CBSE Class 10 Answered

We need oxygen to produce energy in all our cells. We can make energy for a short time without oxygen, but this builds up toxic substances that will eventually kill the cells. So without oxygen all the  cells (as well as the brain and heart) will die. This will happen within a few minutes.

(Going a bit more in detail - Breathing is triggered by rising carbon dioxide levels in the blood rather than diminishing oxygen levels. The central nervous system, specifically those parts involved in consciousness, will not continue to function for more than a few seconds without oxygen. The disruption of cell metabolism and the accumulation of toxic by-products in the body results in patho-physiological consequences such as tissue necrosis, loss of consciousness and death. Carbon dioxide dissolved in blood forms carbonic acid, which acidifies the blood. Too much carbon dioxide in the body causes acidosis, which can kill. )

When man is without food supply, his body can draw from the supply of nutrients already present within the body. Once that is exhausted, the body burns the fat stored in tissues to provide the required amount of energy. (Each pound of body fat contains about 3600 calories, enough energy for the average person for about a day and half) In addition, muscle is broken down by a starving body and used for both fuel and nutrients. Due to this, we can survive without food for several days.