An Apple chip internally code-named 'Phosphorous' has been identified on leaked schematics and thought to replace the M7, a motion coprocessor which debuted inside the iPhone 5s last Fall. (Update: It's looking like a barometer pressure sensor instead.)
It's said to include the M7's motion tracking functions and thought to be able to collect a number of health and fitness data from various health and fitness accessories and specialized medical devices.
This apparently includes heart rates, calories burned, cholesterol levels, blood sugar and more. It's believed the chip works in tandem with iOS 8 and the new Health app, which allows users to enter a number of health and fitness-related data manually, or automatically collect these from various HealthKit-friendly accessories and wearables.