A femoral sheath has two definitions. The first refers to a band of fibrous tissue located in the upper thigh, through which pass the main femoral artery, vein, and lymph vessels. The tissue type is called a fascia and provides protection and guidance to the vessels as they transition from deep within the buttocks area to closer to the skin. A femoral sheath is also a medical device that creates a temporary portal into a vein or artery of the upper thigh, usually for the purpose of introducing devices into the heart to diagnose or treat cardiac disease. The precise placement of this device is critical to its proper functioning and to avoid complications of the procedure.

Fascia refers to any sheet or band of connective tissue that provides support to the organs and tissues within the body. These tough but flexible tissues allow movement within defined ranges, similar to the effect of a girdle or a compression stocking. In exposed areas, such as behind the knees, the inner elbow, or the inner thigh, where nerves and blood vessels must pass close to the skin, fascia also provides protection from injury, as the thick tissues can't easily be penetrated.

The femoral sheath forms a funnel that is oval in shape near the top and more rounded near the bottom. The blood and lymph vessels enter the funnel each in its own separate compartment formed by the fibrous tissue. The purpose of the femoral sheath is to protect these vital vessels from damage as the leg moves relative to the hip.

The medical device known as a femoral sheath is a single-use piece of tubing that is placed into a femoral artery or vein, depending on what part of the heart needs treatment. This specialized type of catheter is 4 to 6 French and a few inches long. French is a measure of the outside diameter of catheters; 4 to 6 French is about 0.0523 to 0.067 inches (1.35 to 2 mm).

The exact placement of this catheter in relation to the femoral sheath anatomical structure is very important. With entry into the vessel that is too high, the catheter will be too stiff to follow the vessel's turn into the body; too low, and it may follow the wrong subvessel. Specialized training is required to master these procedures.

The use of a femoral sheath involves risk to the vascular system, including infection, perforation of tissue, and loss of circulation for an extended period. The risk is mitigated by the important benefits provided, such as the examination of the vessels and valves of the heart, removal of plaque, or the placement of stents. The use of femoral sheaths is considered standard in cardiac surgery.