There are not really any biological uses for Krypton
Krypton is found in trace amounts in the Earth's crust.
It is is the atomsphere, about 1ppm.
Krypton-85 is used to detect abnormal heat openings in a heart. The glow Krypton emits show the leaks that the doctor needs to repair. During the procedure, the patient inhales a krypton-85 air mixture.
In MRI machines Krypton-83 is used to make out the image of the lungs and to see if tobacco smoke deposits are in the lungs.
The compound KrF is used in Krypton fluoride lasers. These lasers have high levels of powers to stop bleeding, prevent diseases and disorders during eye surgery.
Krypton-81 is inhaled by the patient and it shows the how air reaches to the lungs which is recorded on a gamma camera.