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.