Methods and systems are provided for measuring the flow of fluid (e.g., blood) in vasculature of a human by heating the fluid in a portion of the vasculature and detecting the temperature of fluid in and/or tissue proximate to a downstream portion of the vasculature. The blood is heated by emitting energy through an external body surface into the portion of vasculature. The temperature of the downstream portion of vasculature is detected by an infrared sensor receiving infrared light from the body and/or a temperature sensor in thermally conductive contact with the body. The flow of fluid in the portion of vasculature can be determined by determining a latency between a change in the detected temperature relative to a change in the emitted energy, by determining a difference between the detected temperature and a detected temperature of another portion of the body, or by some other method.
