A major cause for feeling hungry lies in physiology of blood circulation.
levels of different substances like glucose are maintained in the blood mainly by digestive system.
"GRELIN" is the hormone secreted in the stmach whhich is cause for hungerpanks.
it is belived that DIENCEPHALAN and VAGUS NERVE(10th cranial nerve)plays an important role in carring these signals(hunger panks sensation) 
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Hypothalamus, thalamus, limbic, cerebrum are the four organ systems
they are not organ systems. they are related to brain and nervous systems