What does a medical professional use to measure blood loss during surgery?

A hemostat is employed by medical professionals during surgery primarily to control bleeding. It is a type of surgical instrument that is designed to clamp blood vessels and tissues to stop blood flow. This ability is crucial when measuring blood loss, as it allows the surgeon to effectively manage and limit any bleeding, thereby providing a clearer assessment of how much blood has been lost throughout the procedure.

Other instruments mentioned, such as a goniometer, which is used to measure the angle of joints for physical therapy or orthopedic assessments, or a retractor, used to hold back tissues for better visibility, do not serve the specific purpose of measuring blood loss. Additionally, a tonometer is utilized to measure the pressure within the eyes and is not related to the measurement of blood loss in surgical settings. Thus, the hemostat is the most appropriate tool among these options for monitoring blood loss during surgery.