A recent Apple patent describes how the front-facing camera, light sensor and proximity sensor found in existing iPhones could be used for health measurements.
The patent was filed on in February 2015 and was granted today:
"The electronic device uses one or more of the camera and the proximity sensor to emit light into a body part of a user touching a surface of the electronic device and one or more of the camera, the ambient light sensor, and the proximity sensor to receive at least part of the emitted light reflected by the body part of the user. The electronic device computes health data of the user based upon sensor data regarding the received light."
The patent also says that more sensors mounted in the same area can take health measurements. For example, it describes electrical contacts similar to those found on some more bathroom scales to measure body fat. The same contacts could also be used to provide an electrocardiogram (ECG) reading, a technique that's already been seen in at least one iPhone case.
"The health data may include one or more of a variety of different wellness, fitness, and/or other parameters relating to the health of a user such as: a blood pressure index, a blood hydration, a body fat content, an oxygen saturation, a pulse rate, a perfusion index, an electrocardiogram, a photoplethysmogram, and/or any other such health data."
Using just a camera and light sensor, the system could measure oxygen saturation, pulse rate, perfusion index and a photoplethysmogram.
As always, we cannot know whether any of these features will be implement in a real product.