The term “auricle” describes a small, projecting, ear-shaped structure or appendage found on a larger body. In human anatomy, this word refers to two separate structures performing different roles in distinct organ systems. The more commonly known auricle is the external, visible part of the ear, responsible for gathering sound energy. Conversely, small, muscular pouches extending from the upper chambers of the heart are also called auricles. While they serve a minor mechanical function, they carry significant clinical importance.
The Outer Ear Structure and Anatomy
The auricle, also known as the pinna, is the visible portion of the external ear that projects from the side of the head. This structure is primarily composed of an irregular plate of flexible elastic cartilage, which gives the ear its complex shape. The cartilage framework is covered by a thin layer of skin and is anchored to the surrounding skull by ligaments and vestigial muscles.
The only part of the auricle lacking cartilaginous support is the earlobe (lobule), a soft, fleshy tissue at the bottom. The auricle’s convoluted shape is defined by several ridges and depressions. These include the prominent outer rim called the helix, and the antihelix, which runs parallel to the helix and divides superiorly into two extensions known as crura.
The deepest depression is the concha, which funnels sound directly into the ear canal (external acoustic meatus). Two small, opposing cartilaginous projections, the tragus and antitragus, guard the concha’s entrance. This unique topography of ridges and valleys is integral to the structure’s function.
How the Auricle Contributes to Hearing
The primary function of the external ear is to collect sound waves from the air and channel them toward the middle ear. The auricle’s large, concave surface acts like a satellite dish, gathering acoustic energy and directing it down the ear canal to strike the eardrum. This funnelling action provides natural sound amplification, particularly for frequencies in the range of human speech, often providing a boost of several decibels.
The complex, curved contours of the helix and concha are important for sound localization—the ability to determine the direction and elevation of a sound source. When sound waves reflect off these ridges and depressions, they are subtly altered. This creates unique spectral patterns that vary depending on the sound’s origin. The brain interprets these modifications to accurately map the sound’s location in three-dimensional space.
One specific modification is the “pinna notch,” a filtering effect that eliminates a narrow band of frequencies, typically around 10 kilohertz. This notch is directionally dependent and helps the listener distinguish between sounds originating from above or below. Without the auricle’s specific filtering and modification of sound waves, a person would struggle to differentiate the vertical location of a noise.
The Auricle of the Heart
The term auricle also describes the left and right atrial appendages, small, ear-shaped pouches extending from the heart’s two upper chambers (the atria). These structures are remnants of the embryonic heart. They are distinguished by a highly trabeculated, or ridged, internal surface formed by pectinate muscles. Both auricles act as minor reservoirs, slightly increasing the atria’s capacity to accommodate returning blood.
Physiologically, these appendages play a minimal role in regulating blood volume, but the left atrial appendage (LAA) is of clinical concern. During normal heart rhythm, blood flows efficiently through the atria and the LAA, preventing pooling. In conditions like atrial fibrillation, the rapid and irregular beating of the atrium causes blood flow to become stagnant within the pouch-like structure of the LAA.
This stasis allows for the formation of blood clots (thrombi). The LAA is the source of thromboembolism in a large majority of patients with nonvalvular atrial fibrillation. If a clot breaks free, it can travel through the bloodstream and lodge in a smaller artery in the brain, causing an ischemic stroke. Because of this risk, the LAA is often the target for therapeutic interventions, such as blood thinning medication or surgical occlusion devices, to prevent stroke in high-risk patients.