right heart to the lungs for gas exchange.

• The Right Heart includes the right atrium and right ventricle; after collecting blood from the systemic circulation, the right atrium sends blood to the right ventricle.

• The Right Ventricle (RV) pumps this deoxygenated blood through the pulmonary trunk to the lungs.

• In the lungs, blood passes through pulmonary capillaries for gas exchange (carbon dioxide release and oxygen absorption).

• After gas exchange, oxygenated blood returns to the left atrium through the pulmonary veins.

Heart Valves and Function

• Valves: Prevent backflow of blood

  • Right atrioventricular (AV)/Tricuspid Valve: Between RA and RV.

  • Pulmonary Semilunar Valve: Between RV and pulmonary trunk.

  • Left AV/Bicuspid/Mitral Valve: Between LA and LV.

  • Aortic Semilunar Valve: Between LV and aorta.

Phases of the Cardiac Cycle and Their Relation to the ECG

The cardiac cycle entails several phases, each of which corresponds to specific parts of the electrocardiogram (ECG) that reflect the electrical activity of the heart. Understanding this relationship provides insight into the synchronous mechanical activity of the heart in relation to its electrical impulses.

Phases of the Cardiac Cycle:

1. Ventricular Filling (Diastole):

   • ECG Relation: This phase occurs primarily during the resting state of the heart, represented on the ECG during the time between the T wave of the previous heartbeat and the beginning of the next P wave.

   • The ventricles fill with blood from the atria, and there is little to no electrical activity.

2. Atrial Systole:

   • ECG Relation: Atrial contraction is represented by the P wave in the ECG, which indicates atrial depolarization.

   • The P wave precedes the mechanical contraction of the atria, ensuring the ventricles are filled with blood as the atria contract.

3. Ventricular Systole:

   • ECG Relation: This phase corresponds to the QRS complex on the ECG, which signifies ventricular depolarization.

   • As the ventricles depolarize (shown by the QRS), they contract, leading to the ejection of blood into the arteries. The ventricles contract just after the QRS complex, enhancing blood flow into the aorta and pulmonary trunk.

4. Isovolumetric Relaxation:

   • ECG Relation: Following the peak of the QRS complex and into the T wave, isovolumetric relaxation occurs as the ventricles begin to repolarize.

   • The T wave on the ECG represents ventricular repolarization. During this period, the valves are closed, and there is no change in volume as the heart prepares for the next filling cycle

Cardiac Output

• Definition: Amount of blood pumped from the left ventricle per minute.

• Formula: Cardiac Output (CO) = Heart Rate (HR) × Stroke Volume (SV).

• Adjusted to meet tissue metabolic needs.

Stroke Volume

• Volume of blood pumped per heartbeat. It is influenced by factors such as preload, afterload, and contractility.

Influencing Factors:

• Preload: Degree of stretch of cardiac muscle.

• Contractility: Strength of contraction.

• Afterload: Resistance faced during ejection.

Conduction System of the Heart

• Components:

  1. Sinoatrial (SA) Node: Primary pacemaker, initiates heartbeat.

  2. Atrioventricular (AV) Node: Delays impulse for ventricular filling.

  3. AV Bundle and Branches: Conduct impulses to ventricles.

  4. Purkinje Fibers: Distribute impulse for contraction.

• Automaticity: Heart's ability to generate impulses without external stimuli.

Electrocardiograms (ECG)

• Summary Table of Events:

  Cardiac Cycle Phase	EKG Event	Atrial Pressure	Ventricular Pressure	Aortic/Pulmonary Pressure
  Atrial Systole	P wave	↑	Slight ↑	No significant change
  Isovolumetric Contraction	QRS complex	↓	↑ rapidly	No significant change
  Ventricular Ejection	ST segment	↓	Peak ↑, then ↓	↑ due to blood ejection
  Isovolumetric Relaxation	T wave	Gradual ↑	Rapid ↓	Dicrotic notch (valve closure)
  Ventricular Filling	No event	Slight ↓, then ↑	Low but slightly rising	Gradual ↓

• P wave → Atrial depolarization → Atrial systole (small pressure increase in atria & ventricles).

• QRS complex → Ventricular depolarization → Ventricular systole (sharp pressure rise, AV valves close).

• T wave → Ventricular repolarization → Isovolumetric relaxation (ventricular pressure drops, semilunar valves close).

• Cycle repeats as atrial pressure rises, AV valves open, and ventricles fill again.