The two upper chambers of the heart, the atria, receive blood before it moves into the lower ventricles for pumping. While the main atrial chambers are relatively smooth-walled, the external surface contains small, muscular pouches that project off the main body of the atria. These structures, which are not the main atrial chambers, play specialized roles in cardiac anatomy and function.
Identifying the Atrial Appendages
The flaps on the front of the atria are formally known as the atrial appendages. They are also commonly referred to as auricles, derived from the Latin auricula (“little ear”), due to their resemblance to a small, ear-like pouch. There is one appendage attached to the right atrium and one to the left atrium; the right appendage is typically larger and more triangular. These muscular extensions act as a reservoir, slightly increasing the overall volume capacity of the atria.
Specific Location and Internal Structure
There is one appendage associated with the right atrium (RAA) and one with the left atrium (LAA), each situated near the major blood vessels entering the heart. The internal walls of both appendages are characterized by the presence of pectinate muscles, or musculi pectinati. These are parallel, comb-like ridges of muscle tissue that make the inner lining of the appendages rough and highly trabeculated.
The pectinate muscles are particularly prominent in the right atrial appendage, extending from a vertical ridge known as the crista terminalis. In contrast, the pectinate muscles of the left atrial appendage are generally fewer and confined mostly to the appendage itself. This muscular architecture allows the appendages to expand significantly, increasing the volume of the atria. The highly textured interior also has significant implications for blood flow dynamics.
Physiological Role and Function
Beyond their role in increasing volume, the atrial appendages contribute to the regulation of the body’s fluid balance and blood pressure. They possess a greater capacity for distensibility compared to the main atrial chambers, allowing them to function as a decompression chamber. This ability is particularly relevant when left atrial pressure increases, such as during ventricular contraction.
Furthermore, the muscular walls of the appendages are rich in granules that contain natriuretic peptides, such as Atrial Natriuretic Peptide (ANP). When the atrial walls are stretched due to high blood volume or pressure, these peptides are secreted into the bloodstream. ANP acts on the kidneys to increase the excretion of salt and water, which helps to lower the overall blood volume and subsequently reduce blood pressure.
Clinical Importance of the Left Atrial Appendage
The left atrial appendage (LAA) holds significant clinical importance, primarily because of its connection to stroke risk in patients with an irregular heart rhythm. The LAA’s narrow opening and its intricate, trabeculated interior create an environment prone to blood stasis, which is a condition where blood flow is slow or stagnant.
In the presence of Atrial Fibrillation (AFib), the atria quiver instead of contracting effectively, causing blood to pool. Because of its complex, recessed anatomy, the LAA becomes the principal site where blood clots, or thrombi, form. In patients with non-valvular AFib, over 90% of stroke-causing blood clots originate within the LAA. If a clot breaks loose from the LAA wall, it can travel to the brain and cause an ischemic stroke. Due to this risk, clinical interventions like LAA occlusion devices are sometimes used to seal off the pouch.