Our body needs energy to do work. Due to this our body requires a supply of oxygen continuously. Thus to supply oxygen continuously respiration goes on continuously. To carry oxygen to all the parts of the body continuously blood needs to circulate. Therefore circulation also needs to go on continuously. When energy is released the food absorbed in the blood gets exhausted. Through the network of nerves the brain gets the message and it signals us that we are hungry and we need to eat food.
The moment we eat food the nervous system brings about co-ordination between the digestive organs and digestion takes place at the right time. Thus respiration and blood circulation go on all the time and the process of digestion happens at the right time.
