A、The volume of blood and blood pressure will drop without it. B、It is the base for digestion and absorption of food. C、It is re

How long can a human survive without food or water? In theory, when you finally run out of body fat, protein and carbohydrates, your body runs out of energy and stops functioning. Jeremy Powell-Tuck, a retired clinician who fed David Blaine after his starvation stunt in London in 2003, isn’t so sure that this is the lethal point. "You’re more likely to die before then," he says.(20)Fat people would only be able to survive for longer if they had enough vital water-soluble B vitamins in their system to help metabolize fat stores. So it is possible that a person could die of starvation and still be fat.(21)The longest recorded starvation was by the Irish hunger-striker Kieran Doherty in 1981, who died after fasting for 73 days. With a supply of vitamins and water, people have been known to survive over a year without eating. "It used to be a very fashionable way of losing weight around 30 years ago," says Powell-Tuck. With vitamins but without water, survival time is sharply reduced. A human can be expected to survive for weeks without food, but a thirsty person deprived of water would last a matter of days. "It depends on the rate of water loss," says Michael Sawka at the US Army Research Institute of Environmental Medicine in Natick, Massachusetts.(22)"Without water, the volume of blood in your body drops, and with it your blood pressure drops. Blood becomes thicker and stickier, making it harder to pump around the body, so your heart rate increases to compensate. " Even in a cool environment, you wouldn’t last for more than a week without water.
20. What can we learn about B vitamins from the passage?
21. What do we know about Kieran Doherty?
22. According to Michael Sawka, why is water vital to human survival?