What systems stop working to cause death?

Cardiovascular, Nervous, Respiratory, Urinary, Digestive, Endocrine, Exocrine... etc.

Ans2. In Patanjali Yog the air we breath is termed as Pran vayu meaning by air for life. Therefore ultimately to cause death at some stage respiratory system has to stop. That's why respirator is one of the last life support system used in ICUs.

It also implies that we must learn how to exercise breathing. There are many ways a person can breath to improve its respiratory sytem. Anulom vilom, kapalbhati etc are few examples of breathing exercise as carried out in Yoga.