Oxygen is the human body's fuel. 

It powers our brains, organs, and bodily tissues.

 Without a proper supply, our bodies can't function. 

Our brains, hearts, and kidneys can only tolerate a lack of oxygen for a limited time before their cells begin to die: 

about five minutes for the brain, 

about thirty for the heart and kidneys. 

When a patient loses oxygen, doctors know the clock is ticking. 

When a patient codes or loses their airway, a doctor's first priority is to get oxygen flowing.

  a limited time before damage sets in, so  spring into action. 

But in life, time limits aren't as clear.

 It can be hard to gauge whether it's better to hurry up or take things slow. 

We don't always know how much time we have before the clock runs out on opportunity.

 Often, we can't even be sure if our time's passed.

 All we can do is cease the present.