Cardiac Anatomy Location

  • The heart is a hollow, muscular organ situated in the central thoracic cavity.
    • Slightly to the left of the midline.
    • Located between the 3rd and 5th intercostal spaces.
    • Anterior to the vertebrae.
    • Posterior to the sternum and lungs.

Pericardium

  • The heart is enclosed within a fluid-filled sac known as the pericardium.
    • Contains approximately 10-50 ml of serous fluid to prevent friction as the heart beats.
  • Functions of the pericardium:
    • Protects the heart against infection, trauma, and friction.
    • Aids in heart function by facilitating a free pumping motion.

Structure of the Pericardium

  • The pericardium is a loose, double-layered sac composed of elastic connective tissue.
    • Outer layer: Fibrous pericardium.
    • Inner layer: Serous pericardium.
    • Composed of two layers:
      • Parietal layer: Lines the inside of the fibrous pericardium.
      • Visceral layer (Epicardium): Adheres to the outside of the heart.

Pericardial Space

  • The pericardial space lies between the visceral and parietal layers.
    • Contains clear fluid released by the serous membrane.
    • This fluid acts as a lubricant to prevent friction during cardiac contraction and relaxation.
    • Normally contains 10-50 ml of fluid.
    • In certain diseases, this space may fill with excess fluid, a condition known as pericardial effusion.

The Heart

  • The heart consists of four chambers, functioning as a two-sided pump (right heart and left heart).
    • Right and Left Atria: Receive blood from the body.
    • Right and Left Ventricles: Pump blood out to the body.
    • The right side functions as a low-pressure system, receiving deoxygenated (venous) blood from the body and pumping it to the lungs (Pulmonary circulatory system).
    • The left side is a high-pressure system that receives oxygenated (arterial) blood from the lungs and pumps it to the body (Systemic circulatory system).

Blood Flow Diagram

  • Superior Vena Cava: From Upper Body
  • Pulmonary Artery: To Right Lung
  • Pulmonary Veins: From Right Lung
  • Pulmonary Artery: To Left Lung
  • Aorta:
  • Pulmonic Valve
  • Right Atrium - Tricuspid Valve
  • Inferior Vena Cava: From Lower Body
  • Right Ventricle
  • Left Atrium - Pulmonary Veins: From Left Lung
  • Left Ventricle
  • Mitral Valve
  • Aortic Valve
  • Aorta: To Lower Body

Atria

  • Each atrium contains appendages, also known as auricles, which are blind pouches extending from the atrial cavity.
    • The appendages increase the surface area of each atrium.
    • Composed of projecting muscles that run parallel, known as Pectinate muscles, which help increase contractility without increasing muscle mass.
  • Right Atrial appendage: Extends from the RA, overlaps with the aortic root.
  • Left Atrial appendage: Projects from the anterior lateral surface alongside the pulmonary artery.

Right Atrium Details

  • The lining of the Right Atrium (RA) is smooth, except for the anterior wall and appendages.
  • A common wall, the Interatrial septum (IAS), divides the right and left atria.
    • The fossa ovalis is a slight depression in the central portion of the IAS, representing the closed foramen ovale from fetal circulation.

Right Atrium's Location and Structure

  • Positioned as the superior portion of the heart, forming the right lateral cardiac border.
  • Located posteriorly and laterally relative to the Right Ventricle (RV).
  • Walls of the RA are relatively thin (posterior) and muscular (anterior and inferior).

Important Structures near Right Atrium

  • The right coronary and non-coronary cusps of the aortic valve are located against the medial right atrial wall, creating a bulge known as Torus aorticus.

Major Veins Entering Right Atrium

  • Superior Vena Cava: Drains blood from the upper body; enters superiorly and laterally into the RA.
  • Coronary Sinus: Drains blood from the myocardium (via coronary veins); enters posterior medially.
    • Protected by a rudimentary valve (Thebesian valve) at its entrance.
  • Inferior Vena Cava: Drains from the abdomen and lower extremities; enters inferiorly and laterally.
    • Protected by the Eustachian valve, which includes a Chiari network (a fenestrated or web-like portion).

Left Atrium

  • Located superiorly and posterior to other cardiac structures, making it the most posterior portion of the heart.
    • Receives oxygenated blood from the pulmonary veins. Typically, there are four pulmonary veins:
    • Right upper pulmonary vein
    • Left upper pulmonary vein
    • Right lower pulmonary vein
    • Left lower pulmonary vein
  • Functions of the left atrium:
    • Serves as a reservoir during ventricular systole.
    • Provides significant blood flow to the left ventricle during left atrial contraction.
    • Acts as a conduit during left ventricular filling.

Left Atrium Structural Details

  • The anterior portion is more muscular, containing the LA appendage, a common site for thrombus formation, better seen on Transesophageal Echo (TEE) than Transthoracic Echo (TTE).
  • The lining is generally smooth, with Pectinate muscles in the appendage and a less prominent fossa ovalis.
  • Slightly smaller than the RA; walls are thicker due to higher pressure on the left side.

Ventricles

  • The ventricles are located inferior to their respective atria.
  • Their walls are thicker than those of the atria, reflecting the higher pressure within the ventricles.
  • Divided by the Interventricular Septum (IVS), which is somewhat triangular in shape.

Interventricular Septum (IVS)

  • Mostly muscular, except for a membranous part located at the heart's base under the Right and Non-coronary cusps of the aorta.
  • This membranous septum has a curved configuration extending superiorly to the base of the semilunar valves.

Functionality of IVS and Ventricles

  • The muscular septum contracts with the Left Ventricle (LV) aiding in efficient ejection of blood.
  • The Right Ventricle (RV) functions by using only the free wall during contraction, as higher pressure is generated by the LV.
  • The upper third of the IVS consists of smooth endocardium, while the LV wall is primarily lined with trabeculae carneae.

Right Ventricle (RV)

  • Receives deoxygenated blood from the right atrium during diastole and pumps it to the lungs during systole.
  • Positioned anteriorly, directly beneath the sternum, crescent-shaped with thinner walls compared to the LV.

Structural Details of Right Ventricle

  • Contains trabeculae carneae, noted for a heavily trabeculated structure when compared with the Left Ventricle.
  • The Crista Supraventricularis separates the outflow tract from the inflow tract and arches over the anterior leaflet of the tricuspid valve.

Right Ventricular Outflow Tract

  • Located superiorly and anteriorly, cone-shaped with relatively smooth walls.
  • Known as the Conus Arteriosus or Infundibulum, ending at the pulmonic valve.
  • Contains a Moderator Band, which extends from the lower IVS to the anterior wall (RV free wall), aiding in preventing over-expansion and carrying the conduction system's right bundle branch.

Left Ventricle (LV)

  • Receives oxygenated blood from the Left Atrium during diastole and pumps it to the Aorta/body during systole.
  • Larger than the RV and has an ellipsoid (conical) shape, divided into inflow and outflow tracts.

LV Inflow and Outflow Tract

  • The inflow tract is funnel-shaped, formed by:
    • Mitral valve annulus
    • Mitral leaflets
    • Chordae tendinae
  • The outflow tract is surrounded by the inferior surface of the anterior medial mitral leaflet, IVS, and LV free wall, ending at the aortic annulus.

Structural Characteristics of the Left Ventricle

  • LV walls are 2 to 3 times thicker than those of the RV due to the requirement of generating higher systemic pressures despite equal chamber volumes.
  • The myocardium in the LV comprises 75% of the total muscle mass of the heart and has less trabeculation compared to the RV.

Heart Valves

  • Four main valves exist in the heart categorized into:
    • Semilunar valves: Aortic valve and Pulmonic valve.
    • Atrioventricular valves: Tricuspid valve and Mitral valve.
  • All valves function to prevent retrograde blood flow within the heart.

Semilunar Valves

  • Comprise the Pulmonic and Aortic valve, serving as outlets to the ventricles.
  • Each semilunar valve consists of three fibrous cusps.
  • They open when ventricular pressure exceeds that of the great vessels, allowing blood flow to pulmonary and systemic circulations, and close when ventricular pressure lowers to prevent retrograde flow.

Aortic Valve Details

  • Located in the Left Ventricular Outflow Tract (LVOT) between the LV and Aorta.
  • Comprised of three cusps: Right coronary cusp, Left coronary cusp, and Non-coronary cusp.
  • Each cusp has a dilated pouch-like area called the sinus of Valsalva, where two sinuses give rise to the coronary arteries:
    • Left sinus → Left Coronary Artery
    • Right sinus → Right Coronary Artery
  • Each cusp's end contains an Arantius nodule to support the cusp and prevent leakage.
    • Normal valve area: 2−4extcm22-4 ext{ cm}^2.

Pulmonic Valve Details

  • The Pulmonic valve is the right-sided semilunar valve within the Right Ventricular Outflow Tract (RVOT) between the RV and Main Pulmonary Artery (MPA).
  • Consists of three cusps: Anterior, Right, and Left posterior cusp.
  • Cusp structure is slightly thinner than that of the Aortic valve.
    • Normal valve area: 2−4extcm22-4 ext{ cm}^2.

Atrioventricular Valves

  • Separate the atria and ventricles; include:
    • Right AV valve: Tricuspid valve.
    • Left AV valve: Mitral or Bicuspid valve.
  • Composed of leaflets supported by the valve annulus, chordae tendinae, and papillary muscles.
  • Function as inlet valves of the ventricles, preventing backflow into the atria.

Tricuspid Valve Details

  • Located between the Right Atrium and Right Ventricle.
  • Larger and thinner than the Mitral valve, more apically positioned.
  • Composed of three leaflets made of fibrous tissue coated by endocardium; differing sizes:
    • Anterior: Largest
    • Septal: Attaches to both membranous and muscular areas of the IVS
    • Posterior: Usually smallest, attaches to the tricuspid ring.
    • Normal valve area: 7−9extcm27-9 ext{ cm}^2.

Mitral Valve (Bicuspid) Details

  • The leaflets are thicker than those of the Tricuspid valve to withstand higher pressures in the LV.
  • Comprised of two leaflets attached at one end to the annulus fibrosis, encircling the valve orifice.
  • Connected to two groups of papillary muscles and chordae tendinae in the LV.
    • Normal valve area: 4−6extcm24-6 ext{ cm}^2.
  • Anterior medial leaflet: Triangular, extends from the posterior medial LV to the anterolateral LV wall, continuous with the posterior Aortic root wall.
  • Posterior lateral leaflet: Quadrangular, longer, encircles approximately two-thirds of the mitral valve orifice.