The term “flank” refers to a distinct anatomical region on the side of the human body, situated in the lower torso. It serves as a protective layer and a point of flexibility for the body’s core. This overview clarifies the physical boundaries of the flank and explains why discomfort in this area is commonly associated with specific internal conditions.
Defining the Human Flank
The human flank is a paired region that wraps around the side of the trunk, beginning where the lower back transitions to the abdomen. Its superior boundary is marked by the costal margin, which is the lower edge of the rib cage. Vertically, the flank extends downward from this point to the iliac crest, the large, curved bone at the top of the pelvis commonly known as the hip bone.
Horizontally, this area starts near the spine and extends toward the front of the torso. Anatomically, the flank is synonymous with the lateral aspect of the lumbar region of the back, which allows for the characteristic curve when the trunk twists or bends sideways.
Beneath the skin and fatty tissue, the flank is composed of several layers of muscle that provide strength and stability to the core. These include the external oblique, internal oblique, and transversus abdominis muscles, which are part of the abdominal wall. These muscles are responsible for trunk rotation, side-bending, and compressing the abdominal contents.
Deep within this muscular and fatty layer lies the retroperitoneal space, which houses several internal organs. The most significant structures are the kidneys, which sit against the posterior abdominal wall, partially protected by the lower ribs and surrounding muscle tissue.
Common Causes of Flank Pain
Discomfort or pain felt in the flank area often originates from the organs located deep within the region, particularly the urinary system. Kidney stones, which are hard deposits that form in the kidney and travel down the ureter, are a frequent cause of severe flank pain. The intense, cramping pain associated with a stone occurs when it obstructs the flow of urine, causing the ureter or renal capsule to distend.
Another common source of pain is a kidney infection, known as pyelonephritis, which is often a complication of a urinary tract infection (UTI). This type of pain tends to be a more constant, dull ache rather than the sharp, intermittent pain of a stone. Kidney infections are typically accompanied by other symptoms, such as fever, chills, and painful or urgent urination.
Not all flank discomfort is related to the kidneys; many musculoskeletal issues can also manifest pain in this area. Strains or spasms in the large muscles of the back and side, such as the latissimus dorsi or the quadratus lumborum, can cause localized tenderness often linked to physical activity, poor posture, or sudden movements.
Issues with the spine, such as arthritis, a herniated disc, or a pinched nerve, can cause referred pain to radiate into the flank. Less frequently, conditions involving the digestive tract, like diverticulitis or gallbladder disease, may also cause pain that is perceived in the flank area. Because flank pain can be a symptom of conditions ranging from a simple muscle strain to a serious kidney issue, persistent or severe discomfort should always be evaluated by a healthcare provider.