Cardiovascular Anatomy - Heart

Heart Anatomy: Detailed Notes

Location of the Heart

  • The heart is located in the thoracic cavity.
  • The diaphragm separates the thoracic cavity from the abdominal pelvic cavity.
  • The heart sits medially between the right and left lungs within a subspace of the thoracic cavity called the mediastinum (or mediastinum).
  • The apex (tip) of the heart leans to the left lateral side, making it easier to hear the heartbeat on the left side.
  • The base of the heart is the posterior and inferior flat surface.

Pericardium (Pericardial Sac)

  • The heart is surrounded by a sac-like membranous structure called the pericardium or pericardial sac.
  • The pericardium consists of two layers:
    • Fibrous pericardium: Made of dense, collagen-rich connective tissue proper (CTP). Collagenous extensions connect it to surrounding structures, anchoring the membrane.
    • Serous pericardium: Subdivided into parietal and visceral pericardia.

Serous Pericardium

  • A single continuous sheet of tissue that doubles back on itself.
    • Parietal pericardium: The external surface.
    • Visceral pericardium: The internal surface.
  • Pericardium: peri (surrounding) + cardium (heart) = thing that surrounds the heart.
  • Serous pericardial layers are simple squamous epithelium.
  • Highly vascular to secrete serous fluid.

Pericardial Cavity and Fluid

  • Serous fluid (pericardial fluid) is secreted into the space between the parietal and visceral pericardia, called the pericardial cavity.
  • Continuous secretion and reabsorption maintain normal fluid volume and composition.
  • Function: Lubricates the heart's external surface to protect it from frictional damage due to movement during the cardiac cycle.
  • The pericardial cavity is a potential space, very small in size.
  • The fluid within doesn't provide cushioning from blunt force trauma.
  • The visceral pericardium is also called the epicardium.

Heart Wall Layers

  • Three layers:
    • Epicardium (outer layer)
    • Myocardium (middle layer)
    • Endocardium (inner layer)

Epicardium

  • Made of simple squamous epithelium (visceral pericardium) anchored to loose connective tissues (areolar and adipose).
  • Contains coronary blood vessels held in place on the surface.
  • Small arteries branch and dive into the heart wall, supplying the myocardial layer.

Myocardium

  • Thickest layer, made of cardiac muscle tissue.
  • Coronary arteries weave through this layer to supply oxygen and nutrients.
  • The thickness ensures oxygen reaches all cells, as diffusion from the heart chamber isn't sufficient.

Endocardium

  • Made of simple squamous epithelium.
  • Has a thin layer of connective tissue for attachment.
  • Contains trabeculae carneae: Irregular extensions of myocardial muscle tissue that line the inner surfaces of the heart chambers (especially ventricles).
    • Trabecula means branch, carne refers to meat.
    • Help decrease the amount of muscular force necessary during contraction of the heart to expel blood.

Ventricular Wall Thickness

  • The myocardial layer of the left ventricle is much thicker than that of the right ventricle.
    • The left ventricle ejects blood into the systemic circuit.
    • The right ventricle pumps blood into the pulmonary circuit.
  • The left ventricle must produce a stronger contraction to pump blood throughout the body.

Internal Anatomy

  • The right and left halves of the heart are separated by a septum (septa is plural).
    • Interventricular septum: Separates right and left ventricles.
    • Interatrial septum: Separates right and left atria.
  • Proper development of the septa during embryonic development is crucial; any gaps or openings can lead to congenital heart defects.
  • If there's an opening some oxygen in the left ventricle will diffuse from high to low and cross over to the right ventricle through that opening.
  • If the opening is large, the stronger left ventricle may push blood into the right ventricle during contraction, reducing oxygen supply to the body.
  • Small openings may have minor consequences, while large ones might require surgical correction.
  • Papillary muscles: Muscular extensions connected to chordae tendineae.
  • Chordae tendineae: String-like structures made of dense connective tissue that connect papillary muscles to atrioventricular valves.

Heart Valves

  • Atrioventricular valves (AV valves): Guard openings, acting as doorways between an atrium and a ventricle.
    • One-way valves to prevent backflow (regurgitation).
  • Chordae tendineae reinforce the valves and prevent eversion (opening in the wrong direction).
  • Defects in valve development can cause eversion or failure to close, leading to regurgitation and heart murmurs (abnormal heartbeat sounds).
  • Semilunar valves: Guard the junctions of a ventricle to its connecting artery.
    • Pulmonary semilunar valve (right semilunar): Connects the right ventricle to the pulmonary trunk.
    • Aortic semilunar valve (left semilunar): Connects the left ventricle to the aorta.
  • Alternate names:
    • Right AV valve: Tricuspid valve (three cusps/flaps).
    • Left AV valve: Bicuspid or mitral valve (two cusps/flaps).
  • All four heart valves should open in one direction and close to prevent regurgitation.

Fibrous Skeleton

  • Extensions of the fibrous connective tissue that the valves are made of, interconnecting the valves.
  • Provides physical support for the valves.
  • Electrically separates or insulates the atria from the ventricles, blocking free-flowing electric impulses.