We feel hungry when the blood gets depleted of nutritive materials. This is because we need to get nutrients to survive. Hunger is the motivation for us to be able to know that we need to get the nutrients in our body.
When the blood gets depleted of nutritive materials, a message is sent to our brain by our body through nerves. The deficiency of nutritive materials is caused in the blood by the consumption of energy in the routine work. Whenever there is a deficiency of nutritive food substances in the blood, a message goes to the hunger centre of the brain. After eating, gradually the blood regains sufficient nutritive substances and therefore the feeling of hunger goes away.